From a6af87660e79b2eabc7e48260bf1afb9ea336530 Mon Sep 17 00:00:00 2001 From: github-actions Date: Sun, 28 Jul 2024 12:08:43 +0000 Subject: [PATCH] Deployed 4239ecf with MkDocs version: 1.2.3 --- CODE_OF_CONDUCT/index.html | 2 +- CONTRIBUTING/index.html | 6 +- index.html | 16 +- level101/big_data/evolution/index.html | 48 +-- level101/big_data/intro/index.html | 12 +- level101/big_data/tasks/index.html | 6 +- level101/databases_nosql/intro/index.html | 24 +- .../databases_nosql/key_concepts/index.html | 50 ++- .../databases_sql/backup_recovery/index.html | 169 ++++---- level101/databases_sql/concepts/index.html | 32 +- level101/databases_sql/conclusion/index.html | 2 +- level101/databases_sql/innodb/index.html | 4 +- level101/databases_sql/intro/index.html | 4 +- level101/databases_sql/lab/index.html | 76 ++-- level101/databases_sql/mysql/index.html | 20 +- level101/databases_sql/operations/index.html | 30 +- .../query_performance/index.html | 109 ++--- level101/databases_sql/replication/index.html | 178 +++++---- .../databases_sql/select_query/index.html | 168 ++++---- level101/git/branches/index.html | 22 +- level101/git/git-basics/index.html | 28 +- level101/git/github-hooks/index.html | 10 +- .../command_line_basics/index.html | 273 +++++++------ level101/linux_basics/conclusion/index.html | 16 +- level101/linux_basics/intro/index.html | 46 +-- .../linux_server_administration/index.html | 292 +++++++------- .../linux_networking/conclusion/index.html | 8 +- level101/linux_networking/dns/index.html | 46 +-- level101/linux_networking/http/index.html | 18 +- level101/linux_networking/intro/index.html | 6 +- level101/linux_networking/ipr/index.html | 22 +- level101/linux_networking/tcp/index.html | 24 +- level101/linux_networking/udp/index.html | 10 +- level101/messagequeue/intro/index.html | 2 +- level101/messagequeue/key_concepts/index.html | 2 +- .../metrics_and_monitoring/alerts/index.html | 10 +- .../best_practices/index.html | 16 +- .../command-line_tools/index.html | 44 +-- .../conclusion/index.html | 4 +- .../introduction/index.html | 52 +-- .../observability/index.html | 4 +- .../third-party_monitoring/index.html | 6 +- level101/python_web/intro/index.html | 32 +- .../python_web/python-concepts/index.html | 22 +- .../python_web/python-web-flask/index.html | 14 +- level101/python_web/sre-conclusion/index.html | 20 +- .../python_web/url-shorten-app/index.html | 15 +- level101/security/conclusion/index.html | 34 +- level101/security/fundamentals/index.html | 101 ++--- level101/security/intro/index.html | 2 +- level101/security/network_security/index.html | 373 ++++++++++-------- .../threats_attacks_defences/index.html | 98 +++-- .../security/writing_secure_code/index.html | 6 +- .../systems_design/availability/index.html | 42 +- level101/systems_design/conclusion/index.html | 2 +- .../systems_design/fault-tolerance/index.html | 47 ++- level101/systems_design/intro/index.html | 7 +- .../systems_design/scalability/index.html | 156 ++++---- search/search_index.json | 2 +- sitemap.xml | 206 +++++----- sitemap.xml.gz | Bin 1226 -> 1227 bytes 61 files changed, 1686 insertions(+), 1410 deletions(-) diff --git a/CODE_OF_CONDUCT/index.html b/CODE_OF_CONDUCT/index.html index 5b2d6dd..fe28caf 100644 --- a/CODE_OF_CONDUCT/index.html +++ b/CODE_OF_CONDUCT/index.html @@ -2177,7 +2177,7 @@

Note: Some LinkedIn-managed communities have codes of conduct that pre-date this document and issue resolution process. While communities are not required to change their code, they are expected to use the resolution process outlined here. The review team will coordinate with the communities involved to address your concerns.

Reporting Code of Conduct Issues

We encourage all communities to resolve issues on their own whenever possible. This builds a broader and deeper understanding and ultimately a healthier interaction. In the event that an issue cannot be resolved locally, please feel free to report your concerns by contacting oss@linkedin.com.

-

In your report please include:

+

In your report, please include:

A query language to interact with and manage data.

-

CRUD operations - create, read, update, delete queries

-

Management operations - create DBs/tables/indexes etc, backup, import/export, users, access controls

-

Exercise: Classify the below queries into the four types - DDL (definition), DML(manipulation), DCL(control) and TCL(transactions) and explain in detail.

+

CRUD operations—create, read, update, delete queries

+

Management operations—create DBs/tables/indexes, backup, import/export, users, access controls, etc

+

Exercise: Classify the below queries into the four types—DDL (definition), DML (manipulation), DCL (control) and TCL (transactions) and explain in detail.

insert, create, drop, delete, update, commit, rollback, truncate, alter, grant, revoke

You can practise these in the lab section.

@@ -2188,19 +2188,19 @@

  • Constraints

    Rules for data that can be stored. Query fails if you violate any of these defined on a table.

    -

    Primary key: one or more columns that contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key. An index on it is created by default.

    -

    Foreign key: links two tables together. Its value(s) match a primary key in a different table \ -Not null: Does not allow null values \ -Unique: Value of column must be unique across all rows \ -Default: Provides a default value for a column if none is specified during insert

    -

    Check: Allows only particular values (like Balance >= 0)

    +

    Primary key: One or more columns that contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key. An index on it is created by default.

    +

    Foreign key: Links two tables together. Its value(s) match a primary key in a different table

    +

    Not null: Does not allow null values

    +

    Unique: Value of column must be unique across all rows

    +

    Default: Provides a default value for a column if none is specified during insert

    +

    Check: Allows only particular values (like Balance >= 0)

  • Indexes

    Most indexes use B+ tree structure.

    -

    Why use them: Speeds up queries (in large tables that fetch only a few rows, min/max queries, by eliminating rows from consideration etc)

    -

    Types of indexes: unique, primary key, fulltext, secondary

    -

    Write-heavy loads, mostly full table scans or accessing large number of rows etc. do not benefit from indexes

    +

    Why use them: Speeds up queries (in large tables that fetch only a few rows, min/max queries, by eliminating rows from consideration, etc)

    +

    Types of indexes: unique, primary key, fulltext, secondary

    +

    Write-heavy loads, mostly full table scans or accessing large number of rows, etc. do not benefit from indexes

  • Joins

    @@ -2208,14 +2208,14 @@ Default: Provides a default value for a column if none is specified during inser

  • Access control

    -

    DBs have privileged accounts for admin tasks, and regular accounts for clients. There are finegrained controls on what actions(DDL, DML etc. discussed earlier )are allowed for these accounts.

    +

    DBs have privileged accounts for admin tasks, and regular accounts for clients. There are fine-grained controls on what actions (DDL, DML, etc. discussed earlier) are allowed for these accounts.

    DB first verifies the user credentials (authentication), and then examines whether this user is permitted to perform the request (authorization) by looking up these information in some internal tables.

    -

    Other controls include activity auditing that allows examining the history of actions done by a user, and resource limits which define the number of queries, connections etc. allowed.

    +

    Other controls include activity auditing that allows examining the history of actions done by a user, and resource limits which define the number of queries, connections, etc. allowed.

    • -

    Commercial, closed source - Oracle, Microsoft SQL Server, IBM DB2

    -

    Open source with optional paid support - MySQL, MariaDB, PostgreSQL

    +

    Commercial, closed source: Oracle, Microsoft SQL Server, IBM DB2

    +

    Open source with optional paid support: MySQL, MariaDB, PostgreSQL

    Individuals and small companies have always preferred open source DBs because of the huge cost associated with commercial software.

    In recent times, even large organizations have moved away from commercial software to open source alternatives because of the flexibility and cost savings associated with it.

    Lack of support is no longer a concern because of the paid support available from the developer and third parties.

    diff --git a/level101/databases_sql/conclusion/index.html b/level101/databases_sql/conclusion/index.html index 1956849..79c1422 100644 --- a/level101/databases_sql/conclusion/index.html +++ b/level101/databases_sql/conclusion/index.html @@ -2151,7 +2151,7 @@

    Conclusion

    -

    We have covered basic concepts of SQL databases. We have also covered some of the tasks that an SRE may be responsible for - there is so much more to learn and do. We hope this course gives you a good start and inspires you to explore further.

    +

    We have covered basic concepts of SQL databases. We have also covered some of the tasks that an SRE may be responsible for—there is so much more to learn and do. We hope this course gives you a good start and inspires you to explore further.

    Further reading

    • More practice with online resources like this one
      • diff --git a/level101/databases_sql/innodb/index.html b/level101/databases_sql/innodb/index.html index 692887b..a008bef 100644 --- a/level101/databases_sql/innodb/index.html +++ b/level101/databases_sql/innodb/index.html @@ -2175,7 +2175,7 @@

      InnoDB

      Why should you use this?

      -

      General purpose, row level locking, ACID support, transactions, crash recovery and multi-version concurrency control etc.

      +

      General purpose, row level locking, ACID support, transactions, crash recovery and multi-version concurrency control, etc.

      Architecture

      alt_text

      Key components:

      @@ -2183,7 +2183,7 @@

    • Memory:

        -
      • Buffer pool: LRU cache of frequently used data(table and index) to be processed directly from memory, which speeds up processing. Important for tuning performance.
        • +
      • Buffer pool: LRU cache of frequently used data (table and index) to be processed directly from memory, which speeds up processing. Important for tuning performance.
      • Change buffer: Caches changes to secondary index pages when those pages are not in the buffer pool and merges it when they are fetched. Merging may take a long time and impact live queries. It also takes up part of the buffer pool. Avoids the extra I/O to read secondary indexes in.
      • Adaptive hash index: Supplements InnoDB’s B-Tree indexes with fast hash lookup tables like a cache. Slight performance penalty for misses, also adds maintenance overhead of updating it. Hash collisions cause AHI rebuilding for large DBs.
      • diff --git a/level101/databases_sql/intro/index.html b/level101/databases_sql/intro/index.html index 246ff82..ebb90db 100644 --- a/level101/databases_sql/intro/index.html +++ b/level101/databases_sql/intro/index.html @@ -2231,7 +2231,7 @@

        What is not covered under this course

        • -

          In depth implementation details

          +

          In-depth implementation details

        • Advanced topics like normalization, sharding

          @@ -2241,7 +2241,7 @@

        Introduction

        -

        The main purpose of database systems is to manage data. This includes storage, adding new data, deleting unused data, updating existing data, retrieving data within a reasonable response time, other maintenance tasks to keep the system running etc.

        +

        The main purpose of database systems is to manage data. This includes storage, adding new data, deleting unused data, updating existing data, retrieving data within a reasonable response time, other maintenance tasks to keep the system running, etc.

        Pre-reads

        RDBMS Concepts

        Course Contents

        diff --git a/level101/databases_sql/lab/index.html b/level101/databases_sql/lab/index.html index c227466..d3e42c8 100644 --- a/level101/databases_sql/lab/index.html +++ b/level101/databases_sql/lab/index.html @@ -2088,24 +2088,26 @@

        Prerequisites

        Install Docker

        Setup

        -

        Create a working directory named sos or something similar, and cd into it.

        -

        Enter the following into a file named my.cnf under a directory named custom.

        -
        sos $ cat custom/my.cnf
+
        Create a working directory named sos or something similar, and cd into it.
        
+
        Enter the following into a file named my.cnf under a directory named custom:
        
+
        sos $ cat custom/my.cnf
 [mysqld]
+
 # These settings apply to MySQL server
 # You can set port, socket path, buffer size etc.
 # Below, we are configuring slow query settings
+
 slow_query_log=1
 slow_query_log_file=/var/log/mysqlslow.log
 long_query_time=1
 
        
 
        Start a container and enable slow query log with the following:
        
-
        sos $ docker run --name db -v custom:/etc/mysql/conf.d -e MYSQL_ROOT_PASSWORD=realsecret -d mysql:8
+
        sos $ docker run --name db -v custom:/etc/mysql/conf.d -e MYSQL_ROOT_PASSWORD=realsecret -d mysql:8
 sos $ docker cp custom/my.cnf $(docker ps -qf "name=db"):/etc/mysql/conf.d/custom.cnf
 sos $ docker restart $(docker ps -qf "name=db")
 
        
-
        Import a sample database
        
-
        sos $ git clone git@github.com:datacharmer/test_db.git
+
        Import a sample database:
        
+
        sos $ git clone git@github.com:datacharmer/test_db.git
 sos $ docker cp test_db $(docker ps -qf "name=db"):/home/test_db/
 sos $ docker exec -it $(docker ps -qf "name=db") bash
 root@3ab5b18b0c7d:/# cd /home/test_db/
@@ -2113,15 +2115,15 @@ root@3ab5b18b0c7d:/# mysql -uroot -prealsecret mysql < employees.sql
 root@3ab5b18b0c7d:/etc# touch /var/log/mysqlslow.log
 root@3ab5b18b0c7d:/etc# chown mysql:mysql /var/log/mysqlslow.log
 
        
-
        Workshop 1: Run some sample queries
-Run the following
        
-
        $ mysql -uroot -prealsecret mysql
+
        Workshop 1: Run some sample queries
        
+
        Run the following:
        
+
        $ mysql -uroot -prealsecret mysql
 mysql>
 
 # inspect DBs and tables
 # the last 4 are MySQL internal DBs
 
-mysql> show databases;
+mysql> SHOW DATABASES;
 +--------------------+
 | Database           |
 +--------------------+
@@ -2132,8 +2134,8 @@ mysql> show databases;
 | sys                |
 +--------------------+
 
-> use employees;
-mysql> show tables;
+mysql> USE employees;
+mysql> SHOW TABLES;
 +----------------------+
 | Tables_in_employees  |
 +----------------------+
@@ -2148,18 +2150,19 @@ mysql> show tables;
 +----------------------+
 
 # read a few rows
-mysql> select * from employees limit 5;
+mysql> SELECT * FROM employees LIMIT 5;
 
 # filter data by conditions
-mysql> select count(*) from employees where gender = 'M' limit 5;
+mysql> SELECT COUNT(*) FROM employees WHERE gender = 'M' LIMIT 5;
 
 # find count of particular data
-mysql> select count(*) from employees where first_name = 'Sachin'; 
+mysql> SELECT COUNT(*) FROM employees WHERE first_name = 'Sachin'; 
 
        
 
        Workshop 2: Use explain and explain analyze to profile a query, identify and add indexes required for improving performance
        
-
        # View all indexes on table 
-#(\G is to output horizontally, replace it with a ; to get table output)
-mysql> show index from employees from employees\G
+
        # View all indexes on table 
+# (\G is to output horizontally, replace it with a ; to get table output)
+
+mysql> SHOW INDEX FROM employees FROM employees\G
 *************************** 1. row ***************************
         Table: employees
    Non_unique: 0
@@ -2177,10 +2180,11 @@ Index_comment:
       Visible: YES
    Expression: NULL
 
-# This query uses an index, idenitfied by 'key' field
+# This query uses an index, identified by 'key' field
 # By prefixing explain keyword to the command, 
 # we get query plan (including key used)
-mysql> explain select * from employees where emp_no < 10005\G
+
+mysql> EXPLAIN SELECT * FROM employees WHERE emp_no < 10005\G
 *************************** 1. row ***************************
            id: 1
   select_type: SIMPLE
@@ -2196,7 +2200,8 @@ possible_keys: PRIMARY
         Extra: Using where
 
 # Compare that to the next query which does not utilize any index
-mysql> explain select first_name, last_name from employees where first_name = 'Sachin'\G
+
+mysql> EXPLAIN SELECT first_name, last_name FROM employees WHERE first_name = 'Sachin'\G
 *************************** 1. row ***************************
            id: 1
   select_type: SIMPLE
@@ -2212,20 +2217,22 @@ possible_keys: NULL
         Extra: Using where
 
 # Let's see how much time this query takes
-mysql> explain analyze select first_name, last_name from employees where first_name = 'Sachin'\G
+
+mysql> EXPLAIN ANALYZE SELECT first_name, last_name FROM employees WHERE first_name = 'Sachin'\G
 *************************** 1. row ***************************
 EXPLAIN: -> Filter: (employees.first_name = 'Sachin')  (cost=30143.55 rows=29911) (actual time=28.284..3952.428 rows=232 loops=1)
     -> Table scan on employees  (cost=30143.55 rows=299113) (actual time=0.095..1996.092 rows=300024 loops=1)
 
 
-# Cost(estimated by query planner) is 30143.55
+# Cost (estimated by query planner) is 30143.55
 # actual time=28.284ms for first row, 3952.428 for all rows
 # Now lets try adding an index and running the query again
-mysql> create index idx_firstname on employees(first_name);
+
+mysql> CREATE INDEX idx_firstname ON employees(first_name);
 Query OK, 0 rows affected (1.25 sec)
 Records: 0  Duplicates: 0  Warnings: 0
 
-mysql> explain analyze select first_name, last_name from employees where first_name = 'Sachin';
+mysql> EXPLAIN ANALYZE SELECT first_name, last_name FROM employees WHERE first_name = 'Sachin';
 +--------------------------------------------------------------------------------------------------------------------------------------------+
 | EXPLAIN                                                                                                                                    |
 +--------------------------------------------------------------------------------------------------------------------------------------------+
@@ -2237,24 +2244,28 @@ mysql> explain analyze select first_name, last_name from employees where firs
 # Actual time=0.551ms for first row
 # 2.934ms for all rows. A huge improvement!
 # Also notice that the query involves only an index lookup,
-# and no table scan (reading all rows of table)
-# ..which vastly reduces load on the DB.
+# and no table scan (reading all rows of the table),
+# which vastly reduces load on the DB.
 
        
 
        Workshop 3: Identify slow queries on a MySQL server
        
-
        # Run the command below in two terminal tabs to open two shells into the container.
-docker exec -it $(docker ps -qf "name=db") bash
+
        # Run the command below in two terminal tabs to open two shells into the container.
 
-# Open a mysql prompt in one of them and execute this command
+$ docker exec -it $(docker ps -qf "name=db") bash
+
+# Open a `mysql` prompt in one of them and execute this command
 # We have configured to log queries that take longer than 1s,
-# so this sleep(3) will be logged
-mysql -uroot -prealsecret mysql
+# so this `sleep(3)` will be logged
+
+$ mysql -uroot -prealsecret mysql
 mysql> select sleep(3);
 
 # Now, in the other terminal, tail the slow log to find details about the query
+
 root@62c92c89234d:/etc# tail -f /var/log/mysqlslow.log
 /usr/sbin/mysqld, Version: 8.0.21 (MySQL Community Server - GPL). started with:
 Tcp port: 3306  Unix socket: /var/run/mysqld/mysqld.sock
 Time                 Id Command    Argument
+
 # Time: 2020-11-26T14:53:44.822348Z
 # User@Host: root[root] @ localhost []  Id:     9
 # Query_time: 5.404938  Lock_time: 0.000000 Rows_sent: 1  Rows_examined: 1
@@ -2262,6 +2273,7 @@ use employees;
 # Time: 2020-11-26T14:53:58.015736Z
 # User@Host: root[root] @ localhost []  Id:     9
 # Query_time: 10.000225  Lock_time: 0.000000 Rows_sent: 1  Rows_examined: 1
+
 SET timestamp=1606402428;
 select sleep(3);
 
        
diff --git a/level101/databases_sql/mysql/index.html b/level101/databases_sql/mysql/index.html
index 42ea65b..f49c854 100644
--- a/level101/databases_sql/mysql/index.html
+++ b/level101/databases_sql/mysql/index.html
@@ -2151,21 +2151,21 @@
 
        MySQL architecture enables you to select the right storage engine for your needs, and abstracts away all implementation details from the end users (application engineers and DBA) who only need to know a consistent stable API.
        
 
        Application layer:
        
 
          
-
        • Connection handling - each client gets its own connection which is cached for the duration of access)
          • 
-
        • Authentication - server checks (username,password,host) info of client and allows/rejects connection
          • 
-
        • Security: server determines whether the client has privileges to execute each query (check with show privileges command)
          • 
+
        • Connection handling: each client gets its own connection which is cached for the duration of access
          • 
+
        • Authentication: server checks (username, password, host) info of client and allows/rejects connection
          • 
+
        • Security: server determines whether the client has privileges to execute each query (check with SHOW PRIVILEGES command)
          • 
 
        
 
        Server layer:
        
 
          
-
        • Services and utilities - backup/restore, replication, cluster etc
          • 
-
        • SQL interface - clients run queries for data access and manipulation
          • 
-
        • SQL parser - creates a parse tree from the query (lexical/syntactic/semantic analysis and code generation)
          • 
-
        • Optimizer - optimizes queries using various algorithms and data available to it(table level stats), modifies queries, order of scanning, indexes to use etc. (check with explain command)
          • 
-
        • Caches and buffers - cache stores query results, buffer pool(InnoDB) stores table and index data in LRU fashion
          • 
+
        • Services and utilities: backup/restore, replication, cluster, etc
          • 
+
        • SQL interface: clients run queries for data access and manipulation
          • 
+
        • SQL parser: creates a parse tree from the query (lexical/syntactic/semantic analysis and code generation)
          • 
+
        • Optimizer: optimizes queries using various algorithms and data available to it (table-level stats), modifies queries, order of scanning, indexes to use, etc. (check with EXPLAIN command)
          • 
+
        • Caches and buffers: cache stores query results, buffer pool (InnoDB) stores table and index data in LRU fashion
          • 
 
        
 
        Storage engine options:
        
 
          
-
        • InnoDB: most widely used, transaction support, ACID compliant, supports row-level locking, crash recovery and multi-version concurrency control. Default since MySQL 5.5+.
          • 
+
        • InnoDB: most-widely used, transaction support, ACID compliant, supports row-level locking, crash recovery and multi-version concurrency control. Default since MySQL 5.5+.
          • 
 
        • MyISAM: fast, does not support transactions, provides table-level locking, great for read-heavy workloads, mostly in web and data warehousing. Default upto MySQL 5.1.
          • 
 
        • Archive: optimised for high speed inserts, compresses data as it is inserted, does not support transactions, ideal for storing and retrieving large amounts of seldom referenced historical, archived data
          • 
 
        • Memory: tables in memory. Fastest engine, supports table-level locking, does not support transactions, ideal for creating temporary tables or quick lookups, data is lost after a shutdown
          • 
@@ -2174,7 +2174,7 @@
 
        
 
        It is possible to migrate from one storage engine to another. But this migration locks tables for all operations and is not online, as it changes the physical layout of the data. It takes a long time and is generally not recommended. Hence, choosing the right storage engine at the beginning is important.
        
 
        General guideline is to use InnoDB unless you have a specific need for one of the other storage engines.
        
-
        Running mysql> SHOW ENGINES;shows you the supported engines on your MySQL server.
        
+
        Running mysql> SHOW ENGINES; shows you the supported engines on your MySQL server.
        
                 
               
               
diff --git a/level101/databases_sql/operations/index.html b/level101/databases_sql/operations/index.html
index b306269..7cd5843 100644
--- a/level101/databases_sql/operations/index.html
+++ b/level101/databases_sql/operations/index.html
@@ -2088,15 +2088,15 @@
                 
          
 
        • 
 
          Explain and explain+analyze
          
-
          EXPLAIN <query> analyzes query plans from the optimizer, including how tables are joined, which tables/rows are scanned etc.
          
-
          Explain analyze shows the above and additional info like execution cost, number of rows returned, time taken etc.
          
+
          EXPLAIN <query> analyzes query plans from the optimizer, including how tables are joined, which tables/rows are scanned, etc.
          
+
          EXPLAIN ANALYZE shows the above and additional info like execution cost, number of rows returned, time taken, etc.
          
 
          This knowledge is useful to tweak queries and add indexes.
          
 
          Watch this performance tuning tutorial video.
          
 
          Checkout the lab section for a hands-on about indexes.
          
 
          • 
 
        • 
 
          Slow query logs
          
-
          Used to identify slow queries (configurable threshold), enabled in config or dynamically with a query
          
+
          Used to identify slow queries (configurable threshold), enabled in config or dynamically with a query.
          
 
          Checkout the lab section about identifying slow queries.
          
 
          • 
 
        • 
@@ -2105,26 +2105,40 @@
 
          • 
 
        • 
 
          Backup and restore strategies, pros and cons
          
-
          Logical backup using mysqldump - slower but can be done online
          
-
          Physical backup (copy data directory or use xtrabackup) -  quick backup/recovery. Copying data directory requires locking or shut down. xtrabackup is an improvement because it supports backups without shutting down (hot backup).
          
+
            
+
          • 
+
            Logical backup using mysqldump - slower but can be done online
            
+
            • 
+
          • 
+
            Physical backup (copy data directory or use XtraBackup) -  quick backup/recovery. Copying data directory requires locking or shut down. XtraBackup is an improvement because it supports backups without shutting down (hot backup).
            
+
            • 
+
          • 
 
            Others - PITR, snapshots etc.
            
 
            • 
+
          
+
          • 
 
        • 
 
          Crash recovery process using redo logs
          
-
          After a crash, when you restart server it reads redo logs and replays modifications to recover
          
+
          After a crash, when you restart server, it reads redo logs and replays modifications to recover
          
 
          • 
 
        • 
 
          Monitoring MySQL
          
+
            
+
          • 
 
            Key MySQL metrics: reads, writes, query runtime, errors, slow queries, connections, running threads, InnoDB metrics
            
+
            • 
+
          • 
 
            Key OS metrics: CPU, load, memory, disk I/O, network
            
 
            • 
+
          
+
          • 
 
        • 
 
          Replication
          
 
          Copies data from one instance to one or more instances. Helps in horizontal scaling, data protection, analytics and performance. Binlog dump thread on primary, replication I/O and SQL threads on secondary. Strategies include the standard async, semi async or group replication.
          
 
          • 
 
        • 
 
          High Availability
          
-
          Ability to cope with failure at software, hardware and network level. Essential for anyone who needs 99.9%+ uptime. Can be implemented with replication or clustering solutions from MySQL, Percona, Oracle etc. Requires expertise to setup and maintain. Failover can be manual, scripted or using tools like Orchestrator.
          
+
          Ability to cope with failure at software, hardware and network level. Essential for anyone who needs 99.9%+ uptime. Can be implemented with replication or clustering solutions from MySQL, Percona, Oracle, etc. Requires expertise to setup and maintain. Failover can be manual, scripted or using tools like Orchestrator.
          
 
          • 
 
        • 
 
          Data directory
          
@@ -2132,7 +2146,7 @@
 
          • 
 
        • 
 
          MySQL configuration
          
-
          This can be done by passing parameters during startup, or in a file. There are a few standard paths where MySQL looks for config files, /etc/my.cnf is one of the commonly used paths. These options are organized under headers (mysqld for server and mysql for client), you can explore them more in the lab that follows.
          
+
          This can be done by passing parameters during startup, or in a file. There are a few standard paths where MySQL looks for config files, /etc/my.cnf is one of the commonly used paths. These options are organized under headers (mysqld for server and mysql for client), you can explore them more in the lab that follows.
          
 
          • 
 
        • 
 
          Logs
          
diff --git a/level101/databases_sql/query_performance/index.html b/level101/databases_sql/query_performance/index.html
index 9596f92..6ab1b9f 100644
--- a/level101/databases_sql/query_performance/index.html
+++ b/level101/databases_sql/query_performance/index.html
@@ -2203,7 +2203,7 @@
                 
          Query Performance Improvement
          
 
          Query Performance is a very crucial aspect of relational databases. If not tuned correctly, the select queries can become slow and painful for the application, and for the MySQL server as well. The important task is to identify the slow queries and try to improve their performance by either rewriting them or creating proper indexes on the tables involved in it.
          
 
          The Slow Query Log
          
-
          The slow query log contains SQL statements that take a longer time to execute then set in the config parameter long_query_time. These queries are the candidates for optimization. There are some good utilities to summarize the slow query logs like, mysqldumpslow (provided by MySQL itself), pt-query-digest (provided by Percona), etc. Following are the config parameters that are used to enable and effectively catch slow queries
          
+
          The slow query log contains SQL statements that take a longer time to execute than set in the config parameter long_query_time. These queries are the candidates for optimization. There are some good utilities to summarize the slow query logs like, mysqldumpslow (provided by MySQL itself), pt-query-digest (provided by Percona), etc. Following are the config parameters that are used to enable and effectively catch slow queries
          
 
@@ -2235,24 +2235,35 @@
 
          
 
 
          
 
 
          
-
          So, for this section, we will be enabling slow_query_log, long_query_time will be kept to 0.3 (300 ms), and log_queries_not_using index will be enabled as well.
          
-
          Below are the queries that we will execute on the employees database.
          
+
          So, for this section, we will be enabling slow_query_log, long_query_time will be kept to 0.3 (300 ms), and log_queries_not_using index will be enabled as well.
          
+
          Below are the queries that we will execute on the employees database.
          
 
            
-
          1. select * from employees where last_name = 'Koblick';
            2. 
-
          2. select * from salaries where salary >= 100000;
            3. 
-
          3. select * from titles where title = 'Manager';
            4. 
-
          4. select * from employees where year(hire_date) = 1995;
            5. 
-
          5. select year(e.hire_date), max(s.salary) from employees e join salaries s on e.emp_no=s.emp_no group by year(e.hire_date);
            6. 
+
          6. 
+
            SELECT * FROM employees WHERE last_name = 'Koblick'
            
+
            7. 
+
          7. 
+
            SELECT * FROM salaries WHERE salary >= 100000
            
+
            8. 
+
          8. 
+
            SELECT * FROM titles WHERE title = 'Manager'
            
+
            9. 
+
          9. 
+
            SELECT * FROM employees WHERE year(hire_date) = 1995
            
+
            10. 
+
          10. 
+
            SELECT year(e.hire_date), max(s.salary) FROM employees e JOIN salaries s ON e.emp_no=s.emp_no GROUP BY year(e.hire_date)
            
+
            11. 
 
          
-
          Now, queries 1, 3 and 4 executed under 300 ms but if we check the slow query logs, we will find these queries logged as they are not using any of the index. Queries 2 and 5 are taking longer than 300ms and also not using any index.
          
-
          Use the following command to get the summary of the slow query log
          
-
          mysqldumpslow /var/lib/mysql/mysql-slow.log
          
+
          Now, queries 1, 3 and 4 executed under 300ms but if we check the slow query logs, we will find these queries logged as they are not using any of the index. Queries 2 and 5 are taking longer than 300ms and also not using any index.
          
+
          Use the following command to get the summary of the slow query log:
          
+
          mysqldumpslow /var/lib/mysql/mysql-slow.log
+
          
 
          slow query log analysis
          
-
          There are some more queries in the snapshot that were along with the queries mentioned. Mysqldumpslow replaces actual values that were used by N (in case of numbers) and S (in case of strings). That can be overridden by -a option, however that will increase the output lines if different values are used in similar queries.
          
+
          There are some more queries in the snapshot that were along with the queries mentioned. mysqldumpslow replaces actual values that were used by N (in case of numbers) and S (in case of strings). That can be overridden by -a option, however, that will increase the output lines if different values are used in similar queries.
          
 
          The EXPLAIN Plan
          
-
          The EXPLAIN command is used with any query that we want to analyze. It describes the query execution plan, how MySQL sees and executes the query. EXPLAIN works with Select, Insert, Update and Delete statements. It tells about different aspects of the query like, how tables are joined, indexes used or not, etc. The important thing here is to understand the basic Explain plan output of a query to determine its performance. 
          
+
          The EXPLAIN command is used with any query that we want to analyze. It describes the query execution plan, how MySQL sees and executes the query. EXPLAIN works with SELECT, INSERT, UPDATE and DELETE statements. It tells about different aspects of the query like, how tables are joined, indexes used or not, etc. The important thing here is to understand the basic EXPLAIN plan output of a query to determine its performance. 
          
 
          Let's take the following query as an example,
          
-
          mysql> explain select * from salaries where salary = 100000;
+
          mysql> EXPLAIN SELECT * FROM salaries WHERE salary = 100000;
 +----+-------------+----------+------------+------+---------------+------+---------+------+---------+----------+-------------+
 | id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra       |
 +----+-------------+----------+------------+------+---------------+------+---------+------+---------+----------+-------------+
@@ -2260,31 +2271,33 @@
 +----+-------------+----------+------------+------+---------------+------+---------+------+---------+----------+-------------+
 1 row in set, 1 warning (0.00 sec)
 
          
-
          The key aspects to understand in the above output are:-
          
+
          The key aspects to understand in the above output are:
          
 
            
 
          • Partitions - the number of partitions considered while executing the query. It is only valid if the table is partitioned.
            • 
 
          • Possible_keys - the list of indexes that were considered during creation of the execution plan.
            • 
 
          • Key - the index that will be used while executing the query.
            • 
 
          • Rows - the number of rows examined during the execution.
            • 
 
          • Filtered - the percentage of rows that were filtered out of the rows examined. The maximum and most optimized result will have 100 in this field. 
            • 
-
          • Extra - this tells some extra information on how MySQL evaluates,  whether the query is using only where clause to match target rows, any index or temporary table, etc.
            • 
+
          • Extra - this tells some extra information on how MySQL evaluates, whether the query is using only WHERE clause to match target rows, any index or temporary table, etc.
            • 
 
          
-
          So, for the above query, we can determine that there are no partitions, there are no candidate indexes to be used and so no index is used at all, over 2M rows are examined and only 10% of them are included in the result, and lastly, only a where clause is used to match the target rows.
          
+
          So, for the above query, we can determine that there are no partitions, there are no candidate indexes to be used and so no index is used at all, over 2M rows are examined and only 10% of them are included in the result, and lastly, only a WHERE clause is used to match the target rows.
          
 
          Creating an Index
          
 
          Indexes are used to speed up selecting relevant rows for a given column value. Without an index, MySQL starts with the first row and goes through the entire table to find matching rows. If the table has too many rows, the operation becomes costly. With indexes, MySQL determines the position to start looking for the data without reading the full table.
          
 
          A primary key is also an index which is also the fastest and is stored along with the table data. Secondary indexes are stored outside of the table data and are used to further enhance the performance of SQL statements. Indexes are mostly stored as B-Trees, with some exceptions like spatial indexes use R-Trees and memory tables use hash indexes.
          
-
          There are 2 ways to create indexes:-
          
+
          There are 2 ways to create indexes:
          
 
            
-
          • While creating a table - if we know beforehand the columns that will drive the most number of where clauses in select queries, then we can put an index over them while creating a table.
            • 
-
          • Altering a Table - To improve the performance of a troubling query, we create an index on a table which already has data in it using ALTER or CREATE INDEX command. This operation does not block the table but might take some time to complete depending on the size of the table.
            • 
+
          • While creating a table - if we know beforehand the columns that will drive the most number of WHERE clauses in SELECT queries, then we can put an index over them while creating a table.
            • 
+
          • Altering a Table - To improve the performance of a troubling query, we create an index on a table which already has data in it using ALTER or CREATE INDEX command. This operation does not block the table but might take some time to complete depending on the size of the table.
            • 
 
          
 
          Let’s look at the query that we discussed in the previous section. It’s clear that scanning over 2M records is not a good idea when only 10% of those records are actually in the resultset. 
          
 
          Hence, we create an index on the salary column of the salaries table.
          
-
          create index idx_salary on salaries(salary)
          
+
          CREATE INDEX idx_salary ON salaries(salary)
+
          
 
          OR
          
-
          alter table salaries add index idx_salary(salary)
          
-
          And the same explain plan now looks like this
          
-
          mysql> explain select * from salaries where salary = 100000;
+
          ALTER TABLE salaries ADD INDEX idx_salary(salary)
+
          
+
          And the same explain plan now looks like this:
          
+
          mysql> EXPLAIN SELECT * FROM salaries WHERE salary = 100000;
 +----+-------------+----------+------------+------+---------------+------------+---------+-------+------+----------+-------+
 | id | select_type | table    | partitions | type | possible_keys | key        | key_len | ref   | rows | filtered | Extra |
 +----+-------------+----------+------------+------+---------------+------------+---------+-------+------+----------+-------+
@@ -2292,9 +2305,9 @@
 +----+-------------+----------+------------+------+---------------+------------+---------+-------+------+----------+-------+
 1 row in set, 1 warning (0.00 sec)
 
          
-
          Now the index used is idx_salary, the one we recently created. The index actually helped examine only 13 records and all of them are in the resultset. Also, the query execution time is also reduced from over 700ms to almost negligible. 
          
-
          Let’s look at another example. Here we are searching for a specific combination of first_name and last_name. But, we might also search based on last_name only.
          
-
          mysql> explain select * from employees where last_name = 'Dredge' and first_name = 'Yinghua';
+
          Now the index used is idx_salary, the one we recently created. The index actually helped examine only 13 records and all of them are in the resultset. Also, the query execution time is also reduced from over 700ms to almost negligible. 
          
+
          Let’s look at another example. Here, we are searching for a specific combination of first_name and last_name. But, we might also search based on last_name only.
          
+
          mysql> EXPLAIN SELECT * FROM employees WHERE last_name = 'Dredge' AND first_name = 'Yinghua';
 +----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
 | id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
 +----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
@@ -2302,9 +2315,10 @@
 +----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
 1 row in set, 1 warning (0.00 sec)
 
          
-
          Now only 1% record out of almost 300K is the resultset. Although the query time is particularly quick as we have only 300K records, this will be a pain if the number of records are over millions. In this case, we create an index on last_name and first_name, not separately, but a composite index including both the columns. 
          
-
          create index idx_last_first on employees(last_name, first_name)
          
-
          mysql> explain select * from employees where last_name = 'Dredge' and first_name = 'Yinghua';
+
          Now only 1% record out of almost 300K is the resultset. Although the query time is particularly quick as we have only 300K records, this will be a pain if the number of records are over millions. In this case, we create an index on last_name and first_name, not separately, but a composite index including both the columns. 
          
+
          CREATE INDEX idx_last_first ON employees(last_name, first_name)
+
          
+
          mysql> EXPLAIN SELECT * FROM employees WHERE last_name = 'Dredge' AND first_name = 'Yinghua';
 +----+-------------+-----------+------------+------+----------------+----------------+---------+-------------+------+----------+-------+
 | id | select_type | table     | partitions | type | possible_keys  | key            | key_len | ref         | rows | filtered | Extra |
 +----+-------------+-----------+------------+------+----------------+----------------+---------+-------------+------+----------+-------+
@@ -2312,8 +2326,8 @@
 +----+-------------+-----------+------------+------+----------------+----------------+---------+-------------+------+----------+-------+
 1 row in set, 1 warning (0.00 sec)
 
          
-
          We chose to put last_name before first_name while creating the index as the optimizer starts from the leftmost prefix of the index while evaluating the query. For example, if we have a 3-column index like idx(c1, c2, c3), then the search capability of the index follows - (c1), (c1, c2) or (c1, c2, c3) i.e. if your where clause has only first_name this index won’t work. 
          
-
          mysql> explain select * from employees where first_name = 'Yinghua';
+
          We chose to put last_name before first_name while creating the index as the optimizer starts from the leftmost prefix of the index while evaluating the query. For example, if we have a 3-column index like idx(c1, c2, c3), then the search capability of the index follows - (c1), (c1, c2) or (c1, c2, c3) i.e. if your WHERE clause has only first_name, this index won’t work.
          
+
          mysql> EXPLAIN SELECT * FROM employees WHERE first_name = 'Yinghua';
 +----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
 | id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
 +----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
@@ -2321,8 +2335,8 @@
 +----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
 1 row in set, 1 warning (0.00 sec)
 
          
-
          But, if you have only the last_name in the where clause, it will work as expected.
          
-
          mysql> explain select * from employees where last_name = 'Dredge';
+
          But, if you have only the last_name in the WHERE clause, it will work as expected.
          
+
          mysql> EXPLAIN SELECT * FROM employees WHERE last_name = 'Dredge';
 +----+-------------+-----------+------------+------+----------------+----------------+---------+-------+------+----------+-------+
 | id | select_type | table     | partitions | type | possible_keys  | key            | key_len | ref   | rows | filtered | Extra |
 +----+-------------+-----------+------------+------+----------------+----------------+---------+-------+------+----------+-------+
@@ -2330,18 +2344,18 @@
 +----+-------------+-----------+------------+------+----------------+----------------+---------+-------+------+----------+-------+
 1 row in set, 1 warning (0.00 sec)
 
          
-
          For another example, use the following queries:-
          
-
          create table employees_2 like employees;
-create table salaries_2 like salaries;
-alter table salaries_2 drop primary key;
+
          For another example, use the following queries:
          
+
          CREATE TABLE employees_2 LIKE employees;
+CREATE TABLE salaries_2 LIKE salaries;
+ALTER TABLE salaries_2 DROP PRIMARY KEY;
 
          
-
          We made copies of employees and salaries tables without the Primary Key of salaries table to understand an example of Select with Join.
          
+
          We made copies of employees and salaries tables without the Primary Key of salaries table to understand an example of SELECT with JOIN.
          
 
          When you have queries like the below, it becomes tricky to identify the pain point of the query.
          
-
          mysql> select e.first_name, e.last_name, s.salary, e.hire_date from employees_2 e join salaries_2 s on e.emp_no=s.emp_no where e.last_name='Dredge';
+
          mysql> SELECT e.first_name, e.last_name, s.salary, e.hire_date FROM employees_2 e JOIN salaries_2 s ON e.emp_no=s.emp_no WHERE e.last_name='Dredge';
 1860 rows in set (4.44 sec)
 
          
 
          This query is taking about 4.5 seconds to complete with 1860 rows in the resultset. Let’s look at the Explain plan. There will be 2 records in the Explain plan as 2 tables are used in the query.
          
-
          mysql> explain select e.first_name, e.last_name, s.salary, e.hire_date from employees_2 e join salaries_2 s on e.emp_no=s.emp_no where e.last_name='Dredge';
+
          mysql> EXPLAIN SELECT e.first_name, e.last_name, s.salary, e.hire_date FROM employees_2 e JOIN salaries_2 s ON e.emp_no=s.emp_no WHERE e.last_name='Dredge';
 +----+-------------+-------+------------+--------+------------------------+---------+---------+--------------------+---------+----------+-------------+
 | id | select_type | table | partitions | type   | possible_keys          | key     | key_len | ref                | rows    | filtered | Extra       |
 +----+-------------+-------+------------+--------+------------------------+---------+---------+--------------------+---------+----------+-------------+
@@ -2350,10 +2364,11 @@ alter table salaries_2 drop primary key;
 +----+-------------+-------+------------+--------+------------------------+---------+---------+--------------------+---------+----------+-------------+
 2 rows in set, 1 warning (0.00 sec)
 
          
-
          These are in order of evaluation i.e. salaries_2 will be evaluated first and then employees_2 will be joined to it. As it looks like, it scans almost all the rows of salaries_2 table and tries to match the employees_2 rows as per the join condition. Though where clause is used in fetching the final resultset, but the index corresponding to the where clause is not used for the employees_2 table. 
          
-
          If the join is done on two indexes which have the same data-types, it will always be faster. So, let’s create an index on the emp_no column of salaries_2 table and analyze the query again.
          
-
          create index idx_empno on salaries_2(emp_no);
          
-
          mysql> explain select e.first_name, e.last_name, s.salary, e.hire_date from employees_2 e join salaries_2 s on e.emp_no=s.emp_no where e.last_name='Dredge';
+
          These are in order of evaluation, i.e. salaries_2 will be evaluated first and then employees_2 will be joined to it. As it looks like, it scans almost all the rows of salaries_2 table and tries to match the employees_2 rows as per the JOIN condition. Though WHERE clause is used in fetching the final resultset, but the index corresponding to the WHERE clause is not used for the employees_2 table. 
          
+
          If the join is done on two indexes which have the same data-types, it will always be faster. So, let’s create an index on the emp_no column of salaries_2 table and analyze the query again.
          
+
          CREATE INDEX idx_empno ON salaries_2(emp_no)
+
          
+
          mysql> EXPLAIN SELECT e.first_name, e.last_name, s.salary, e.hire_date FROM employees_2 e JOIN salaries_2 s ON e.emp_no=s.emp_no WHERE e.last_name='Dredge';
 +----+-------------+-------+------------+------+------------------------+----------------+---------+--------------------+------+----------+-------+
 | id | select_type | table | partitions | type | possible_keys          | key            | key_len | ref                | rows | filtered | Extra |
 +----+-------------+-------+------------+------+------------------------+----------------+---------+--------------------+------+----------+-------+
@@ -2362,8 +2377,8 @@ alter table salaries_2 drop primary key;
 +----+-------------+-------+------------+------+------------------------+----------------+---------+--------------------+------+----------+-------+
 2 rows in set, 1 warning (0.00 sec)
 
          
-
          Now, not only did the index help the optimizer to examine only a few rows in both tables, it reversed the order of the tables in evaluation. The employees_2 table is evaluated first and rows are selected as per the index respective to the where clause. Then the records are joined to salaries_2 table as per the index used due to the join condition. The execution time of the query came down from 4.5s to 0.02s.
          
-
          mysql> select e.first_name, e.last_name, s.salary, e.hire_date from employees_2 e join salaries_2 s on e.emp_no=s.emp_no where e.last_name='Dredge'\G
+
          Now, not only did the index help the optimizer to examine only a few rows in both tables, it reversed the order of the tables in evaluation. The employees_2 table is evaluated first and rows are selected as per the index respective to the WHERE clause. Then, the records are joined to salaries_2 table as per the index used due to the JOIN condition. The execution time of the query came down from 4.5s to 0.02s.
          
+
          mysql> SELECT e.first_name, e.last_name, s.salary, e.hire_date FROM employees_2 e JOIN salaries_2 s ON e.emp_no=s.emp_no WHERE e.last_name='Dredge'\G
 1860 rows in set (0.02 sec)
 
          
                 
diff --git a/level101/databases_sql/replication/index.html b/level101/databases_sql/replication/index.html
index b0c58c5..38575e6 100644
--- a/level101/databases_sql/replication/index.html
+++ b/level101/databases_sql/replication/index.html
@@ -2188,42 +2188,44 @@
                 
                 
          MySQL Replication
          
 
          Replication enables data from one MySQL host (termed as Primary) to be copied to another MySQL host (termed as Replica). MySQL Replication is asynchronous in nature by default, but it can be changed to semi-synchronous with some configurations.
          
-
          Some common applications of MySQL replication are:-
          
+
          Some common applications of MySQL replication are:
          
 
            
 
          • Read-scaling - as multiple hosts can replicate the data from a single primary host, we can set up as many replicas as we need and scale reads through them, i.e. application writes will go to a single primary host and the reads can balance between all the replicas that are there. Such a setup can improve the write performance as well, as the primary is dedicated to only updates and not reads.
            • 
 
          • Backups using replicas - the backup process can sometimes be a little heavy. But if we have replicas configured, then we can use one of them to get the backup without affecting the primary data at all.
            • 
 
          • Disaster Recovery - a replica in some other geographical region paves a proper path to configure disaster recovery.
            • 
 
          
-
          MySQL supports different types of synchronizations as well:-
          
+
          MySQL supports different types of synchronizations as well:
          
 
            
 
          • Asynchronous - this is the default synchronization method. It is one-way, i.e. one host serves as primary and one or more hosts as replica. We will discuss this method throughout the replication topic.
            • 
 
          
 
          replication topologies
          
 
            
 
          • Semi-Synchronous - in this type of synchronization, a commit performed on the primary host is blocked until at least one replica acknowledges it. Post the acknowledgement from any one replica, the control is returned to the session that performed the transaction. This ensures strong consistency but the replication is slower than asynchronous.
            • 
-
          • Delayed - we can deliberately lag the replica in a typical MySQL replication by the number of seconds desired by the use case. This type of replication safeguards from severe human errors of dropping or corrupting the data on the primary, for example, in the above diagram for Delayed Replication, if a DROP DATABASE is executed by mistake on the primary, we still have 30 minutes to recover the data from R2 as that command has not been replicated on R2 yet.
            • 
+
          • Delayed - we can deliberately lag the replica in a typical MySQL replication by the number of seconds desired by the use case. This type of replication safeguards from severe human errors of dropping or corrupting the data on the primary, for example, in the above diagram for Delayed Replication, if a DROP DATABASE is executed by mistake on the primary, we still have 30 minutes to recover the data from R2 as that command has not been replicated on R2 yet.
            • 
 
          
 
          Pre-Requisites
          
-
          Before we dive into setting up replication, we should know about the binary logs. Binary logs play a very important role in MySQL replication. Binary logs, or commonly known as binlogs contain events about the changes done to the database, like table structure changes, data changes via DML operations, etc. They are not used to log SELECT statements. For replication, the primary sends the information to the replicas using its binlogs about the changes done to the database, and the replicas make the same data changes. 
          
-
          With respect to MySQL replication, the binary log format can be of two types that decides the main type of replication:-
-- Statement-Based Replication or SBR
-- Row-Based Replication or RBR
          
-
          Statement Based Binlog Format
          
-
          Originally, the replication in MySQL was based on SQL statements getting replicated and executed on the replica from the primary. This is called statement based logging. The binlog contains the exact SQL statement run by the session. 
          
+
          Before we dive into setting up replication, we should know about the binary logs. Binary logs play a very important role in MySQL replication. Binary logs, or commonly known as binlogs contain events about the changes done to the database, like table structure changes, data changes via DML operations, etc. They are not used to log SELECT statements. For replication, the primary sends the information to the replicas using its binlogs about the changes done to the database, and the replicas make the same data changes. 
          
+
          With respect to MySQL replication, the binary log format can be of two types that decides the main type of replication:
          
+
            
+
          • Statement-Based Replication or SBR
            • 
+
          • Row-Based Replication or RBR
            • 
+
          
+
          Statement-Based Binlog Format
          
+
          Originally, the replication in MySQL was based on SQL statements getting replicated and executed on the replica from the primary. This is called statement-based logging. The binlog contains the exact SQL statement run by the session. 
          
 
          SBR update example
          
-
          So If we run the above statements to insert 3 records and the update 3 in a single update statement, they will be logged exactly the same as when we executed them.
          
+
          So, if we run the above statements to insert 3 records and the update 3 in a single update statement, they will be logged exactly the same as when we executed them.
          
 
          SBR binlog
          
-
          Row Based Binlog Format
          
-
          The Row based is the default one in the latest MySQL releases. This is a lot different from the Statement format as here, row events are logged instead of statements. By that we mean, in the above example one update statement affected 3 records, but binlog had only one update statement; if it is a row based format, binlog will have an event for each record updated.
          
+
          Row-Based Binlog Format
          
+
          The row-based is the default one in the latest MySQL releases. This is a lot different from the Statement format as here, row events are logged instead of statements. By that we mean, in the above example one update statement affected 3 records, but binlog had only one UPDATE statement; if it is a row-based format, binlog will have an event for each record updated.
          
 
          RBR update example
          
 
          RBR binlog
          
-
          Statement Based v/s Row Based binlogs
          
+
          Statement-Based v/s Row-Based binlogs
          
 
          Let’s have a look at the operational differences between statement-based and row-based binlogs. 
          
 
-
-
+
+
@@ -2240,35 +2242,35 @@
 
-
+
-
+
          
 
 
          Statement BasedRow BasedStatement-BasedRow-Based
 
          
           
 
          Restoring using binlogs is slower
 
          
 
          When used for replication, if any statement has a predefined function that has its own value, like sysdate(), uuid() etc, the output could be different on the replica which makes it inconsistent.When used for replication, if any statement has a predefined function that has its own value, like sysdate(), uuid() etc, the output could be different on the replica which makes it inconsistent.
 Whatever is executed becomes a row event with values, so there will be no problem if such functions are used in SQL statements.
 
          
 
          
 Only statements are logged so no other row events are generated.A lot of events are generated when a table is copied into another using INSERT INTO SELECT.A lot of events are generated when a table is copied into another using INSERT INTO SELECT.
 
          
           
 
          
-
          Note - There is another type of binlog format called Mixed. With mixed logging, statement based is used by default but it switches to row based in certain cases. If MySQL cannot guarantee that statement based logging is safe for the statements executed, it issues a warning and switches to row based for those statements.
          
+
          Note: There is another type of binlog format called Mixed. With mixed logging, statement-based is used by default but it switches to row-based in certain cases. If MySQL cannot guarantee that statement-based logging is safe for the statements executed, it issues a warning and switches to row-based for those statements.
          
 
          We will be using binary log format as Row for the entire replication topic.
          
 
          Replication in Motion
          
 
          replication in motion
          
 
          The above figure indicates how a typical MySQL replication works.
          
 
            
-
          1. Replica_IO_Thread is responsible to fetch the binlog events from the primary binary logs to the replica
            2. 
+
          2. Replica_IO_Thread is responsible to fetch the binlog events from the primary binary logs to the replica.
            3. 
 
          3. On the Replica host, relay logs are created which are exact copies of the binary logs. If the binary logs on primary are in row format, the relay logs will be the same.
            4. 
-
          4. Replica_SQL_Thread applies the relay logs on the replica MySQL server.
            5. 
-
          5. If log-bin is enabled on the replica, then the replica will have its own binary logs as well. If log-slave-updates is enabled, then it will have the updates from the primary logged in the binlogs as well.
            6. 
+
          6. Replica_SQL_Thread applies the relay logs on the replica MySQL server.
            7. 
+
          7. If log-bin is enabled on the replica, then the replica will have its own binary logs as well. If log-slave-updates is enabled, then it will have the updates from the primary logged in the binlogs as well.
            8. 
 
          
 
          Setting up Replication
          
-
          In this section, we will set up a simple asynchronous replication. The binlogs will be in row based format. The replication will be set up on two fresh hosts with no prior data present. There are two different ways in which we can set up replication. 
          
+
          In this section, we will set up a simple asynchronous replication. The binlogs will be in row-based format. The replication will be set up on two fresh hosts with no prior data present. There are two different ways in which we can set up replication. 
          
 
            
 
          • Binlog based - Each replica keeps a record of the binlog coordinates on the primary - current binlog and position in the binlog till where it has read and processed. So, at a time different replicas might be reading different parts of the same binlog.
            • 
-
          • GTID based - Every transaction gets an identifier called global transaction identifier or GTID. There is no need to keep the record of binlog coordinates, as long as the replica has all the GTIDs executed on the primary, it is consistent with the primary. A typical GTID is the server_uuid:# positive integer.
            • 
+
          • GTID based - Every transaction gets an identifier called global transaction identifier or GTID. There is no need to keep the record of binlog coordinates, as long as the replica has all the GTIDs executed on the primary, it is consistent with the primary. A typical GTID is the server_uuid:# positive integer.
            • 
 
          
-
          We will set up a GTID based replication in the following section but will also discuss binlog based replication setup as well.
          
+
          We will set up a GTID-based replication in the following section but will also discuss binlog-based replication setup as well.
          
 
          Primary Host Configurations
          
-
          The following config parameters should be present in the primary my.cnf file for setting up GTID based replication.
          
+
          The following config parameters should be present in the primary my.cnf file for setting up GTID-based replication.
          
 
          server-id - a unique ID for the mysql server
 log-bin - the binlog location
 binlog-format - ROW | STATEMENT (we will use ROW)
@@ -2276,7 +2278,7 @@ gtid-mode - ON
 enforce-gtid-consistency - ON (allows execution of only those statements which can be logged using GTIDs)
 
          
 
          Replica Host Configurations
          
-
          The following config parameters should be present in the replica my.cnf file for setting up replication.
          
+
          The following config parameters should be present in the replica my.cnf file for setting up replication.
          
 
          server-id - different than the primary host
 log-bin - (optional, if you want replica to log its own changes as well)
 binlog-format - depends on the above
@@ -2285,15 +2287,15 @@ enforce-gtid-consistency - ON
 log-slave-updates - ON (if binlog is enabled, then we can enable this. This enables the replica to log the changes coming from the primary along with its own changes. Helps in setting up chain replication)
 
          
 
          Replication User
          
-
          Every replica connects to the primary using a mysql user for replicating. So there must be a mysql user account for the same on the primary host. Any user can be used for this purpose provided it has REPLICATION SLAVE privilege. If the sole purpose is replication then we can have a user with only the required privilege.
          
-
          On the primary host
          
-
          mysql> create user repl_user@<replica_IP> identified by 'xxxxx';
+
          Every replica connects to the primary using a mysql user for replicating. So there must be a mysql user account for the same on the primary host. Any user can be used for this purpose provided it has REPLICATION SLAVE privilege. If the sole purpose is replication, then we can have a user with only the required privilege.
          
+
          On the primary host:
          
+
          mysql> CREATE USER repl_user@<replica_IP> IDENTIFIED BY 'xxxxx';
 
-mysql> grant replication slave on *.* to repl_user@'<replica_IP>';
+mysql> GRANT REPLICATION SLAVE ON *.* TO repl_user@'<replica_IP>';
 
          
 
          Obtaining Starting position from Primary
          
-
          Run the following command on the primary host
          
-
          mysql> show master status\G
+
          Run the following command on the primary host:
          
+
          mysql> SHOW MASTER STATUS\G
 *************************** 1. row ***************************
              File: mysql-bin.000001
          Position: 73
@@ -2302,32 +2304,36 @@ mysql> grant replication slave on *.* to repl_user@'<replica_IP>';
 Executed_Gtid_Set: e17d0920-d00e-11eb-a3e6-000d3aa00f87:1-3
 1 row in set (0.00 sec)
 
          
-
          If we are working with binary log based replication, the top two output lines are the most important ones. That tells the current binlog on the primary host and till what position it has written. For fresh hosts we know that no data is written so we can directly set up replication using the very first binlog file and position 4. If we are setting up a replication from a backup, then that changes the way we obtain the starting position. For GTIDs, the executed_gtid_set is the value where primary is right now. Again, for a fresh setup, we don’t have to specify anything about the starting point and it will start from the transaction id 1, but when we set up from a backup, the backup will contain the GTID positions till where backup has been taken.
          
+
          If we are working with binary log-based replication, the top two output lines are the most important ones. That tells the current binlog on the primary host and till what position it has written. For fresh hosts we know that no data is written, so we can directly set up replication using the very first binlog file and position 4. If we are setting up a replication from a backup, then that changes the way we obtain the starting position. For GTIDs, the executed_gtid_set is the value where primary is right now. Again, for a fresh setup, we don’t have to specify anything about the starting point and it will start from the transaction id 1, but when we set up from a backup, the backup will contain the GTID positions till where backup has been taken.
          
 
          Setting up Replica
          
-
          The replication setup must know about the primary host, the user and password to connect, the binlog coordinates (for binlog based replication) or the GTID auto-position parameter.
-The following command is used for setting up
          
-
          change master to
+
          The replication setup must know about the primary host, the user and password to connect, the binlog coordinates (for binlog-based replication) or the GTID auto-position parameter.
+The following command is used for setting up:
          
+
          CHANGE MASTER TO
 master_host = '<primary host IP>',
 master_port = <primary host port - default=3306>,
 master_user = 'repl_user',
 master_password = 'xxxxx',
 master_auto_position = 1;
 
          
-
          Note - the Change Master To command has been replaced with Change Replication Source To from Mysql 8.0.23 onwards, also all the master and slave keywords are replaced with source and replica.
          
-
          If it is binlog based replication, then instead of master_auto_position, we need to specify the binlog coordinates.
          
+
          Note: The CHANGE MASTER TO command has been replaced with CHANGE REPLICATION SOURCE TO from Mysql 8.0.23 onwards, also all the master and slave keywords are replaced with source and replica.
          
+
          If it is binlog-based replication, then instead of master_auto_position, we need to specify the binlog coordinates.
          
 
          master_log_file = 'mysql-bin.000001',
 master_log_pos = 4
 
          
 
          Starting Replication and Check Status
          
 
          Now that everything is configured, we just need to start the replication on the replica via the following command
          
-
          start slave;
          
+
          START SLAVE;
+
          
 
          OR from MySQL 8.0.23 onwards,
          
-
          start replica;
          
-
          Whether or not the replication is running successfully, we can determine by running the following command
          
-
          show slave status\G
          
+
          START REPLICA;
+
          
+
          Whether or not the replication is running successfully, we can determine by running the following command:
          
+
          SHOW SLAVE STATUS\G
+
          
 
          OR from MySQL 8.0.23 onwards,
          
-
          show replica status\G
          
-
          mysql> show replica status\G
+
          SHOW REPLICA STATUS\G
+
          
+
          mysql> SHOW REPLICA STATUS\G
 *************************** 1. row ***************************
              Replica_IO_State: Waiting for master to send event
                   Source_Host: <primary IP>
@@ -2391,30 +2397,66 @@ Source_SSL_Verify_Server_Cert: No
             Network_Namespace:
 1 row in set (0.00 sec)
 
          
-
          Some of the parameters are explained below:-
          
-
            
-
          • Relay_Source_Log_File - the primary’s file where replica is currently reading from
            • 
-
          • Execute_Source_Log_Pos - for the above file on which position is the replica reading currently from. These two parameters are of utmost importance when binlog based replication is used.
            • 
-
          • Replica_IO_Running - IO thread of replica is running or not
            • 
-
          • Replica_SQL_Running - SQL thread of replica is running or not
            • 
-
          • Seconds_Behind_Source - the difference of seconds when a statement was executed on Primary and then on Replica. This indicates how much replication lag is there.
            • 
-
          • Source_UUID - the uuid of the primary host
            • 
-
          • Retrieved_Gtid_Set - the GTIDs fetched from the primary host by the replica to be executed.
            • 
-
          • Executed_Gtid_Set - the GTIDs executed on the replica. This set remains the same for the entire cluster if the replicas are in sync.
            • 
-
          • Auto_Position - it directs the replica to fetch the next GTID automatically
            • 
-
          
+
          Some of the parameters are explained below:
          
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
          ParametersDescription
          Relay_Source_Log_Filethe primary’s file where replica is currently reading from
          Execute_Source_Log_Posfor the above file on which position is the replica reading currently from. These two parameters are of utmost importance when binlog based replication is used
          Replica_IO_RunningIO thread of replica is running or not
          Replica_SQL_RunningSQL thread of replica is running or not
          Seconds_Behind_Sourcethe difference of seconds when a statement was executed on Primary and then on Replica. This indicates how much replication lag is there
          Source_UUIDthe uuid of the primary host
          Retrieved_Gtid_Setthe GTIDs fetched from the primary host by the replica to be executed
          Executed_Gtid_Setthe GTIDs executed on the replica. This set remains the same for the entire cluster if the replicas are in sync
          Auto_Positionit directs the replica to fetch the next GTID automatically
          
 
          Create a Replica for the already setup cluster
          
 
          The steps discussed in the previous section talks about the setting up replication on two fresh hosts. When we have to set up a replica for a host which is already serving applications, then the backup of the primary is used, either fresh backup taken for the replica (should only be done if the traffic it is serving is less) or use a recently taken backup.
          
-
          If the size of the databases on the MySQL primary server is small, less than 100G recommended, then mysqldump can be used to take backup along with the following options.
          
-
          mysqldump -uroot -p -hhost_ip -P3306 --all-databases --single-transaction --master-data=1 > primary_host.bkp
          
+
          If the size of the databases on the MySQL primary server is small, less than 100G recommended, then mysqldump can be used to take backup along with the following options.
          
+
          mysqldump -uroot -p -hhost_ip -P3306 --all-databases --single-transaction --master-data=1 > primary_host.bkp
+
          
 
            
 
          • --single-transaction - this option starts a transaction before taking the backup which ensures it is consistent. As transactions are isolated from each other, so no other writes affect the backup.
            • 
-
          • --master-data - this option is required if binlog based replication is desired to be set up. It includes the binary log file and log file position in the backup file.
            • 
+
          • --master-data - this option is required if binlog-based replication is desired to be set up. It includes the binary log file and log file position in the backup file.
            • 
 
          
-
          When GTID mode is enabled and mysqldump is executed, it includes the GTID executed to be used to start the replica after the backup position. The contents of the mysqldump output file will have the following
          
+
          When GTID mode is enabled and mysqldump is executed, it includes the GTID executed to be used to start the replica after the backup position. The contents of the mysqldump output file will have the following
          
 
          GTID info in mysqldump
          
-
          It is recommended to comment these before restoring otherwise they could throw errors. Also, using master-data=2 will automatically comment the master_log_file line.
          
-
          Similarly, when taking backup of the host using xtrabackup, the file xtrabckup_info file contains the information about binlog file and file position, as well as the GTID executed set.
          
+
          It is recommended to comment these before restoring otherwise they could throw errors. Also, using master-data=2 will automatically comment the master_log_file line.
          
+
          Similarly, when taking backup of the host using xtrabackup, the file xtrabckup_info file contains the information about binlog file and file position, as well as the GTID executed set.
          
 
          server_version = 8.0.25
 start_time = 2021-06-22 03:45:17
 end_time = 2021-06-22 03:45:20
@@ -2428,8 +2470,8 @@ format = file
 compressed = N
 encrypted = N
 
          
-
          Now, after setting MySQL server on the desired host, restore the backup taken from any one of the above methods. If the intended way is binlog based replication, then use the binlog file and position info in the following command
          
-
          change Replication Source to 
+
          Now, after setting MySQL server on the desired host, restore the backup taken from any one of the above methods. If the intended way is binlog-based replication, then use the binlog file and position info in the following command:
          
+
          CHANGE REPLICATION SOURCE TO 
 source_host = ‘primary_ip’,
 source_port = 3306,
 source_user = ‘repl_user’,
@@ -2437,19 +2479,19 @@ source_password = ‘xxxxx’,
 source_log_file = ‘mysql-bin.000007’,
 source_log_pos = ‘196’;
 
          
-
          If the replication needs to be set via GITDs, then run the below command to tell the replica about the GTIDs already executed. On the Replica host, run th following commands
          
-
          reset master;
+
          If the replication needs to be set via GITDs, then run the below command to tell the replica about the GTIDs already executed. On the Replica host, run the following commands:
          
+
          RESET MASTER;
 
 set global gtid_purged = ‘e17d0920-d00e-11eb-a3e6-000d3aa00f87:1-5’
 
-change replication source to
+CHANGE REPLICATION SOURCE TO
 source_host = ‘primary_ip’,
 source_port = 3306,
 source_user = ‘repl_user’,
 source_password = ‘xxxxx’,
 source_auto_position = 1
 
          
-
          The reset master command resets the position of the binary log to initial. It can be skipped if the host is a freshly installed MySQL, but we restored a backup so it is necessary. The gtid_purged global variable lets the replica know the GTIDs that have already been executed, so that the replication can start after that. Then in the change source command, we set the auto-position to 1 which automatically gets the next GTID to proceed.
          
+
          The reset master command resets the position of the binary log to initial. It can be skipped if the host is a freshly installed MySQL, but we restored a backup so it is necessary. The gtid_purged global variable lets the replica know the GTIDs that have already been executed, so that the replication can start after that. Then in the change source command, we set the auto-position to 1 which automatically gets the next GTID to proceed.
          
 
          Further Reading
          
 
            
 
          • More applications of Replication
            • 
diff --git a/level101/databases_sql/select_query/index.html b/level101/databases_sql/select_query/index.html
index e790d82..5cb1aa9 100644
--- a/level101/databases_sql/select_query/index.html
+++ b/level101/databases_sql/select_query/index.html
@@ -2187,8 +2187,8 @@
                   
            Select Query
            
                 
                 
            SELECT Query
            
-
            The most commonly used command while working with MySQL is SELECT. It is used to fetch the result set from one or more tables. 
-The general form of a typical select query looks like:-
            
+
            The most commonly used command while working with MySQL is SELECT. It is used to fetch the resultset from one or more tables. 
+The general form of a typical select query looks like:
            
 
            SELECT expr
 FROM table1
 [WHERE condition]
@@ -2196,17 +2196,17 @@ FROM table1
 [ORDER BY column_list ASC|DESC]
 [LIMIT #]
 
            
-
            The above general form contains some commonly used clauses of a SELECT query:-
            
+
            The above general form contains some commonly used clauses of a SELECT query:
            
 
              
 
            • expr - comma-separated column list or * (for all columns)
              • 
 
            • WHERE - a condition is provided, if true, directs the query to select only those records.
              • 
-
            • GROUP BY - groups the entire result set based on the column list provided. An aggregate function is recommended to be present in the select expression of the query. HAVING supports grouping by putting a condition on the selected or any other aggregate function.
              • 
-
            • ORDER BY - sorts the result set based on the column list in ascending or descending order.
              • 
+
            • GROUP BY - groups the entire resultset based on the column list provided. An aggregate function is recommended to be present in the select expression of the query. HAVING supports grouping by putting a condition on the selected or any other aggregate function.
              • 
+
            • ORDER BY - sorts the resultset based on the column list in ascending or descending order.
              • 
 
            • LIMIT - commonly used to limit the number of records.
              • 
 
            
 
            Let’s have a look at some examples for a better understanding of the above. The dataset used for the examples below is available here and is free to use.
            
 
            Select all records
            
-
            mysql> select * from employees limit 5;
+
            mysql> SELECT * FROM employees LIMIT 5;
 +--------+------------+------------+-----------+--------+------------+
 | emp_no | birth_date | first_name | last_name | gender | hire_date  |
 +--------+------------+------------+-----------+--------+------------+
@@ -2219,7 +2219,7 @@ FROM table1
 5 rows in set (0.00 sec)
 
            
 
            Select specific fields for all records
            
-
            mysql> select first_name, last_name, gender from employees limit 5;
+
            mysql> SELECT first_name, last_name, gender FROM employees LIMIT 5;
 +------------+-----------+--------+
 | first_name | last_name | gender |
 +------------+-----------+--------+
@@ -2232,7 +2232,7 @@ FROM table1
 5 rows in set (0.00 sec)
 
            
 
            Select all records Where hire_date >= January 1, 1990
            
-
            mysql> select * from employees where hire_date >= '1990-01-01' limit 5;
+
            mysql> SELECT * FROM employees WHERE hire_date >= '1990-01-01' LIMIT 5;
 +--------+------------+------------+-------------+--------+------------+
 | emp_no | birth_date | first_name | last_name   | gender | hire_date  |
 +--------+------------+------------+-------------+--------+------------+
@@ -2245,7 +2245,7 @@ FROM table1
 5 rows in set (0.01 sec)
 
            
 
            Select first_name and last_name from all records Where birth_date >= 1960 AND gender = ‘F’
            
-
            mysql> select first_name, last_name from employees where year(birth_date) >= 1960 and gender='F' limit 5;
+
            mysql> SELECT first_name, last_name FROM employees WHERE year(birth_date) >= 1960 AND gender='F' LIMIT 5;
 +------------+-----------+
 | first_name | last_name |
 +------------+-----------+
@@ -2258,18 +2258,18 @@ FROM table1
 5 rows in set (0.00 sec)
 
            
 
            Display the total number of records
            
-
            mysql> select count(*) from employees;
+
            mysql> SELECT COUNT(*) FROM employees;
 +----------+
-| count(*) |
+| COUNT(*) |
 +----------+
 |   300024 |
 +----------+
 1 row in set (0.05 sec)
 
            
 
            Display gender-wise count of all records
            
-
            mysql> select gender, count(*) from employees group by gender;
+
            mysql> SELECT gender, COUNT(*) FROM employees GROUP BY gender;
 +--------+----------+
-| gender | count(*) |
+| gender | COUNT(*) |
 +--------+----------+
 | M      |   179973 |
 | F      |   120051 |
@@ -2277,9 +2277,9 @@ FROM table1
 2 rows in set (0.14 sec)
 
            
 
            Display the year of hire_date and number of employees hired that year, also only those years where more than 20k employees were hired
            
-
            mysql> select year(hire_date), count(*) from employees group by year(hire_date) having count(*) > 20000;
+
            mysql> SELECT year(hire_date), COUNT(*) FROM employees GROUP BY year(hire_date) HAVING COUNT(*) > 20000;
 +-----------------+----------+
-| year(hire_date) | count(*) |
+| year(hire_date) | COUNT(*) |
 +-----------------+----------+
 |            1985 |    35316 |
 |            1986 |    36150 |
@@ -2292,8 +2292,8 @@ FROM table1
 +-----------------+----------+
 8 rows in set (0.14 sec)
 
            
-
            Display all records ordered by their hire_date in descending order. If hire_date is the same then in order of their birth_date ascending order
            
-
            mysql> select * from employees order by hire_date desc, birth_date asc limit 5;
+
            Display all records ordered by their hire_date in descending order. If hire_date is the same, then in order of their birth_date ascending order
            
+
            mysql> SELECT * FROM employees ORDER BY hire_date DESC, birth_date ASC LIMIT 5;
 +--------+------------+------------+-----------+--------+------------+
 | emp_no | birth_date | first_name | last_name | gender | hire_date  |
 +--------+------------+------------+-----------+--------+------------+
@@ -2306,20 +2306,20 @@ FROM table1
 5 rows in set (0.12 sec)
 
            
 
            SELECT - JOINS
            
-
            JOIN statement is used to produce a combined result set from two or more tables based on certain conditions. It can be also used with Update and Delete statements but we will be focussing on the select query. 
-Following is a basic general form for joins
            
+
            JOIN statement is used to produce a combined resultset from two or more tables based on certain conditions. It can be also used with UPDATE and DELETE statements, but we will be focussing on the select query.
            
+
            Following is a basic general form for joins:
            
 
            SELECT table1.col1, table2.col1, ... (any combination)
 FROM
 table1 <join_type> table2
 ON (or USING depends on join_type) table1.column_for_joining = table2.column_for_joining
 WHERE …
 
            
-
            Any number of columns can be selected, but it is recommended to select only those which are relevant to increase the readability of the resultset. All other clauses like where, group by are not mandatory.
+
            Any number of columns can be selected, but it is recommended to select only those which are relevant to increase the readability of the resultset. All other clauses like WHERE, GROUP BY are not mandatory.
 Let’s discuss the types of JOINs supported by MySQL Syntax.
            
 
            Inner Join
            
 
            This joins table A with table B on a condition. Only the records where the condition is True are selected in the resultset.
            
-
            Display some details of employees along with their salary
            
-
            mysql> select e.emp_no,e.first_name,e.last_name,s.salary from employees e join salaries s on e.emp_no=s.emp_no limit 5;
+
            Display some details of employees along with their salary:
            
+
            mysql> SELECT e.emp_no,e.first_name,e.last_name,s.salary FROM employees e JOIN salaries s ON e.emp_no=s.emp_no LIMIT 5;
 +--------+------------+-----------+--------+
 | emp_no | first_name | last_name | salary |
 +--------+------------+-----------+--------+
@@ -2331,8 +2331,8 @@ Let’s discuss the types of JOINs supported by MySQL Syntax.
            
 +--------+------------+-----------+--------+
 5 rows in set (0.00 sec)
 
            
-
            Similar result can be achieved by
            
-
            mysql> select e.emp_no,e.first_name,e.last_name,s.salary from employees e join salaries s using (emp_no) limit 5;
+
            Similar result can be achieved by:
            
+
            mysql> SELECT e.emp_no,e.first_name,e.last_name,s.salary FROM employees e JOIN salaries s USING (emp_no) LIMIT 5;
 +--------+------------+-----------+--------+
 | emp_no | first_name | last_name | salary |
 +--------+------------+-----------+--------+
@@ -2344,8 +2344,8 @@ Let’s discuss the types of JOINs supported by MySQL Syntax.
            
 +--------+------------+-----------+--------+
 5 rows in set (0.00 sec)
 
            
-
            And also by
            
-
            mysql> select e.emp_no,e.first_name,e.last_name,s.salary from employees e natural join salaries s limit 5;
+
            And also by:
            
+
            mysql> SELECT e.emp_no,e.first_name,e.last_name,s.salary FROM employees e NATURAL JOIN salaries s LIMIT 5;
 +--------+------------+-----------+--------+
 | emp_no | first_name | last_name | salary |
 +--------+------------+-----------+--------+
@@ -2358,11 +2358,13 @@ Let’s discuss the types of JOINs supported by MySQL Syntax.
            
 5 rows in set (0.00 sec)
 
            
 
            Outer Join
            
-
            Majorly of two types:-
-- LEFT - joining complete table A with table B on a condition. All the records from table A are selected, but from table B, only those records are selected where the condition is True.
-- RIGHT - Exact opposite of the left join.
            
-
            Let us assume the below tables for understanding left join better.
            
-
            mysql> select * from dummy1;
+
            Majorly of two types:
            
+
              
+
            • LEFT - joining complete table A with table B on a condition. All the records from table A are selected, but from table B, only those records are selected where the condition is True.
              • 
+
            • RIGHT - Exact opposite of the LEFT JOIN.
              • 
+
            
+
            Let us assume the below tables for understanding LEFT JOIN better.
            
+
            mysql> SELECT * FROM dummy1;
 +----------+------------+
 | same_col | diff_col_1 |
 +----------+------------+
@@ -2371,7 +2373,7 @@ Let’s discuss the types of JOINs supported by MySQL Syntax.
            
 |        3 | C          |
 +----------+------------+
 
-mysql> select * from dummy2;
+mysql> SELECT * FROM dummy2;
 +----------+------------+
 | same_col | diff_col_2 |
 +----------+------------+
@@ -2379,8 +2381,8 @@ mysql> select * from dummy2;
 |        3 | Y          |
 +----------+------------+
 
            
-
            A simple select join will look like the one below.
            
-
            mysql> select * from dummy1 d1 left join dummy2 d2 on d1.same_col=d2.same_col;
+
            A simple SELECT JOIN will look like the one below:
            
+
            mysql> SELECT * FROM dummy1 d1 LEFT JOIN dummy2 d2 ON d1.same_col=d2.same_col;
 +----------+------------+----------+------------+
 | same_col | diff_col_1 | same_col | diff_col_2 |
 +----------+------------+----------+------------+
@@ -2390,8 +2392,8 @@ mysql> select * from dummy2;
 +----------+------------+----------+------------+
 3 rows in set (0.00 sec)
 
            
-
            Which can also be written as
            
-
            mysql> select * from dummy1 d1 left join dummy2 d2 using(same_col);
+
            Which can also be written as:
            
+
            mysql> SELECT * FROM dummy1 d1 LEFT JOIN dummy2 d2 USING(same_col);
 +----------+------------+------------+
 | same_col | diff_col_1 | diff_col_2 |
 +----------+------------+------------+
@@ -2401,8 +2403,8 @@ mysql> select * from dummy2;
 +----------+------------+------------+
 3 rows in set (0.00 sec)
 
            
-
            And also as
            
-
            mysql> select * from dummy1 d1 natural left join dummy2 d2;
+
            And also as:
            
+
            mysql> SELECT * FROM dummy1 d1 NATURAL LEFT JOIN dummy2 d2;
 +----------+------------+------------+
 | same_col | diff_col_1 | diff_col_2 |
 +----------+------------+------------+
@@ -2414,8 +2416,8 @@ mysql> select * from dummy2;
 
            
 
            Cross Join
            
 
            This does a cross product of table A and table B without any condition. It doesn’t have a lot of applications in the real world.
            
-
            A Simple Cross Join looks like this
            
-
            mysql> select * from dummy1 cross join dummy2;
+
            A Simple CROSS JOIN looks like this:
            
+
            mysql> SELECT * FROM dummy1 CROSS JOIN dummy2;
 +----------+------------+----------+------------+
 | same_col | diff_col_1 | same_col | diff_col_2 |
 +----------+------------+----------+------------+
@@ -2428,8 +2430,8 @@ mysql> select * from dummy2;
 +----------+------------+----------+------------+
 6 rows in set (0.01 sec)
 
            
-
            One use case that can come in handy is when you have to fill in some missing entries. For example, all the entries from dummy1 must be inserted into a similar table dummy3, with each record must have 3 entries with statuses 1, 5 and 7.
            
-
            mysql> desc dummy3;
+
            One use case that can come in handy is when you have to fill in some missing entries. For example, all the entries from dummy1 must be inserted into a similar table dummy3, with each record must have 3 entries with statuses 1, 5 and 7.
            
+
            mysql> DESC dummy3;
 +----------+----------+------+-----+---------+-------+
 | Field    | Type     | Null | Key | Default | Extra |
 +----------+----------+------+-----+---------+-------+
@@ -2439,11 +2441,11 @@ mysql> select * from dummy2;
 +----------+----------+------+-----+---------+-------+
 3 rows in set (0.02 sec)
 
            
-
            Either you create an insert query script with as many entries as in dummy1 or use cross join to produce the required resultset.
            
-
            mysql> select * from dummy1 
-cross join 
-(select 1 union select 5 union select 7) T2 
-order by same_col;
+
            Either you create an INSERT query script with as many entries as in dummy1 or use CROSS JOIN to produce the required resultset.
            
+
            mysql> SELECT * FROM dummy1 
+CROSS JOIN 
+(SELECT 1 UNION SELECT 5 UNION SELECT 7) T2 
+ORDER BY same_col;
 +----------+------------+---+
 | same_col | diff_col_1 | 1 |
 +----------+------------+---+
@@ -2462,7 +2464,7 @@ order by same_col;
 
            The T2 section in the above query is called a sub-query. We will discuss the same in the next section.
            
 
            Natural Join
            
 
            This implicitly selects the common column from table A and table B and performs an inner join.
            
-
            mysql> select e.emp_no,e.first_name,e.last_name,s.salary from employees e natural join salaries s limit 5;
+
            mysql> SELECT e.emp_no,e.first_name,e.last_name,s.salary FROM employees e NATURAL JOIN salaries s LIMIT 5;
 +--------+------------+-----------+--------+
 | emp_no | first_name | last_name | salary |
 +--------+------------+-----------+--------+
@@ -2474,17 +2476,17 @@ order by same_col;
 +--------+------------+-----------+--------+
 5 rows in set (0.00 sec)
 
            
-
            Notice how natural join and using takes care that the common column is displayed only once if you are not explicitly selecting columns for the query.
            
-
            Some More Examples
            
-
            Display emp_no, salary, title and dept of the employees where salary > 80000
            
-
            mysql> select e.emp_no, s.salary, t.title, d.dept_no 
-from  
+
            Notice how NATURAL JOIN and using takes care that the common column is displayed only once if you are not explicitly selecting columns for the query.
            
+
            Some More Examples
            
+
            Display emp_no, salary, title and dept of the employees where salary > 80000.
            
+
            mysql> SELECT e.emp_no, s.salary, t.title, d.dept_no 
+FROM  
 employees e 
-join salaries s using (emp_no) 
-join titles t using (emp_no) 
-join dept_emp d using (emp_no) 
-where s.salary > 80000 
-limit 5;
+JOIN salaries s USING (emp_no) 
+JOIN titles t USING (emp_no) 
+JOIN dept_emp d USING (emp_no) 
+WHERE s.salary > 80000 
+LIMIT 5;
 +--------+--------+--------------+---------+
 | emp_no | salary | title        | dept_no |
 +--------+--------+--------------+---------+
@@ -2496,15 +2498,15 @@ limit 5;
 +--------+--------+--------------+---------+
 5 rows in set (0.00 sec)
 
            
-
            Display title-wise count of employees in each department order by dept_no
            
-
            mysql> select d.dept_no, t.title, count(*) 
-from titles t 
-left join dept_emp d using (emp_no) 
-group by d.dept_no, t.title 
-order by d.dept_no 
-limit 10;
+
            Display title-wise count of employees in each department ordered by dept_no:
            
+
            mysql> SELECT d.dept_no, t.title, COUNT(*) 
+FROM titles t 
+LEFT JOIN dept_emp d USING (emp_no) 
+GROUP BY d.dept_no, t.title 
+ORDER BY d.dept_no 
+LIMIT 10;
 +---------+--------------------+----------+
-| dept_no | title              | count(*) |
+| dept_no | title              | COUNT(*) |
 +---------+--------------------+----------+
 | d001    | Manager            |        2 |
 | d001    | Senior Staff       |    13940 |
@@ -2520,14 +2522,14 @@ limit 10;
 10 rows in set (1.32 sec)
 
            
 
            SELECT - Subquery
            
-
            A subquery is generally a smaller resultset that can be used to power a select query in many ways. It can be used in a ‘where’ condition, can be used in place of join mostly where a join could be an overkill. 
-These subqueries are also termed as derived tables. They must have a table alias in the select query.
            
+
            A subquery is generally a smaller resultset that can be used to power a SELECT query in many ways. It can be used in a WHERE condition, can be used in place of JOIN mostly where a JOIN could be an overkill. 
+These subqueries are also termed as derived tables. They must have a table alias in the SELECT query.
            
 
            Let’s look at some examples of subqueries.
            
-
            Here we got the department name from the departments table by a subquery which used dept_no from dept_emp table.
            
-
            mysql> select e.emp_no, 
-(select dept_name from departments where dept_no=d.dept_no) dept_name from employees e 
-join dept_emp d using (emp_no) 
-limit 5;
+
            Here, we got the department name from the departments table by a subquery which used dept_no from dept_emp table.
            
+
            mysql> SELECT e.emp_no, 
+(SELECT dept_name FROM departments WHERE dept_no=d.dept_no) dept_name FROM employees e 
+JOIN dept_emp d USING (emp_no) 
+LIMIT 5;
 +--------+-----------------+
 | emp_no | dept_name       |
 +--------+-----------------+
@@ -2539,23 +2541,23 @@ limit 5;
 +--------+-----------------+
 5 rows in set (0.01 sec)
 
            
-
            Here, we used the ‘avg’ query above (which got the avg salary) as a subquery to list the employees whose latest salary is more than the average. 
            
-
            mysql> select avg(salary) from salaries;
+
            Here, we used the AVG query above (which got the avg salary) as a subquery to list the employees whose latest salary is more than the average. 
            
+
            mysql> SELECT AVG(salary) FROM salaries;
 +-------------+
-| avg(salary) |
+| AVG(salary) |
 +-------------+
 |  63810.7448 |
 +-------------+
 1 row in set (0.80 sec)
 
-mysql> select e.emp_no, max(s.salary) 
-from employees e 
-natural join salaries s 
-group by e.emp_no 
-having max(s.salary) > (select avg(salary) from salaries) 
-limit 10;
+mysql> SELECT e.emp_no, MAX(s.salary) 
+FROM employees e 
+NATURAL JOIN salaries s 
+GROUP BY e.emp_no 
+HAVING MAX(s.salary) > (SELECT AVG(salary) FROM salaries) 
+LIMIT 10;
 +--------+---------------+
-| emp_no | max(s.salary) |
+| emp_no | MAX(s.salary) |
 +--------+---------------+
 |  10001 |         88958 |
 |  10002 |         72527 |
diff --git a/level101/git/branches/index.html b/level101/git/branches/index.html
index 2e20901..83f28b3 100644
--- a/level101/git/branches/index.html
+++ b/level101/git/branches/index.html
@@ -2151,7 +2151,7 @@
                 
                 
                 
            Working With Branches
            
-
            Coming back to our local repo which has two commits. So far, what we have is a single line of history. Commits are chained in a single line. But sometimes you may have a need to work on two different features in parallel in the same repo. Now one option here could be making a new folder/repo with the same code and use that for another feature development. But there's a better way. Use branches. Since git follows tree like structure for commits, we can use branches to work on different sets of features. From a commit, two or more branches can be created and branches can also be merged.
            
+
            Coming back to our local repo which has two commits. So far, what we have is a single line of history. Commits are chained in a single line. But sometimes you may have a need to work on two different features in parallel in the same repo. Now one option here could be making a new folder/repo with the same code and use that for another feature development. But there's a better way. Use branches. Since git follows tree-like structure for commits, we can use branches to work on different sets of features. From a commit, two or more branches can be created and branches can also be merged.
            
 
            Using branches, there can exist multiple lines of histories and we can checkout to any of them and work on it. Checking out, as we discussed earlier, would simply mean replacing contents of the directory (repo) with the snapshot at the checked out version.
            
 
            Let's create a branch and see how it looks like:
            
 
            $ git branch b1
@@ -2159,14 +2159,14 @@ $ git log --oneline --graph
 * 7f3b00e (HEAD -> master, b1) adding file 2
 * df2fb7a adding file 1
 
            
-
            We create a branch called b1. Git log tells us that b1 also points to the last commit (7f3b00e) but the HEAD is still pointing to master. If you remember, HEAD points to the commit/reference wherever you are checkout to. So if we checkout to b1, HEAD should point to that. Let's confirm:
            
+
            We create a branch called b1. Git log tells us that b1 also points to the last commit (7f3b00e) but the HEAD is still pointing to master. If you remember, HEAD points to the commit/reference wherever you are checkout to. So if we checkout to b1, HEAD should point to that. Let's confirm:
            
 
            $ git checkout b1
 Switched to branch 'b1'
 $ git log --oneline --graph
 * 7f3b00e (HEAD -> b1, master) adding file 2
 * df2fb7a adding file 1
 
            
-
            b1 still points to the same commit but HEAD now points to b1. Since we create a branch at commit 7f3b00e, there will be two lines of histories starting this commit. Depending on which branch you are checked out on, the line of history will progress.
            
+
            b1 still points to the same commit but HEAD now points to b1. Since we create a branch at commit 7f3b00e, there will be two lines of histories starting this commit. Depending on which branch you are checked out on, the line of history will progress.
            
 
            At this moment, we are checked out on branch b1, so making a new commit will advance branch reference b1 to that commit and current b1 commit will become its parent. Let's do that.
            
 
            # Creating a file and making a commit
 $ echo "I am a file in b1 branch" > b1.txt
@@ -2183,7 +2183,7 @@ $ git log --oneline --graph
 * df2fb7a adding file 1
 $
 
            
-
            Do note that master is still pointing to the old commit it was pointing to. We can now checkout to master branch and make commits there. This will result in another line of history starting from commit 7f3b00e.
            
+
            Do note that master is still pointing to the old commit it was pointing to. We can now checkout to master branch and make commits there. This will result in another line of history starting from commit 7f3b00e.
            
 
            # checkout to master branch
 $ git checkout master
 Switched to branch 'master'
@@ -2202,7 +2202,7 @@ $ git log --oneline --graph
 * 7f3b00e adding file 2
 * df2fb7a adding file 1
 
            
-
            Notice how branch b1 is not visible here since we are on the master. Let's try to visualize both to get the whole picture:
            
+
            Notice how branch b1 is not visible here since we are on the master. Let's try to visualize both to get the whole picture:
            
 
            $ git log --oneline --graph --all
 * 60dc441 (HEAD -> master) adding master.txt file
 | * 872a38f (b1) adding b1 file
@@ -2210,10 +2210,10 @@ $ git log --oneline --graph
 * 7f3b00e adding file 2
 * df2fb7a adding file 1
 
            
-
            Above tree structure should make things clear. Notice a clear branch/fork on commit 7f3b00e. This is how we create branches. Now they both are two separate lines of history on which feature development can be done independently.
            
+
            Above tree structure should make things clear. Notice a clear branch/fork on commit 7f3b00e. This is how we create branches. Now they both are two separate lines of history on which feature development can be done independently.
            
 
            To reiterate, internally, git is just a tree of commits. Branch names (human readable) are pointers to those commits in the tree. We use various git commands to work with the tree structure and references. Git accordingly modifies contents of our repo.
            
 
            Merges
            
-
            Now say the feature you were working on branch b1 is complete and you need to merge it on master branch, where all the final version of code goes. So first you will checkout to branch master and then you pull the latest code from upstream (eg: GitHub). Then you need to merge your code from b1 into master. There could be two ways this can be done.
            
+
            Now say the feature you were working on branch b1 is complete and you need to merge it on master branch, where all the final version of code goes. So first, you will checkout to branch master and then you pull the latest code from upstream (eg: GitHub). Then you need to merge your code from b1 into master. There could be two ways this can be done.
            
 
            Here is the current history:
            
 
            $ git log --oneline --graph --all
 * 60dc441 (HEAD -> master) adding master.txt file
@@ -2222,7 +2222,7 @@ $ git log --oneline --graph
 * 7f3b00e adding file 2
 * df2fb7a adding file 1
 
            
-
            Option 1: Directly merge the branch. Merging the branch b1 into master will result in a new merge commit. This will merge changes from two different lines of history and create a new commit of the result.
            
+
            Option 1: Directly merge the branch. Merging the branch b1 into master will result in a new merge commit. This will merge changes from two different lines of history and create a new commit of the result.
            
 
            $ git merge b1
 Merge made by the 'recursive' strategy.
 b1.txt | 1 +
@@ -2237,8 +2237,8 @@ $ git log --oneline --graph --all
 * 7f3b00e adding file 2
 * df2fb7a adding file 1
 
            
-
            You can see a new merge commit created (8fc28f9). You will be prompted for the commit message. If there are a lot of branches in the repo, this result will end-up with a lot of merge commits. Which looks ugly compared to a single line of history of development. So let's look at an alternative approach
            
-
            First let's reset our last merge and go to the previous state.
            
+
            You can see a new merge commit created (8fc28f9). You will be prompted for the commit message. If there are a lot of branches in the repo, this result will end-up with a lot of merge commits. Which looks ugly compared to a single line of history of development. So let's look at an alternative approach.
            
+
            First, let's reset our last merge and go to the previous state.
            
 
            $ git reset --hard 60dc441
 HEAD is now at 60dc441 adding master.txt file
 $ git log --oneline --graph --all
@@ -2248,7 +2248,7 @@ $ git log --oneline --graph --all
 * 7f3b00e adding file 2
 * df2fb7a adding file 1
 
            
-
            Option 2: Rebase. Now, instead of merging two branches which has a similar base (commit: 7f3b00e), let us rebase branch b1 on to current master. What this means is take branch b1 (from commit 7f3b00e to commit 872a38f) and rebase (put them on top of) master (60dc441).
            
+
            Option 2: Rebase. Now, instead of merging two branches which has a similar base (commit: 7f3b00e), let us rebase branch b1 on to current master. What this means is take branch b1 (from commit 7f3b00e to commit 872a38f) and rebase (put them on top of) master (60dc441).
            
 
            # Switch to b1
 $ git checkout b1
 Switched to branch 'b1'
diff --git a/level101/git/git-basics/index.html b/level101/git/git-basics/index.html
index 788029c..e8f9149 100644
--- a/level101/git/git-basics/index.html
+++ b/level101/git/git-basics/index.html
@@ -2348,7 +2348,7 @@
 
            Prerequisites
            
 
              
 
            1. Have Git installed https://git-scm.com/downloads
              2. 
-
            2. Have taken any git high level tutorial or following LinkedIn learning courses
            
 
            What to expect from this course
            
-
            As an engineer in the field of computer science, having knowledge of version control tools becomes almost a requirement. While there are a lot of version control tools that exist today like SVN, Mercurial, etc, Git perhaps is the most used one and this course we will be working with Git. While this course does not start with Git 101 and expects basic knowledge of git as a prerequisite, it will reintroduce the git concepts known by you with details covering what is happening under the hood as you execute various git commands. So that next time you run a git command, you will be able to press enter more confidently!
            
+
            As an engineer in the field of computer science, having knowledge of version control tools becomes almost a requirement. While there are a lot of version control tools that exist today like SVN, Mercurial, etc, Git perhaps is the most used one and this course we will be working with Git. While this course does not start with Git 101 and expects basic knowledge of git as a prerequisite, it will reintroduce the git concepts known by you with details covering what is happening under the hood as you execute various git commands. So that next time you run a git command, you will be able to press enter more confidently!
            
 
            What is not covered under this course
            
 
            Advanced usage and specifics of internal implementation details of Git.
            
 
            Course Contents
            
@@ -2383,9 +2383,9 @@ Initialized empty Git repository in /private/tmp/school-of-sre/.git/
 
            $ ls .git/
 HEAD        config      description hooks       info        objects     refs
 
            
-
            There are a bunch of folders and files in the .git folder. As I said, all these enables git to do its magic. We will look into some of these folders and files. But for now, what we have is an empty git repository.
            
+
            There are a bunch of folders and files in the .git folder. As I said, all these enable git to do its magic. We will look into some of these folders and files. But for now, what we have is an empty git repository.
            
 
            Tracking a File
            
-
            Now as you might already know, let us create a new file in our repo (we will refer to the folder as repo now.) And see git status
            
+
            Now as you might already know, let us create a new file in our repo (we will refer to the folder as repo now.) And see git status:
            
 
            $ echo "I am file 1" > file1.txt
 $ git status
 On branch master
@@ -2399,7 +2399,7 @@ Untracked files:
 
 nothing added to commit but untracked files present (use "git add" to track)
 
            
-
            The current git status says No commits yet and there is one untracked file. Since we just created the file, git is not tracking that file. We explicitly need to ask git to track files and folders. (also checkout gitignore) And how we do that is via git add command as suggested in the above output. Then we go ahead and create a  commit.
            
+
            The current git status says No commits yet and there is one untracked file. Since we just created the file, git is not tracking that file. We explicitly need to ask git to track files and folders. (Also checkout gitignore) And how we do that is via git add command as suggested in the above output. Then, we go ahead and create a commit.
            
 
            $ git add file1.txt
 $ git status
 On branch master
@@ -2416,7 +2416,7 @@ $ git commit -m "adding file 1"
 1 file changed, 1 insertion(+)
 create mode 100644 file1.txt
 
            
-
            Notice how after adding the file, git status says Changes to be committed:. What it means is whatever is listed there, will be included in the next commit. Then we go ahead and create a commit, with an attached messaged via -m.
            
+
            Notice how after adding the file, git status says Changes to be committed:. What it means is whatever is listed there, will be included in the next commit. Then, we go ahead and create a commit, with an attached message via -m.
            
 
            More About a Commit
            
 
            Commit is a snapshot of the repo. Whenever a commit is made, a snapshot of the current state of repo (the folder) is taken and saved. Each commit has a unique ID. (df2fb7a for the commit we made in the previous step). As we keep adding/changing more and more contents and keep making commits, all those snapshots are stored by git. Again, all this magic happens inside the .git folder. This is where all this snapshot or versions are stored in an efficient manner.
            
 
            Adding More Changes
            
@@ -2437,11 +2437,11 @@ create mode 100644 file2.txt
 * df2fb7a adding file 1
 
            
 
            git log, as the name suggests, prints the log of all the git commits. Here you see two additional arguments, --oneline prints the shorter version of the log, ie: the commit message only and not the person who made the commit and when. --graph prints it in graph format.
            
-
            Now at this moment the commits might look like just one in each line but all commits are stored as a tree like data structure internally by git. That means there can be two or more children commits of a given commit. And not just a single line of commits. We will look more into this part when we get to the Branches section. For now this is our commit history:
            
+
            Now at this moment, the commits might look like just one in each line but all commits are stored as a tree like data structure internally by git. That means there can be two or more children commits of a given commit. And not just a single line of commits. We will look more into this part when we get to the Branches section. For now, this is our commit history:
            
 
               df2fb7a ===> 7f3b00e
 
            
 
            Are commits really linked?
            
-
            As I just said, the two commits we just made are linked via tree like data structure and we saw how they are linked. But let's actually verify it. Everything in git is an object. Newly created files are stored as an object. Changes to file are stored as an objects and even commits are objects. To view contents of an object we can use the following command with the object's ID. We will take a look at the contents of the second commit
            
+
            As I just said, the two commits we just made are linked via tree like data structure and we saw how they are linked. But let's actually verify it. Everything in git is an object. Newly created files are stored as an object. Changes to file are stored as an objects and even commits are objects. To view contents of an object, we can use the following command with the object's ID. We will take a look at the contents of the second commit:
            
 
            $ git cat-file -p 7f3b00e
 tree ebf3af44d253e5328340026e45a9fa9ae3ea1982
 parent df2fb7a61f5d40c1191e0fdeb0fc5d6e7969685a
@@ -2450,13 +2450,13 @@ committer Sanket Patel <spatel1@linkedin.com> 1603273316 -0700
 
 adding file 2
 
            
-
            Take a note of parent attribute in the above output. It points to the commit id of the first commit we made. So this proves that they are linked! Additionally you can see the second commit's message in this object. As I said all this magic is enabled by .git folder and the object to which we are looking at also is in that folder.
            
+
            Take a note of parent attribute in the above output. It points to the commit id of the first commit we made. So this proves that they are linked! Additionally, you can see the second commit's message in this object. As I said all this magic is enabled by .git folder and the object to which we are looking at also is in that folder.
            
 
            $ ls .git/objects/7f/3b00eaa957815884198e2fdfec29361108d6a9
 .git/objects/7f/3b00eaa957815884198e2fdfec29361108d6a9
 
            
 
            It is stored in .git/objects/ folder. All the files and changes to them as well are stored in this folder.
            
 
            The Version Control part of Git
            
-
            We already can see two commits (versions) in our git log. One thing a version control tool gives you is ability to browse back and forth in history. For example: some of your users are running an old version of code and they are reporting an issue. In order to debug the issue, you need access to the old code. The one in your current repo is the latest code. In this example, you are working on the second commit (7f3b00e) and someone reported an issue with the code snapshot at commit (df2fb7a). This is how you would get access to the code at any older commit
            
+
            We already can see two commits (versions) in our git log. One thing a version control tool gives you is ability to browse back and forth in history. For example: some of your users are running an old version of code and they are reporting an issue. In order to debug the issue, you need access to the old code. The one in your current repo is the latest code. In this example, you are working on the second commit (7f3b00e) and someone reported an issue with the code snapshot at commit (df2fb7a). This is how you would get access to the code at any older commit.
            
 
            # Current contents, two files present
 $ ls
 file1.txt file2.txt
@@ -2480,10 +2480,10 @@ HEAD is now at df2fb7a adding file 1
 $ ls
 file1.txt
 
            
-
            So this is how we would get access to old versions/snapshots. All we need is a reference to that snapshot. Upon executing git checkout ..., what git does for you is use the .git folder, see what was the state of things (files and folders) at that version/reference and replace the contents of current directory with those contents. The then-existing content will no longer be present in the local dir (repo) but we can and will still get access to them because they are tracked via git commit and .git folder has them stored/tracked.
            
+
            So this is how we would get access to old versions/snapshots. All we need is a reference to that snapshot. Upon executing git checkout ..., what git does for you is use the .git folder, see what was the state of things (files and folders) at that version/reference and replace the contents of current directory with those contents. The then-existing content will no longer be present in the local dir (repo) but we can and will still get access to them because they are tracked via git commit and .git folder has them stored/tracked.
            
 
            Reference
            
 
            I mention in the previous section that we need a reference to the version. By default, git repo is made of tree of commits. And each commit has a unique IDs. But the unique ID is not the only thing we can reference commits via. There are multiple ways to reference commits. For example: HEAD is a reference to current commit. Whatever commit your repo is checked out at, HEAD will point to that. HEAD~1 is reference to previous commit. So while checking out previous version in section above, we could have done git checkout HEAD~1.
            
-
            Similarly, master is also a reference (to a branch). Since git uses tree like structure to store commits, there of course will be branches. And the default branch is called master. Master (or any branch reference) will point to the latest commit in the branch. Even though we have checked out to the previous commit in out repo, master still points to the latest commit. And we can get back to the latest version by checkout at master reference
            
+
            Similarly, master is also a reference (to a branch). Since git uses tree like structure to store commits, there of course will be branches. And the default branch is called master. Master (or any branch reference) will point to the latest commit in the branch. Even though we have checked out to the previous commit in out repo, master still points to the latest commit. And we can get back to the latest version by checkout at master reference
            
 
            $ git checkout master
 Previous HEAD position was df2fb7a adding file 1
 Switched to branch 'master'
@@ -2503,7 +2503,7 @@ file1.txt file2.txt
 
            $ cat .git/refs/heads/master
 7f3b00eaa957815884198e2fdfec29361108d6a9
 
            
-
            Viola! Where master is pointing to is stored in a file. Whenever git needs to know where master reference is pointing to, or if git needs to update where master points, it just needs to update the file above. So when you create a new commit, a new commit is created on top of the current commit and the master file is updated with the new commit's ID.
            
+
            Viola! Where master is pointing to is stored in a file. Whenever git needs to know where master reference is pointing to, or if git needs to update where master points, it just needs to update the file above. So when you create a new commit, a new commit is created on top of the current commit and the master file is updated with the new commit's ID.
            
 
            Similary, for HEAD reference:
            
 
            $ cat .git/HEAD
 ref: refs/heads/master
@@ -2515,7 +2515,7 @@ ref: refs/heads/master
 * 7f3b00e (HEAD -> master) adding file 2
 * df2fb7a adding file 1
 
            
-
            Now let's change master to point to the previous/first commit.
            
+
            Now, let's change master to point to the previous/first commit.
            
 
            $ echo df2fb7a61f5d40c1191e0fdeb0fc5d6e7969685a > .git/refs/heads/master
 $ git log --oneline --graph
 * df2fb7a (HEAD -> master) adding file 1
diff --git a/level101/git/github-hooks/index.html b/level101/git/github-hooks/index.html
index e4b89ab..7e59ed7 100644
--- a/level101/git/github-hooks/index.html
+++ b/level101/git/github-hooks/index.html
@@ -2151,13 +2151,13 @@
                 
                 
                 
            Git with GitHub
            
-
            Till now all the operations we did were in our local repo while git also helps us in a collaborative environment. GitHub is one place on the internet where you can centrally host your git repos and collaborate with other developers.
            
+
            Till now all the operations we did were in our local repo while git also helps us in a collaborative environment. GitHub is one place on the Internet where you can centrally host your git repos and collaborate with other developers.
            
 
            Most of the workflow will remain the same as we discussed, with addition of couple of things:
            
 
              
-
            1. Pull: to pull latest changes from github (the central) repo
              2. 
-
            2. Push: to push your changes to github repo so that it's available to all people
              3. 
+
            3. Pull: to pull latest changes from GitHub (the central) repo
              4. 
+
            4. Push: to push your changes to GitHub repo so that it's available to all people
              5. 
 
            
-
            GitHub has written nice guides and tutorials about this and you can refer them here:
            
+
            GitHub has written nice guides and tutorials about this and you can refer to them here:
            
 
              
 
            • GitHub Hello World
              • 
 
            • Git Handbook
              • 
@@ -2168,7 +2168,7 @@
 applypatch-msg.sample     fsmonitor-watchman.sample pre-applypatch.sample     pre-push.sample           pre-receive.sample        update.sample
 commit-msg.sample         post-update.sample        pre-commit.sample         pre-rebase.sample         prepare-commit-msg.sample
 
            
-
            Names are self explanatory. These hooks are useful when you want to do certain things when a certain event happens. If you want to run tests before pushing code, you would want to setup pre-push hooks. Let's try to create a pre commit hook.
            
+
            Names are self-explanatory. These hooks are useful when you want to do certain things when a certain event happens. If you want to run tests before pushing code, you would want to setup pre-push hooks. Let's try to create a pre commit hook.
            
 
            $ echo "echo this is from pre commit hook" > .git/hooks/pre-commit
 $ chmod +x .git/hooks/pre-commit
 
            
diff --git a/level101/linux_basics/command_line_basics/index.html b/level101/linux_basics/command_line_basics/index.html
index e537755..c4f11f5 100644
--- a/level101/linux_basics/command_line_basics/index.html
+++ b/level101/linux_basics/command_line_basics/index.html
@@ -2522,40 +2522,83 @@
                 
                 
            Command Line Basics
            
 
            Lab Environment Setup
            
-
            One can use an online bash interpreter to run all the commands that are provided as examples in this course. This will also help you in getting a hands-on experience of various linux commands.
            
-
            REPL is one of the popular online bash interpreters for running linux commands. We will be using it for running all the commands mentioned in this course.
            
+
            One can use an online Bash interpreter to run all the commands that are provided as examples in this course. This will also help you in getting a hands-on experience of various Linux commands.
            
+
            REPL is one of the popular online Bash interpreters for running Linux commands. We will be using it for running all the commands mentioned in this course.
            
 
            What is a Command
            
 
            A command is a program that tells the operating system to perform
-specific work. Programs are stored as files in linux. Therefore, a
+specific work. Programs are stored as files in Linux. Therefore, a
 command is also a file which is stored somewhere on the disk.
            
 
            Commands may also take additional arguments as input from the user.
 These arguments are called command line arguments. Knowing how to use
 the commands is important and there are many ways to get help in Linux,
 especially for commands. Almost every command will have some form of
-documentation, most commands will have a command-line argument -h or
---help that will display a reasonable amount of documentation. But the
-most popular documentation system in Linux is called man pages - short
+documentation, most commands will have a command-line argument -h or 
+--help that will display a reasonable amount of documentation. But the
+most popular documentation system in Linux is called man pages—short
 for manual pages.
            
-
            Using --help to show the documentation for ls command.
            
+
            Using --help to show the documentation for ls command.
            
 
            
 
            File System Organization
            
-
            The linux file system has a hierarchical (or tree-like) structure with
-its highest level directory called root ( denoted by / ). Directories
-present inside the root directory stores file related to the system.
+
            The Linux file system has a hierarchical (or tree-like) structure with
+its highest-level directory called root (denoted by /). Directories
+present inside the root directory stores files related to the system.
 These directories in turn can either store system files or application
-files or user related files.
            
+files or user-related files.
            
 
            
-
            bin   | The executable program of most commonly used commands reside in bin directory
            
-
            dev   | This directory contains files related to devices on the system  
            
-
            etc   | This directory contains all the system configuration files
            
-
            home  | This directory contains user related files and directories.
            
-
            lib   | This directory contains all the library files
            
-
            mnt   | This directory contains files related to mounted devices on the system
            
-
            proc  | This directory contains files related to the running processes on the system
            
-
            root  | This directory contains root user related files and directories.
            
-
            sbin  | This directory contains programs used for system administration.
            
-
            tmp   | This directory is used to store temporary files on the system
            
-
            usr   |  This directory is used to store application programs on the system
            
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
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+
+
+
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+
            DirectoryDescription
            binThe executable program of most commonly used commands reside in bin directory
            devThis directory contains files related to devices on the system
            etcThis directory contains all the system configuration files
            homeThis directory contains user-related files and directories
            libThis directory contains all the library files
            mntThis directory contains files related to mounted devices on the system
            procThis directory contains files related to the running processes on the system
            rootThis directory contains root user-related files and directories
            sbinThis directory contains programs used for system administration
            tmpThis directory is used to store temporary files on the system
            usrThis directory is used to store application programs on the system
            
 
            Commands for Navigating the File System
            
 
            There are three basic commands which are used frequently to navigate the
 file system:
            
@@ -2572,28 +2615,28 @@ file system:
            
 
          
 
          We will now try to understand what each command does and how to use
 these commands. You should also practice the given examples on the
-online bash shell.
          
+online Bash shell.
          
 
          pwd (print working directory)
          
 
          At any given moment of time, we will be standing in a certain directory.
 To get the name of the directory in which we are standing, we can use
-the pwd command in linux.
          
+the pwd command in Linux.
          
 
          
-
          We will now use the cd command to move to a different directory and then
+
          We will now use the cd command to move to a different directory and then
 print the working directory.
          
 
          
 
          cd (change directory)
          
-
          The cd command can be used to change the working directory. Using the
+
          The cd command can be used to change the working directory. Using the
 command, you can move from one directory to another.
          
-
          In the below example, we are initially in the root directory. we have
-then used the cd command to change the directory.
          
+
          In the below example, we are initially in the root directory. We have
+then used the cd command to change the directory.
          
 
          
 
          ls (list files and directories)**
          
-
          The ls command is used to list the contents of a directory. It will list
+
          The ls command is used to list the contents of a directory. It will list
 down all the files and folders present in the given directory.
          
-
          If we just type ls in the shell, it will list all the files and
+
          If we just type ls in the shell, it will list all the files and
 directories present in the current directory.
          
 
          
-
          We can also provide the directory name as argument to ls command. It
+
          We can also provide the directory name as argument to ls command. It
 will then list all the files and directories inside the given directory.
          
 
          
 
          Commands for Manipulating Files
          
@@ -2618,80 +2661,80 @@ files:
          
 
        
 
        We will now try to understand what each command does and how to use
 these commands. You should also practice the given examples on the
-online bash shell.
        
+online Bash shell.
        
 
        touch (create new file)
        
-
        The touch command can be used to create an empty new file.
-This command is very useful for many other purposes but we will discuss
+
        The touch command can be used to create an empty new file.
+This command is very useful for many other purposes, but we will discuss
 the simplest use case of creating a new file.
        
-
        General syntax of using touch command
        
-
        touch <file_name>
+
        General syntax of using touch command:
        
+
        touch <file_name>
 
        
 
        
 
        mkdir (create new directories)
        
-
        The mkdir command is used to create directories.You can use ls command
+
        The mkdir command is used to create directories. You can use ls command
 to verify that the new directory is created.
        
-
        General syntax of using mkdir command
        
-
        mkdir <directory_name>
+
        General syntax of using mkdir command:
        
+
        mkdir <directory_name>
 
        
 
        
 
        rm (delete files and directories)
        
-
        The rm command can be used to delete files and directories. It is very
+
        The rm command can be used to delete files and directories. It is very
 important to note that this command permanently deletes the files and
 directories. It's almost impossible to recover these files and
-directories once you have executed rm command on them successfully. Do
+directories once you have executed rm command on them successfully. Do
 run this command with care.
        
-
        General syntax of using rm command:
        
-
        rm <file_name>
+
        General syntax of using rm command:
        
+
        rm <file_name>
 
        
-
        Let's try to understand the rm command with an example. We will try to
-delete the file and directory we created using touch and mkdir command
+
        Let's try to understand the rm command with an example. We will try to
+delete the file and directory we created using touch and mkdir command
 respectively.
        
 
        
 
        cp (copy files and directories)
        
-
        The cp command is used to copy files and directories from one location
-to another. Do note that the cp command doesn't do any change to the
+
        The cp command is used to copy files and directories from one location
+to another. Do note that the cp command doesn't do any change to the
 original files or directories. The original files or directories and
-their copy both co-exist after running cp command successfully.
        
-
        General syntax of using cp command:
        
-
        cp <source_path> <destination_path>
+their copy both co-exist after running cp command successfully.
        
+
        General syntax of using cp command:
        
+
        cp <source_path> <destination_path>
 
        
-
        We are currently in the '/home/runner' directory. We will use the mkdir
-command to create a new directory named "test_directory". We will now
-try to copy the "_test_runner.py" file to the directory we created just
+
        We are currently in the /home/runner directory. We will use the mkdir
+command to create a new directory named test_directory. We will now
+try to copy the _test_runner.py file to the directory we created just
 now.
        
 
        
-
        Do note that nothing happened to the original "_test_runner.py" file.
+
        Do note that nothing happened to the original _test_runner.py file.
 It's still there in the current directory. A new copy of it got created
-inside the "test_directory".
        
+inside the test_directory.
        
 
        
-
        We can also use the cp command to copy the whole directory from one
+
        We can also use the cp command to copy the whole directory from one
 location to another. Let's try to understand this with an example.
        
 
        
-
        We again used the mkdir command to create a new directory called
-"another_directory". We then used the cp command along with an
-additional argument '-r' to copy the "test_directory".
        
+
        We again used the mkdir command to create a new directory called
+another_directory. We then used the cp command along with an
+additional argument -r to copy the test_directory.
        
 
        mv (move files and directories)
        
-
        The mv command can either be used to move files or directories from one
+
        The mv command can either be used to move files or directories from one
 location to another or it can be used to rename files or directories. Do
 note that moving files and copying them are very different. When you
 move the files or directories, the original copy is lost.
        
-
        General syntax of using mv command:
        
-
        mv <source_path> <destination_path>
+
        General syntax of using mv command:
        
+
        mv <source_path> <destination_path>
 
        
-
        In this example, we will use the mv command to move the
-"_test_runner.py" file to "test_directory". In this case, this file
-already exists in "test_directory". The mv command will just replace it.
+
        In this example, we will use the mv command to move the
+_test_runner.py file to test_directory. In this case, this file
+already exists in test_directory. The mv command will just replace it.
 Do note that the original file doesn't exist in the current directory
-after mv command ran successfully.
        
+after mv command ran successfully.
        
 
        
-
        We can also use the mv command to move a directory from one location to
-another. In this case, we do not need to use the '-r' flag that we did
-while using the cp command. Do note that the original directory will not
-exist if we use mv command.
        
-
        One of the important uses of the mv command is to rename files and
+
        We can also use the mv command to move a directory from one location to
+another. In this case, we do not need to use the -r flag that we did
+while using the cp command. Do note that the original directory will not
+exist if we use mv command.
        
+
        One of the important uses of the mv command is to rename files and
 directories. Let's see how we can use this command for renaming.
        
-
        We have first changed our location to "test_directory". We then use the
-mv command to rename the ""_test_runner.py" file to "test.py".
        
+
        We have first changed our location to test_directory. We then use the
+mv command to rename the _test_runner.py file to test.py.
        
 
        
 
        Commands for Viewing Files
        
 
        There are five basic commands which are used frequently to view the
@@ -2715,8 +2758,8 @@ files:

      We will now try to understand what each command does and how to use these commands. You should also practice the given examples on the -online bash shell.

      -

      We will create a new file called "numbers.txt" and insert numbers from 1 +online Bash shell.

      +

      We will create a new file called numbers.txt and insert numbers from 1 to 100 in this file. Each number will be in a separate line.

      Do not worry about the above command now. It's an advanced command which @@ -2724,7 +2767,7 @@ is used to generate numbers. We have then used a redirection operator to push these numbers to the file. We will be discussing I/O redirection in the later sections.

      cat

      -

      The most simplest use of cat command is to print the contents of the file on +

      The most simplest use of cat command is to print the contents of the file on your output screen. This command is very useful and can be used for many other purposes. We will study about other use cases later.

      @@ -2732,42 +2775,42 @@ other purposes. We will study about other use cases later.

      printed from 1 to 100 on your screen. You will need to scroll up to view all the numbers.

      -

      The head command displays the first 10 lines of the file by default. We +

      The head command displays the first 10 lines of the file by default. We can include additional arguments to display as many lines as we want from the top.

      In this example, we are only able to see the first 10 lines from the -file when we use the head command.

      +file when we use the head command.

      -

      By default, head command will only display the first 10 lines. If we +

      By default, head command will only display the first 10 lines. If we want to specify the number of lines we want to see from start, use the -'-n' argument to provide the input.

      +-n argument to provide the input.

      tail

      -

      The tail command displays the last 10 lines of the file by default. We +

      The tail command displays the last 10 lines of the file by default. We can include additional arguments to display as many lines as we want from the end of the file.

      -

      By default, the tail command will only display the last 10 lines. If we -want to specify the number of lines we want to see from the end, use '-n' +

      By default, the tail command will only display the last 10 lines. If we +want to specify the number of lines we want to see from the end, use -n argument to provide the input.

      In this example, we are only able to see the last 5 lines from the file -when we use the tail command with explicit -n option.

      +when we use the tail command with explicit -n option.

      more

      -

      More command displays the contents of a file or a command output, +

      The more command displays the contents of a file or a command output, displaying one screen at a time in case the file is large (Eg: log files). It also allows forward navigation and limited backward navigation in the file.

      -

      More command displays as much as can fit on the current screen and waits for user input to advance. Forward navigation can be done by pressing Enter, which advances the output by one line and Space, which advances the output by one screen.

      +

      The more command displays as much as can fit on the current screen and waits for user input to advance. Forward navigation can be done by pressing Enter, which advances the output by one line and Space, which advances the output by one screen.

      less

      -

      Less command is an improved version of more. It displays the contents of a file or a command output, one page at a time. -It allows backward navigation as well as forward navigation in the file and also has search options. We can use arrow keys for advancing backward or forward by one line. For moving forward by one page, press Space and for moving backward by one page, press b on your keyboard. +

      The less command is an improved version of more. It displays the contents of a file or a command output, one page at a time. +It allows backward navigation as well as forward navigation in the file and also has search options. We can use arrow keys for advancing backward or forward by one line. For moving forward by one page, press Space and for moving backward by one page, press b on your keyboard. You can go to the beginning and the end of a file instantly.

      Echo Command in Linux

      -

      The echo command is one of the simplest commands that is used in the -shell. This command is equivalent to what we have in other +

      The echo command is one of the simplest commands that is used in the +shell. This command is equivalent to print in other programming languages.

      -

      The echo command prints the given input string on the screen.

      +

      The echo command prints the given input string on the screen.

      Text Processing Commands

      In the previous section, we learned how to view the content of a file. @@ -2798,39 +2841,39 @@ texts:

    We will now try to understand what each command does and how to use these commands. You should also practice the given examples on the -online bash shell.

    -

    We will create a new file called "numbers.txt" and insert numbers from 1 +online Bash shell.

    +

    We will create a new file called numbers.txt and insert numbers from 1 to 10 in this file. Each number will be in a separate line.

    grep

    -

    The grep command in its simplest form can be used to search particular +

    The grep command in its simplest form can be used to search particular words in a text file. It will display all the lines in a file that contains a particular input. The word we want to search is provided as -an input to the grep command.

    -

    General syntax of using grep command:

    -
    grep <word_to_search> <file_name>
+an input to the grep command.
    
+
    General syntax of using grep command:
    
+
    grep <word_to_search> <file_name>
 
    
 
    In this example, we are trying to search for a string "1" in this file.
-The grep command outputs the lines where it found this string.
    
+The grep command outputs the lines where it found this string.
    
 
    
 
    sed
    
-
    The sed command in its simplest form can be used to replace a text in a
+
    The sed command in its simplest form can be used to replace a text in a
 file.
    
-
    General syntax of using the sed command for replacement:
    
-
    sed 's/<text_to_replace>/<replacement_text>/' <file_name>
+
    General syntax of using the sed command for replacement:
    
+
    sed 's/<text_to_replace>/<replacement_text>/' <file_name>
 
    
 
    Let's try to replace each occurrence of "1" in the file with "3" using
-sed command.
    
+sed command.
    
 
    
 
    The content of the file will not change in the above
-example. To do so, we have to use an extra argument '-i' so that the
+example. To do so, we have to use an extra argument -i so that the
 changes are reflected back in the file.
    
 
    sort
    
-
    The sort command can be used to sort the input provided to it as an
+
    The sort command can be used to sort the input provided to it as an
 argument. By default, it will sort in increasing order.
    
 
    Let's first see the content of the file before trying to sort it.
    
 
    
-
    Now, we will try to sort the file using the sort command. The sort
+
    Now, we will try to sort the file using the sort command. The sort
 command sorts the content in lexicographical order.
    
 
    
 
    The content of the file will not change in the above
@@ -2838,28 +2881,28 @@ example.
    
 
    I/O Redirection
    
 
    Each open file gets assigned a file descriptor. A file descriptor is an
 unique identifier for open files in the system. There are always three
-default files open, stdin (the keyboard), stdout (the screen), and
-stderr (error messages output to the screen). These files can be
+default files open, stdin (the keyboard), stdout (the screen), and
+stderr (error messages output to the screen). These files can be
 redirected.
    
-
    Everything is a file in linux -
+
    Everything is a file in Linux -
 https://unix.stackexchange.com/questions/225537/everything-is-a-file
    
 
    Till now, we have displayed all the output on the screen which is the
 standard output. We can use some special operators to redirect the
 output of the command to files or even to the input of other commands.
 I/O redirection is a very powerful feature.
    
-
    In the below example, we have used the '>' operator to redirect the
-output of ls command to output.txt file.
    
+
    In the below example, we have used the > operator to redirect the
+output of ls command to output.txt file.
    
 
    
-
    In the below example, we have redirected the output from echo command to
+
    In the below example, we have redirected the output from echo command to
 a file.
    
 
    
 
    We can also redirect the output of a command as an input to another
 command. This is possible with the help of pipes.
    
-
    In the below example, we have passed the output of cat command as an
-input to grep command using pipe(|) operator.
    
+
    In the below example, we have passed the output of cat command as an
+input to grep command using pipe (|) operator.
    
 
    
-
    In the below example, we have passed the output of sort command as an
-input to uniq command using pipe(|) operator. The uniq command only
+
    In the below example, we have passed the output of sort command as an
+input to uniq command using pipe (|) operator. The uniq command only
 prints the unique numbers from the input.
    
 
    
 
    I/O redirection -
diff --git a/level101/linux_basics/conclusion/index.html b/level101/linux_basics/conclusion/index.html
index 7450a75..0254b4d 100644
--- a/level101/linux_basics/conclusion/index.html
+++ b/level101/linux_basics/conclusion/index.html
@@ -334,7 +334,7 @@
       
         
  • 
   
-    Useful Courses and tutorials
+    Useful Courses and Tutorials
   
   
 
    • 
@@ -2146,7 +2146,7 @@
       
         
  • 
   
-    Useful Courses and tutorials
+    Useful Courses and Tutorials
   
   
 
    • 
@@ -2165,7 +2165,7 @@
                 
                 
                 
    Conclusion
    
-
    We have covered the basics of Linux operating systems and basic commands used in linux.
+
    We have covered the basics of Linux operating systems and basic commands used in Linux.
 We have also covered the Linux server administration commands.
    
 
    We hope that this course will make it easier for you to operate on the command line.
    
 
    Applications in SRE Role
    
@@ -2176,12 +2176,12 @@ We have also covered the Linux server administration commands.
    
 
  • tail command is very useful to view the latest data in the log file.
    • 
 
  • Different users will have different permissions depending on their roles. We will also not want everyone in the company to access our servers for security reasons. Users permissions can be restricted with chown, chmod and chgrp commands.
    • 
 
  • ssh is one of the most frequently used commands for a SRE. Logging into servers and troubleshooting along with performing basic administration tasks will only be possible if we are able to login into the server.
    • 
-
  • What if we want to run an apache server or nginx on a server? We will first install it using the package manager. Package management commands become important here.
    • 
-
  • Managing services on servers is another critical responsibility of a SRE. Systemd related commands can help in troubleshooting issues. If a service goes down, we can start it using systemctl start command. We can also stop a service in case it is not needed.
    • 
-
  • Monitoring is another core responsibility of a SRE. Memory and CPU are two important system level metrics which should be monitored. Commands like top and free are quite helpful here.
    • 
-
  • If a service is throwing an error, how do we find out the root cause of the error ? We will certainly need to check logs to find out the whole stack trace of the error. The log file will also tell us the number of times the error has occurred along with time when it started.
    • 
+
  • What if we want to run an Apache server or NGINX on a server? We will first install it using the package manager. Package management commands become important here.
    • 
+
  • Managing services on servers is another critical responsibility of a SRE. systemd-related commands can help in troubleshooting issues. If a service goes down, we can start it using systemctl start command. We can also stop a service in case it is not needed.
    • 
+
  • Monitoring is another core responsibility of a SRE. Memory and CPU are two important system-level metrics which should be monitored. Commands like top and free are quite helpful here.
    • 
+
  • If a service throws an error, how do we find out the root cause of the error? We will certainly need to check logs to find out the whole stack trace of the error. The log file will also tell us the number of times the error has occurred along with time when it started.
    • 
 
-
    Useful Courses and tutorials
    
+
    Useful Courses and Tutorials
    
 
      
 
    • Edx basic linux commands course
      • 
 
    • Edx Red Hat Enterprise Linux Course
      • 
diff --git a/level101/linux_basics/intro/index.html b/level101/linux_basics/intro/index.html
index d12edb6..c528a77 100644
--- a/level101/linux_basics/intro/index.html
+++ b/level101/linux_basics/intro/index.html
@@ -2306,7 +2306,7 @@
 
      Introduction
      
 
      Prerequisites
      
 
        
-
      • Should be comfortable in using any operating systems like Windows, Linux or Mac
        • 
+
      • Should be comfortable in using any operating systems like Windows, Linux or
        • 
 
      • Expected to have fundamental knowledge of operating systems
        • 
 
      
 
      What to expect from this course
      
@@ -2316,13 +2316,13 @@ Linux distributions and uses of Linux operating systems. We will also talk about
 difference between GUI and CLI.
      
 
      In the second part, we cover some basic commands used in Linux. 
 We will focus on commands used for navigating the file system, viewing and manipulating files,
-I/O redirection etc.
      
-
      In the third part, we cover Linux system administration. This includes day to day tasks 
+I/O redirection, etc.
      
+
      In the third part, we cover Linux system administration. This includes day-to-day tasks 
 performed by Linux admins, like managing users/groups, managing file permissions, 
 monitoring system performance, log files etc.
      
-
      In the second and third part, we will be taking examples to understand the concepts.
      
+
      In the second and third part, we will be showing examples to understand the concepts.
      
 
      What is not covered under this course
      
-
      We are not covering advanced Linux commands and bash scripting in this
+
      We are not covering advanced Linux commands and Bash scripting in this
 course. We will also not be covering Linux internals. 
      
 
      Course Contents
      
 
      The following topics has been covered in this course:
      
@@ -2370,26 +2370,26 @@ course. We will also not be covering Linux internals. 
      
 
      Most of us are familiar with the Windows operating system used in more than
  75% of the personal computers. The Windows operating systems
 are based on Windows NT kernel. 
      
-
      A kernel is the most important part of
-an operating system - it performs important functions like process
-management, memory management, filesystem management etc.
      
-
      Linux operating systems are based on the Linux kernel. A Linux based
+
      A kernel is the most important part of
+an operating system—it performs important functions like process
+management, memory management, filesystem management, etc.
      
+
      Linux operating systems are based on the Linux kernel. A Linux-based
 operating system will consist of Linux kernel, GUI/CLI, system libraries
 and system utilities. The Linux kernel was independently developed and
-released by Linus Torvalds. The Linux kernel is free and open-source -
-https://github.com/torvalds/linux
      
-
      Linux is a kernel and not a complete operating system. Linux kernel is combined with GNU system to make a complete operating system. Therefore, linux based operating systems are also called as GNU/Linux systems. GNU is an extensive collection of free softwares like compiler, debugger, C library etc.
-Linux and the GNU  System
      
+released by Linus Torvalds. The Linux kernel is free and open-source (See 
+https://github.com/torvalds/linux).
      
+
      Linux is a kernel and not a complete operating system. Linux kernel is combined with GNU system to make a complete operating system. Therefore, Linux-based operating systems are also called as GNU/Linux systems. GNU is an extensive collection of free softwares like compiler, debugger, C library etc. (See
+Linux and the GNU  System)
      
 
      History of Linux -
 https://en.wikipedia.org/wiki/History_of_Linux
      
 
-
      A Linux distribution(distro) is an operating system based on
+
      A Linux distribution (distro) is an operating system based on
 the Linux kernel and a package management system. A package management
 system consists of tools that help in installing, upgrading,
 configuring and removing softwares on the operating system.
      
 
      Software are usually adopted to a distribution and are packaged in a
-distro specific format. These packages are available through a distro
-specific repository. Packages are installed and managed in the operating
+distro-specific format. These packages are available through a distro-specific 
+repository. Packages are installed and managed in the operating
 system by a package manager.
      
 
      List of popular Linux distributions:
      
 
        
@@ -2425,12 +2425,12 @@ system by a package manager.
        
 
 
 
-Debian style (.deb)
+Debian style (.deb)
 Debian, Ubuntu
 APT
 
 
-Red Hat style (.rpm)
+Red Hat style (.rpm)
 Fedora, CentOS, Red Hat Enterprise Linux
 YUM
 
@@ -2465,13 +2465,13 @@ system by a package manager.
        
 
        Mobile phones - Android is based on Linux operating system
        
 
 
      • 
-
        Embedded devices - watches, televisions, traffic lights etc
        
+
        Embedded devices - watches, televisions, traffic lights, etc.
        
 
        • 
 
      • 
 
        Satellites
        
 
        • 
 
      • 
-
        Network devices - routers, switches etc.
        
+
        Network devices - routers, switches, etc.
        
 
        • 
 
      
 
      Graphical user interface (GUI) vs Command line interface (CLI)
      
@@ -2493,9 +2493,9 @@ example of a CLI (command line interface). Bash is one of the most popular shell
 programs available on Linux servers. Other popular shell programs are
 zsh, ksh and tcsh.
      
 
      Terminal is a program that opens a window and lets you interact with the
-shell. Some popular examples of terminals are gnome-terminal, xterm,
-konsole etc.
      
-
      Linux users do use the terms shell, terminal, prompt, console etc.
+shell. Some popular examples of terminals are GNOME-terminal, xterm,
+Konsole, etc.
      
+
      Linux users do use the terms shell, terminal, prompt, console, etc.
 interchangeably. In simple terms, these all refer to a way of taking
 commands from the user.
      
                 
diff --git a/level101/linux_basics/linux_server_administration/index.html b/level101/linux_basics/linux_server_administration/index.html
index f8d28a9..6ad21a3 100644
--- a/level101/linux_basics/linux_server_administration/index.html
+++ b/level101/linux_basics/linux_server_administration/index.html
@@ -2709,7 +2709,7 @@
                 
                 
                 
      Linux Server Administration
      
-
      In this course will try to cover some of the common tasks that a linux
+
      In this course, will try to cover some of the common tasks that a Linux
 server administrator performs. We will first try to understand what a
 particular command does and then try to understand the commands using
 examples. Do keep in mind that it's very important to practice the Linux
@@ -2717,7 +2717,7 @@ commands on your own.
      
 
      Lab Environment Setup
      
 
 
      Multi-User Operating Systems
      
-
      An operating system is considered as multi-user if it allows multiple people/users to use a computer and not affect each other's files and preferences. Linux based operating systems are multi-user in nature as it allows multiple users to access the system at the same time. A typical computer will only have one keyboard and monitor but multiple users can log in via SSH if the computer is connected to the network. We will cover more about SSH later.
      
+
      An operating system is considered as multi-user if it allows multiple people/users to use a computer and not affect each other's files and preferences. Linux-based operating systems are multi-user in nature as it allows multiple users to access the system at the same time. A typical computer will only have one keyboard and monitor but multiple users can log in via SSH if the computer is connected to the network. We will cover more about SSH later.
      
 
      As a server administrator, we are mostly concerned with the Linux servers which are physically present at a very large distance from us. We can connect to these servers with the help of remote login methods like SSH.
      
 
      Since Linux supports multiple users, we need to have a method which can protect the users from each other. One user should not be able to access and modify files of other users
      
 
      User/Group Management
      
@@ -2747,25 +2747,29 @@ commands on your own.
      
 
 
    
 
    id command
    
-
    id command can be used to find the uid and gid associated with an user.
+
    id command can be used to find the uid and gid associated with an user.
 It also lists down the groups to which the user belongs to.
    
-
    The uid and gid associated with the root user is 0.
-
    
-
    A good way to find out the current user in Linux is to use the whoami
+
    The uid and gid associated with the root user is 0.
    
+
    
+
    A good way to find out the current user in Linux is to use the whoami
 command.
    
 
    
-
    "root" user or superuser is the most privileged user with
+
    root user or superuser is the most privileged user with
 unrestricted access to all the resources on the system. It has UID 0
    
 
    Important files associated with users/groups
    
 
-
-
+
+
+
+
+
+
@@ -2778,7 +2782,7 @@ command.
    
-
    If you want to understand each filed discussed in the above outputs, you can go
+
    If you want to understand each field discussed in the above outputs, you can go
 through below links:
      
 
    • 
@@ -2792,25 +2796,17 @@ through below links:
      
 
      Some of the commands which are used frequently to manage users/groups
 on Linux are following:
      
 
        
-
      • 
-
        useradd - Creates a new user
        
-
        • 
-
      • 
-
        passwd - Adds or modifies passwords for a user
        
-
        • 
-
      • 
-
        usermod - Modifies attributes of an user
        
-
        • 
-
      • 
-
        userdel - Deletes an user
        
-
        • 
+
      • useradd - Creates a new user
        • 
+
      • passwd - Adds or modifies passwords for a user
        • 
+
      • usermod - Modifies attributes of an user
        • 
+
      • userdel - Deletes an user
        • 
 
      
 
      useradd
      
-
      The useradd command adds a new user in Linux.
      
-
      We will create a new user 'shivam'. We will also verify that the user
-has been created by tailing the /etc/passwd file. The uid and gid are
+
      The useradd command adds a new user in Linux.
      
+
      We will create a new user shivam. We will also verify that the user
+has been created by tailing the /etc/passwd file. The uid and gid are
 1000 for the newly created user. The home directory assigned to the user
-is /home/shivam and the login shell assigned is /bin/bash. Do note that
+is /home/shivam and the login shell assigned is /bin/bash. Do note that
 the user home directory and login shell can be modified later on.
      
 
      
 
      If we do not specify any value for attributes like home directory or
@@ -2818,13 +2814,13 @@ login shell, default values will be assigned to the user. We can also
 override these default values when creating a new user.
      
 
      
 
      passwd
      
-
      The passwd command is used to create or modify passwords for a user.
      
+
      The passwd command is used to create or modify passwords for a user.
      
 
      In the above examples, we have not assigned any password for users
-'shivam' or 'amit' while creating them.
      
-
      "!!" in an account entry in shadow means the account of an user has
+shivam or amit while creating them.
      
+
      !! in an account entry in shadow means the account of an user has
 been created, but not yet given a password.
      
 
      
-
      Let's now try to create a password for user "shivam".
      
+
      Let's now try to create a password for user shivam.
      
 
      
 
      Do remember the password as we will be later using examples
 where it will be useful.
      
@@ -2833,112 +2829,116 @@ from a normal user to root user, it will request you for a password.
 Also, when you login using root user, the password will be asked.
      
 
      
 
      usermod
      
-
      The usermod command is used to modify the attributes of an user like the
+
      The usermod command is used to modify the attributes of an user like the
 home directory or the shell.
      
-
      Let's try to modify the login shell of user "amit" to "/bin/bash".
      
+
      Let's try to modify the login shell of user amit to /bin/bash.
      
 
      
 
      In a similar way, you can also modify many other attributes for a user.
-Try 'usermod -h' for a list of attributes you can modify.
      
+Try usermod -h for a list of attributes you can modify.
      
 
      userdel
      
-
      The userdel command is used to remove a user on Linux. Once we remove a
+
      The userdel command is used to remove a user on Linux. Once we remove a
 user, all the information related to that user will be removed.
      
-
      Let's try to delete the user "amit". After deleting the user, you will
-not find the entry for that user in "/etc/passwd" or "/etc/shadow" file.
      
+
      Let's try to delete the user amit. After deleting the user, you will
+not find the entry for that user in /etc/passwd or /etc/shadow file.
      
 
      
 
      Important commands for managing groups
      
-
      Commands for managing groups are quite similar to the commands used for managing users. Each command is not explained  in detail here as they are quite similar. You can try running these commands on your system.
      
+
      Commands for managing groups are quite similar to the commands used for managing users. Each command is not explained in detail here as they are quite similar. You can try running these commands on your system.
      
 
    
 
 
    /etc/passwdStores the user name, the uid, the gid, the home directory, the login shell etcFilesDescription
 
    
     
 
 
    /etc/passwdStores the user name, the uid, the gid, the home directory, the login shell etc
    
 /etc/shadow
 Stores the password associated with the users
 
    
 
 
 
 
    
-
-
+
+
-
+
+
+
+
+
-
+
-
+
    
 
 
    groupadd \<group_name>Creates a new groupCommandDescription
 
    
     
 
 
    groupmod \<group_name>groupadd <group_name>Creates a new group
    groupmod <group_name>
 Modifies attributes of a group
 
    
 
    groupdel \<group_name>groupdel <group_name>
 Deletes a group
 
    
 
    gpasswd \<group_name>gpasswd  <group_name>
 Modifies password for group
 
    
     
 
    
 
    
-
    We will now try to add user "shivam" to the group we have created above.
    
+
    We will now try to add user shivam to the group we have created above.
    
 
    
 
    Becoming a Superuser
    
 
    Before running the below commands, do make sure that you have set up a
-password for user "shivam" and user "root" using the passwd command
+password for user shivam and user root using the passwd command
 described in the above section.
    
-
    The su command can be used to switch users in Linux. Let's now try to
-switch to user "shivam".
    
+
    The su command can be used to switch users in Linux. Let's now try to
+switch to user shivam.
    
 
    
-
    Let's now try to open the "/etc/shadow" file.
    
+
    Let's now try to open the /etc/shadow file.
    
 
    
-
    The operating system didn't allow the user "shivam" to read the content
-of the "/etc/shadow" file. This is an important file in Linux which
-stores the passwords of users. This file can only be accessed by root or
-users who have the superuser privileges.
    
-
    The sudo command allows a user to run commands with the security
+
    The operating system didn't allow the user shivam to read the content
+of the /etc/shadow file. This is an important file in Linux which
+stores the passwords of users. This file can only be accessed by root or
+users who have the superuser privileges.
    
+
    The sudo command allows a user to run commands with the security
 privileges of the root user. Do remember that the root user has all
-the privileges on a system. We can also use su command to switch to the
+the privileges on a system. We can also use su command to switch to the
 root user and open the above file but doing that will require the
 password of the root user. An alternative way which is preferred on most
-modern operating systems is to use sudo command for becoming a
+modern operating systems is to use sudo command for becoming a
 superuser. Using this way, a user has to enter his/her password and they
-need to be a part of the sudo group.
    
+need to be a part of the sudo group.
    
 
    How to provide superpriveleges to other users ?
    
-
    Let's first switch to the root user using su command. Do note that using
+
    Let's first switch to the root user using su command. Do note that using
 the below command will need you to enter the password for the root user.
    
 
    
-
    In case, you forgot to set a password for the root user, type "exit" and
+
    In case, you forgot to set a password for the root user, type exit and
 you will be back as the root user. Now, set up a password using the
-passwd command.
    
-
    The file /etc/sudoers holds the names of users permitted to invoke
-sudo. In redhat operating systems, this file is not present by
-default. We will need to install sudo.
    
+passwd command.
    
+
    The file /etc/sudoers holds the names of users permitted to invoke
+sudo. In Red Hat operating systems, this file is not present by
+default. We will need to install sudo.
    
 
    
-
    We will discuss the yum command in detail in later sections.
    
-
    Try to open the "/etc/sudoers" file on the system. The file has a lot of
+
    We will discuss the yum command in detail in later sections.
    
+
    Try to open the /etc/sudoers file on the system. The file has a lot of
 information. This file stores the rules that users must follow when
-running the sudo command. For example, root is allowed to run any
+running the sudo command. For example, root is allowed to run any
 commands from anywhere.
    
 
    
 
    One easy way of providing root access to users is to add them to a group
-which has permissions to run all the commands. "wheel" is a group in
-redhat Linux with such privileges.
    
+which has permissions to run all the commands. wheel is a group in
+Red Hat Linux with such privileges.
    
 
    
-
    Let's add the user "shivam" to this group so that it also has sudo
+
    Let's add the user shivam to this group so that it also has sudo
 privileges.
    
 
    
-
    Let's now switch back to user "shivam" and try to access the
-"/etc/shadow" file.
    
+
    Let's now switch back to user shivam and try to access the
+/etc/shadow file.
    
 
    
-
    We need to use sudo before running the command since it can only be
-accessed with the sudo privileges. We have already given sudo privileges
-to user “shivam” by adding him to the group “wheel”.
    
+
    We need to use sudo before running the command since it can only be
+accessed with the sudo privileges. We have already given sudo privileges
+to user shivam by adding him to the group wheel.
    
 
    File Permissions
    
 
    On a Linux operating system, each file and directory is assigned access
 permissions for the owner of the file, the members of a group of related
 users and everybody else. This is to make sure that one user is not
 allowed to access the files and resources of another user.
    
-
    To see the permissions of a file, we can use the ls command. Let's look
-at the permissions of /etc/passwd file.
    
+
    To see the permissions of a file, we can use the ls command. Let's look
+at the permissions of /etc/passwd file.
    
 
    
 
    Let's go over some of the important fields in the output that are
 related to file permissions.
    
 
    
 
    
 
    Chmod command
    
-
    The chmod command is used to modify files and directories permissions in
+
    The chmod command is used to modify files and directories permissions in
 Linux.
    
-
    The chmod command accepts permissions in as a numerical argument. We can
+
    The chmod command accepts permissions in as a numerical argument. We can
 think of permission as a series of bits with 1 representing True or
 allowed and 0 representing False or not allowed.
    
 
@@ -3004,56 +3004,56 @@ allowed and 0 representing False or not allowed.
    We will now create a new file and check the permission of the file.
    The group owner doesn't have the permission to write to this file. Let's
-give the group owner or root the permission to write to it using chmod
+give the group owner or root the permission to write to it using chmod
 command.
    
-
    Chmod command can be also used to change the permissions of a directory
+
    chmod command can be also used to change the permissions of a directory
 in the similar way.
    Chown command
    
-
    The chown command is used to change the owner of files or
+
    The chown command is used to change the owner of files or
 directories in Linux.
    
-
    Command syntax: chown \<new_owner> \<file_name>
    
+
    Command syntax: chown \<new_owner\> \<file_name\>
    
-
    In case, we do not have sudo privileges, we need to use sudo
-command. Let's switch to user 'shivam' and try changing the owner. We
-have also changed the owner of the file to root before running the below
+
    In case, we do not have sudo privileges, we need to use sudo
+command. Let's switch to user shivam and try changing the owner. We
+have also changed the owner of the file to root before running the below
 command.
    Chown command can also be used to change the owner of a directory in the
 similar way.
    Chgrp command
    
-
    The chgrp command can be used to change the group ownership of files or
-directories in Linux. The syntax is very similar to that of chown
+
    The chgrp command can be used to change the group ownership of files or
+directories in Linux. The syntax is very similar to that of chown
 command.
    
-
    Chgrp command can also be used to change the owner of a directory in the
+
    chgrp command can also be used to change the owner of a directory in the
 similar way.
    SSH Command
    
-
    The ssh command is used for logging into the remote systems, transfer files between systems and for executing commands on a remote machine. SSH stands for secure shell and is used to provide an encrypted secured connection between two hosts over an insecure network like the internet.
    
+
    The ssh command is used for logging into the remote systems, transfer files between systems and for executing commands on a remote machine. SSH stands for secure shell and is used to provide an encrypted secured connection between two hosts over an insecure network like the internet.
    Reference:
 https://www.ssh.com/ssh/command/
    We will now discuss passwordless authentication which is secure and most
-commonly used for ssh authentication.
    
+commonly used for ssh authentication.
    Passwordless Authentication Using SSH
    
-
    Using this method, we can ssh into hosts without entering the password.
+
    Using this method, we can ssh into hosts without entering the password.
 This method is also useful when we want some scripts to perform
 ssh-related tasks.
    Passwordless authentication requires the use of a public and private key pair. As the name implies, the public key can be shared with anyone but the private key should be kept private.
-Lets not get  into the details of how this authentication works. You can read more about it
+Let's not get into the details of how this authentication works. You can read more about it
 here
    Steps for setting up a passwordless authentication with a remote host:
      
 
    1. 
 
      Generating public-private key pair  
      
-
      If we already have a key pair stored in \~/.ssh directory, we will not need to generate keys again.
      
-
      Install openssh package which contains all the commands related to ssh.
      
+
      If we already have a key pair stored in ~/.ssh directory, we will not need to generate keys again.
      
+
      Install openssh package which contains all the commands related to ssh.
      
 
      
-
      Generate a key pair using the ssh-keygen command. One can choose the
+
      Generate a key pair using the ssh-keygen command. One can choose the
 default values for all prompts.
      
 
      
-
      After running the ssh-keygen command successfully, we should see two
-keys present in the \~/.ssh directory. Id_rsa is the private key and
-id_rsa.pub is the public key. Do note that the private key can only be
+
      After running the ssh-keygen command successfully, we should see two
+keys present in the ~/.ssh directory. id_rsa is the private key and
+id_rsa.pub is the public key. Do note that the private key can only be
 read and modified by you.
      
 
      
 
      2. 
@@ -3061,26 +3061,30 @@ read and modified by you.
      
 
      Transferring the public key to the remote host
      
 
      There are multiple ways to transfer the public key to the remote server.
 We will look at one of the most common ways of doing it using the
-ssh-copy-id command.
      
+ssh-copy-id command.
      
 
      
-
      Install the openssh-clients package to use ssh-copy-id command.
      
+
      Install the openssh-clients package to use ssh-copy-id command.
      
 
      
-
      Use the ssh-copy-id command to copy your public key to the remote host.
      
+
      Use the ssh-copy-id command to copy your public key to the remote host.
      
 
      
-
      Now, ssh into the remote host using the password authentication.
      
+
      Now, ssh into the remote host using the password authentication.
      
 
      
-
      Our public key should be there in \~/.ssh/authorized_keys now.
      
+
      Our public key should be there in ~/.ssh/authorized_keys now.
      
 
      
-
      \~/.ssh/authorized_key contains a list of public keys. The users
-associated with these public keys have the ssh access into the remote
+
      ~/.ssh/authorized_key contains a list of public keys. The users
+associated with these public keys have the ssh access into the remote
 host.
      
 
 
    How to run commands on a remote host ?
    
-
    General syntax: ssh \<user>@\<hostname/hostip> \<command>
    
+
    General syntax: 
    
+
    ssh \<user\>@\<hostname/hostip\> \<command\>
+
    How to transfer files from one host to another host ?
    
-
    General syntax: scp \<source> \<destination>
    
+
    General syntax:
    
+
    scp \<source\> \<destination\>
+
    Package Management
    Package management is the process of installing and managing software on
@@ -3096,11 +3100,11 @@ systems.
    
-
+
-
+
@@ -3115,110 +3119,110 @@ systems.
    
-
+
-
-
+
+
-
+
-
+
    
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
    Debian style (.deb)Debian style (.deb)
 Debian, Ubuntu
 
    
 
    Red Hat style (.rpm)Red Hat style (.rpm)
 Fedora, CentOS, Red Hat Enterprise Linux
 
    
     
 
 
 
    yum install \<package_name>yum install <package_name>
 Installs a package on your system
 
    
 
    yum update \<package_name>Updates a package to it's latest available versionyum update <package_name>Updates a package to its latest available version
 
    
 
    yum remove \<package_name>yum remove <package_name>
 Removes a package from your system
 
    
 
    yum search \<keyword>yum search <keyword>
 Searches for a particular keyword
 
    
     
 
    
 
    DNF is
 the successor to YUM which is now used in Fedora for installing and
-managing packages. DNF may replace YUM in the future on all RPM based
+managing packages. DNF may replace YUM in the future on all RPM-based
 Linux distributions.
    
 
    
-
    We did find an exact match for the keyword httpd when we searched using
-yum search command. Let's now install the httpd package.
    
+
    We did find an exact match for the keyword httpd when we searched using
+yum search command. Let's now install the httpd package.
    
 
    
-
    After httpd is installed, we will use the yum remove command to remove
-httpd package.
    
+
    After httpd is installed, we will use the yum remove command to remove
+httpd package.
    
 
    
 
    Process Management
    
 
    In this section, we will study about some useful commands that can be
 used to monitor the processes on Linux systems.
    
 
    ps (process status)
    
-
    The ps command is used to know the information of a process or list of
+
    The ps command is used to know the information of a process or list of
 processes.
    
 
    
-
    If you get an error "ps command not found" while running ps command, do
-install procps package.
    
-
    ps without any arguments is not very useful. Let's try to list all the
+
    If you get an error "ps command not found" while running ps command, do
+install procps package.
    
+
    ps without any arguments is not very useful. Let's try to list all the
 processes on the system by using the below command.
    
 
    Reference:
 https://unix.stackexchange.com/questions/106847/what-does-aux-mean-in-ps-aux
    
 
    
-
    We can use an additional argument with ps command to list the
-information about the process with a specific process ID.
    
+
    We can use an additional argument with ps command to list the
+information about the process with a specific process ID (PID).
    
 
    
-
    We can use grep in combination with ps command to list only specific
+
    We can use grep in combination with ps command to list only specific
 processes.
    
 
    
 
    top
    
-
    The top command is used to show information about Linux processes
+
    The top command is used to show information about Linux processes
 running on the system in real time. It also shows a summary of the
 system information.
    
 
    
-
    For each process, top lists down the process ID, owner, priority, state,
-cpu utilization, memory utilization and much more information. It also
-lists down the memory utilization and cpu utilization of the system as a
-whole along with system uptime and cpu load average.
    
+
    For each process, top lists down the process ID, owner, priority, state,
+CPU utilization, memory utilization and much more information. It also
+lists down the memory utilization and CPU utilization of the system as a
+whole along with system uptime and CPU load average.
    
 
    Memory Management
    
 
    In this section, we will study about some useful commands that can be
 used to view information about the system memory.
    
 
    free
    
-
    The free command is used to display the memory usage of the system. The
+
    The free command is used to display the memory usage of the system. The
 command displays the total free and used space available in the RAM
 along with space occupied by the caches/buffers.
    
 
    
-
    free command by default shows the memory usage in kilobytes. We can use
+
    free command by default shows the memory usage in kilobytes. We can use
 an additional argument to get the data in human-readable format.
    
 
    
 
    vmstat
    
-
    The vmstat command can be used to display the memory usage along with
-additional information about io and cpu usage.
    
+
    The vmstat command can be used to display the memory usage along with
+additional information about IO and CPU usage.
    
 
    
 
    Checking Disk Space
    
 
    In this section, we will study about some useful commands that can be
 used to view disk space on Linux.
    
 
    df (disk free)
    
-
    The df command is used to display the free and available space for each
+
    The df command is used to display the free and available space for each
 mounted file system.
    
 
    
 
    du (disk usage)
    
-
    The du command is used to display disk usage of files and directories on
+
    The du command is used to display disk usage of files and directories on
 the system.
    
 
    
 
    The below command can be used to display the top 5 largest directories
-in the root directory.
    
+in the root directory.
    
 
    
 
    Daemons
    
-
    A computer program that runs as a background process is called a daemon.
-Traditionally, the name of daemon processes ended with d - sshd, httpd
+
    A computer program that runs as a background process is called a daemon.
+Traditionally, the name of daemon processes ends with d - sshd, httpd,
 etc. We cannot interact with a daemon process as they run in the
 background.
    
 
    Services and daemons are used interchangeably most of the time.
    
 
    Systemd
    
-
    Systemd is a system and service manager for Linux operating systems.
-Systemd units are the building blocks of systemd. These units are
+
    systemd is a system and service manager for Linux operating systems.
+systemd units are the building blocks of systemd. These units are
 represented by unit configuration files.
    
 
    The below examples shows the unit configuration files available at
-/usr/lib/systemd/system which are distributed by installed RPM packages.
+/usr/lib/systemd/system which are distributed by installed RPM packages.
 We are more interested in the configuration file that ends with service
 as these are service units.
    
 
    
 
    Managing System Services
    
-
    Service units end with .service file extension. Systemctl command can be
-used to start/stop/restart the services managed by systemd.
    
+
    Service units end with .service file extension. systemctl command can be
+used to start/stop/restart the services managed by systemd.
    
 
diff --git a/level101/linux_networking/conclusion/index.html b/level101/linux_networking/conclusion/index.html
index 4e76e93..947e449 100644
--- a/level101/linux_networking/conclusion/index.html
+++ b/level101/linux_networking/conclusion/index.html
@@ -2107,13 +2107,13 @@
                 
                 
                 
    Conclusion
    
-
    With this we have traversed through the TCP/IP stack completely. We hope there will be a different perspective when one opens any website in the browser post the course.
    
+
    With this, we have traversed through the TCP/IP stack completely. We hope there will be a different perspective when one opens any website in the browser post the course.
    During the course we have also dissected what are common tasks in this pipeline which falls under the ambit of SRE.
    Post Training Exercises
      
-
    1. Setup own DNS resolver in the dev environment which acts as an authoritative DNS server for example.com and forwarder for other domains. Update resolv.conf to use the new DNS resolver running in localhost
      2. 
-
    2. Set up a site dummy.example.com in localhost and run a webserver with a self signed certificate. Update the trusted CAs or pass self signed CA’s public key as a parameter so that curl https://dummy.example.com -v works properly without self signed cert warning
      3. 
-
    3. Update the routing table to use another host(container/VM) in the same network as a gateway for 8.8.8.8/32 and run ping 8.8.8.8. Do the packet capture on the new gateway to see L3 hop is working as expected(might need to disable icmp_redirect)
      4. 
+
    4. Set up your own DNS resolver in the dev environment which acts as an authoritative DNS server for example.com and forwarder for other domains. Update resolv.conf to use the new DNS resolver running in localhost.
      5. 
+
    5. Set up a site dummy.example.com in localhost and run a webserver with a self-signed certificate. Update the trusted CAs or pass self-signed CA’s public key as a parameter so that curl https://dummy.example.com -v works properly without self-signed cert warning.
      6. 
+
    6. Update the routing table to use another host (container/VM) in the same network as a gateway for 8.8.8.8/32 and run ping 8.8.8.8. Do the packet capture on the new gateway to see L3 hop is working as expected (might need to disable icmp_redirect).
      7. 
 
    
                 
               
diff --git a/level101/linux_networking/dns/index.html b/level101/linux_networking/dns/index.html
index e312415..2cff576 100644
--- a/level101/linux_networking/dns/index.html
+++ b/level101/linux_networking/dns/index.html
@@ -2151,12 +2151,12 @@
                 
                 
                 
    DNS
    
-
    Domain Names are the simple human-readable names for websites. The Internet understands only IP addresses, but since memorizing incoherent numbers is not practical, domain names are used instead. These domain names are translated into IP addresses by the DNS infrastructure. When somebody tries to open www.linkedin.com in the browser, the browser tries to convert www.linkedin.com to an IP Address. This process is called DNS resolution. A simple pseudocode depicting this process looks this
    
+
    Domain Names are the simple human-readable names for websites. The Internet understands only IP addresses, but since memorizing incoherent numbers is not practical, domain names are used instead. These domain names are translated into IP addresses by the DNS infrastructure. When somebody tries to open www.linkedin.com in the browser, the browser tries to convert www.linkedin.com to an IP Address. This process is called DNS resolution. A simple pseudocode depicting this process looks this:
    ip, err = getIPAddress(domainName)
 if err:
-  print(“unknown Host Exception while trying to resolve:%s”.format(domainName))
+  print("unknown Host Exception while trying to resolve:%s".format(domainName))
 
    
-
    Now let’s try to understand what happens inside the getIPAddress function. The browser would have a DNS cache of its own where it checks if there is a mapping for the domainName to an IP Address already available, in which case the browser uses that IP address. If no such mapping exists, the browser calls gethostbyname syscall to ask the operating system to find the IP address for the given domainName
    
+
    Now, let’s try to understand what happens inside the getIPAddress function. The browser would have a DNS cache of its own where it checks if there is a mapping for the domainName to an IP Address already available, in which case the browser uses that IP address. If no such mapping exists, the browser calls gethostbyname syscall to ask the operating system to find the IP address for the given domainName.
    def getIPAddress(domainName):
     resp, fail = lookupCache(domainName)
     If not fail:
@@ -2168,19 +2168,19 @@ if err:
        else:
           return resp
 
    
-
    Now lets understand what operating system kernel does when the gethostbyname function is called. The Linux operating system looks at the file /etc/nsswitch.conf file which usually has a line
    
+
    Now, let's understand what operating system kernel does when the gethostbyname function is called. The Linux operating system looks at the file /etc/nsswitch.conf file which usually has a line.
    hosts:      files dns
 
    
-
    This line means the OS has to look up first in file (/etc/hosts) and then use DNS protocol to do the resolution if there is no match in /etc/hosts.
    
-
    The file /etc/hosts is of format
    
+
    This line means the OS has to look up first in file (/etc/hosts) and then use DNS protocol to do the resolution if there is no match in /etc/hosts.
    
+
    The file /etc/hosts is of format:
    IPAddress FQDN [FQDN].*
    127.0.0.1 localhost.localdomain localhost
 ::1 localhost.localdomain localhost
 
    
-
    If a match exists for a domain in this file then that IP address is returned by the OS. Lets add a line to this file
    
+
    If a match exists for a domain in this file, then that IP address is returned by the OS. Let's add a line to this file:
    127.0.0.1 test.linkedin.com
 
    
-
    And then do ping test.linkedin.com
    
+
    And then do ping test.linkedin.com.
    ping test.linkedin.com -n
 
    PING test.linkedin.com (127.0.0.1) 56(84) bytes of data.
@@ -2189,11 +2189,11 @@ if err:
 64 bytes from 127.0.0.1: icmp_seq=3 ttl=64 time=0.037 ms
 
 
    
-
    As mentioned earlier, if no match exists in /etc/hosts, the OS tries to do a DNS resolution using the DNS protocol. The linux system makes a DNS request to the first IP in /etc/resolv.conf. If there is no response, requests are sent to subsequent servers in resolv.conf. These servers in resolv.conf are called DNS resolvers. The DNS resolvers are populated by DHCP or statically configured by an administrator.
+
    As mentioned earlier, if no match exists in /etc/hosts, the OS tries to do a DNS resolution using the DNS protocol. The Linux system makes a DNS request to the first IP in /etc/resolv.conf. If there is no response, requests are sent to subsequent servers in resolv.conf. These servers in resolv.conf are called DNS resolvers. The DNS resolvers are populated by DHCP or statically configured by an administrator.
 Dig is a userspace DNS system which creates and sends request to DNS resolvers and prints the response it receives to the console.
    
-
    #run this command in one shell to capture all DNS requests
+
    # run this command in one shell to capture all DNS requests
 sudo tcpdump -s 0 -A -i any port 53
-#make a dig request from another shell
+# make a dig request from another shell
 dig linkedin.com
 
    
 
    13:19:54.432507 IP 172.19.209.122.56497 > 172.23.195.101.53: 527+ [1au] A? linkedin.com. (41)
@@ -2203,21 +2203,21 @@ dig linkedin.com
 
 ..)........
 
    
-
    The packet capture shows a request is made to 172.23.195.101:53 (this is the resolver in /etc/resolv.conf) for linkedin.com and a response is received from 172.23.195.101 with the IP address of linkedin.com 108.174.10.10
    
-
    Now let's try to understand how DNS resolver tries to find the IP address of linkedin.com. DNS resolver first looks at its cache. Since many devices in the network can query for the domain name linkedin.com, the name resolution result may already exist in the cache. If there is a cache miss, it starts the DNS resolution process. The DNS server breaks “linkedin.com” to “.”, “com.” and “linkedin.com.” and starts DNS resolution from “.”. The “.” is called root domain and those IPs are known to the DNS resolver software. DNS resolver queries the root domain nameservers to find the right top-level domain (TLD) nameservers which could respond regarding details for "com.". The address of the TLD nameserver of “com.” is returned. Now the DNS resolution service contacts the TLD nameserver for “com.” to fetch the authoritative nameserver for “linkedin.com”. Once an authoritative nameserver of “linkedin.com” is known, the resolver contacts Linkedin’s nameserver to provide the IP address of “linkedin.com”. This whole process can be visualized by running the following -
    
+
    The packet capture shows a request is made to 172.23.195.101:53 (this is the resolver in /etc/resolv.conf) for linkedin.com and a response is received from 172.23.195.101 with the IP address of linkedin.com 108.174.10.10.
    
+
    Now, let's try to understand how DNS resolver tries to find the IP address of linkedin.com. DNS resolver first looks at its cache. Since many devices in the network can query for the domain name linkedin.com, the name resolution result may already exist in the cache. If there is a cache miss, it starts the DNS resolution process. The DNS server breaks “linkedin.com” to “.”, “com.” and “linkedin.com.” and starts DNS resolution from “.”. The “.” is called root domain and those IPs are known to the DNS resolver software. DNS resolver queries the root domain nameservers to find the right top-level domain (TLD) nameservers which could respond regarding details for "com.". The address of the TLD nameserver of “com.” is returned. Now the DNS resolution service contacts the TLD nameserver for “com.” to fetch the authoritative nameserver for “linkedin.com”. Once an authoritative nameserver of “linkedin.com” is known, the resolver contacts LinkedIn’s nameserver to provide the IP address of “linkedin.com”. This whole process can be visualized by running the following:
    
 
    dig +trace linkedin.com
 
    
 
    linkedin.com.       3600    IN  A   108.174.10.10
 
    
-
    This DNS response has 5 fields where the first field is the request and the last field is the response. The second field is the Time to Live which says how long the DNS response is valid in seconds. In this case this mapping of linkedin.com is valid for 1 hour. This is how the resolvers and application(browser) maintain their cache. Any request for linkedin.com beyond 1 hour will be treated as a cache miss as the mapping has expired its TTL and the whole process has to be redone.
+
    This DNS response has 5 fields where the first field is the request and the last field is the response. The second field is the Time-to-Live (TTL) which says how long the DNS response is valid in seconds. In this case, this mapping of linkedin.com is valid for 1 hour. This is how the resolvers and application (browser) maintain their cache. Any request for linkedin.com beyond 1 hour will be treated as a cache miss as the mapping has expired its TTL and the whole process has to be redone.
 The 4th field says the type of DNS response/request. Some of the various DNS query types are
 A, AAAA, NS, TXT, PTR, MX and CNAME.
    
 
      
 
    • A record returns IPV4 address of the domain name
      • 
 
    • AAAA record returns the IPV6 address of the domain Name
      • 
 
    • NS record returns the authoritative nameserver for the domain name
      • 
-
    • CNAME records are aliases to the domain names. Some domains point to other domain names and resolving the latter domain name gives an IP which is used as an IP for the former domain name as well. Example www.linkedin.com’s IP address is the same as 2-01-2c3e-005a.cdx.cedexis.net.
      • 
-
    • For the brevity we are not discussing other DNS record types, the RFC of each of these records are available here.
      • 
+
    • CNAME records are aliases to the domain names. Some domains point to other domain names and resolving the latter domain name gives an IP which is used as an IP for the former domain name as well. Example www.linkedin.com’s IP address is the same as 2-01-2c3e-005a.cdx.cedexis.net.
      • 
+
    • For the brevity, we are not discussing other DNS record types, the RFC of each of these records are available here.
      • 
 
    
 
    dig A linkedin.com +short
 108.174.10.10
@@ -2240,16 +2240,16 @@ dns1.p09.nsone.net.
 dig www.linkedin.com CNAME +short
 2-01-2c3e-005a.cdx.cedexis.net.
 
    
-
    Armed with these fundamentals of DNS lets see usecases where DNS is used by SREs.
    
+
    Armed with these fundamentals of DNS lets see use cases where DNS is used by SREs.
    
 
    Applications in SRE role
    
-
    This section covers some of the common solutions SRE can derive from DNS
    
+
    This section covers some of the common solutions SRE can derive from DNS.
    
 
      
-
    1. Every company has to have its internal DNS infrastructure for intranet sites and internal services like databases and other internal applications like wiki. So there has to be a DNS infrastructure maintained for those domain names by the infrastructure team. This DNS infrastructure has to be optimized and scaled so that it doesn’t become a single point of failure. Failure of the internal DNS infrastructure can cause API calls of microservices to fail and other cascading effects.
      2. 
-
    2. DNS can also be used for discovering services. For example the hostname serviceb.internal.example.com could list instances which run service b internally in example.com company. Cloud providers provide options to enable DNS discovery(example)
      3. 
-
    3. DNS is used by cloud providers and CDN providers to scale their services. In Azure/AWS, Load Balancers are given a CNAME instead of IPAddress. They update the IPAddress of the Loadbalancers as they scale by changing the IP Address of alias domain names. This is one of the reasons why A records of such alias domains are short lived like 1 minute.
      4. 
+
    4. Every company has to have its internal DNS infrastructure for intranet sites and internal services like databases and other internal applications like Wiki. So there has to be a DNS infrastructure maintained for those domain names by the infrastructure team. This DNS infrastructure has to be optimized and scaled so that it doesn’t become a single point of failure. Failure of the internal DNS infrastructure can cause API calls of microservices to fail and other cascading effects.
      5. 
+
    5. DNS can also be used for discovering services. For example the hostname serviceb.internal.example.com could list instances which run service b internally in example.com company. Cloud providers provide options to enable DNS discovery (example).
      6. 
+
    6. DNS is used by cloud providers and CDN providers to scale their services. In Azure/AWS, Load Balancers are given a CNAME instead of IPAddress. They update the IPAddress of the Loadbalancers as they scale by changing the IP Address of alias domain names. This is one of the reasons why A records of such alias domains are short-lived like 1 minute.
      7. 
 
    7. DNS can also be used to make clients get IP addresses closer to their location so that their HTTP calls can be responded faster if the company has a presence geographically distributed.
      8. 
-
    8. SRE also has to understand since there is no verification in DNS infrastructure, these responses can be spoofed. This is safeguarded by other protocols like HTTPS(dealt later). DNSSEC protects from forged or manipulated DNS responses.
      9. 
-
    9. Stale DNS cache can be a problem. Some apps might still be using expired DNS records for their api calls. This is something SRE has to be wary of when doing maintenance.
      10. 
+
    10. SRE also has to understand since there is no verification in DNS infrastructure, these responses can be spoofed. This is safeguarded by other protocols like HTTPS (dealt later). DNSSEC protects from forged or manipulated DNS responses.
      11. 
+
    11. Stale DNS cache can be a problem. Some apps might still be using expired DNS records for their API calls. This is something SRE has to be wary of when doing maintenance.
      12. 
 
    12. DNS Loadbalancing and service discovery also has to understand TTL and the servers can be removed from the pool only after waiting till TTL post the changes are made to DNS records. If this is not done, a certain portion of the traffic will fail as the server is removed before the TTL.
      13. 
 
    
                 
diff --git a/level101/linux_networking/http/index.html b/level101/linux_networking/http/index.html
index 2d16c3d..b26fc55 100644
--- a/level101/linux_networking/http/index.html
+++ b/level101/linux_networking/http/index.html
@@ -2107,9 +2107,9 @@
                 
                 
                 
    HTTP
    
-
    Till this point we have only got the IP address of linkedin.com. The HTML page of linkedin.com is served by HTTP protocol which the browser renders. Browser sends a HTTP request to the IP of the server determined above.
-Request has a verb GET, PUT, POST followed by a path and query parameters and lines of key value pair which gives information about the client and capabilities of the client like contents it can accept and a body (usually in POST or PUT)
    
-
    # Eg run the following in your container and have a look at the headers 
+
    Till this point we have only got the IP address of linkedin.com. The HTML page of linkedin.com is served by HTTP protocol which the browser renders. Browser sends a HTTP request to the IP of the server determined above.
+Request has a verb GET, PUT, POST followed by a path and query parameters and lines of key-value pair which gives information about the client and capabilities of the client like contents it can accept and a body (usually in POST or PUT).
    
+
    # Eg. run the following in your container and have a look at the headers 
 curl linkedin.com -v
 
    
 
    * Connected to linkedin.com (108.174.10.10) port 80 (#0)
@@ -2128,7 +2128,7 @@ curl linkedin.com -v
 * Connection #0 to host linkedin.com left intact
 * Closing connection 0
 
    
-
    Here, in the first line GET is the verb, / is the path and 1.1 is the HTTP protocol version. Then there are key value pairs which give client capabilities and some details to the server. The server responds back with HTTP version, Status Code and Status message. Status codes 2xx means success, 3xx denotes redirection, 4xx denotes client side errors and 5xx server side errors.
    
+
    Here, in the first line GET is the verb, / is the path and 1.1 is the HTTP protocol version. Then there are key-value pairs which give client capabilities and some details to the server. The server responds back with HTTP version, Status Code and Status message. Status codes 2xx means success, 3xx denotes redirection, 4xx denotes client-side errors and 5xx server-side errors.
    
 
    We will now jump in to see the difference between HTTP/1.0 and HTTP/1.1. 
    
 
    #On the terminal type
 telnet  www.linkedin.com 80
@@ -2138,10 +2138,10 @@ HOST:linkedin.com
 USER-AGENT: curl
 
 
    
-
    This would get server response and waits for next input as the underlying connection to www.linkedin.com can be reused for further queries. While going through TCP, we can understand the benefits of this. But in HTTP/1.0 this connection will be immediately closed after the response meaning new connection has to be opened for each query. HTTP/1.1 can have only one inflight request in an open connection but connection can be reused for multiple requests one after another. One of the benefits of HTTP/2.0 over HTTP/1.1 is we can have multiple inflight requests on the same connection. We are restricting our scope to generic HTTP and not jumping to the intricacies of each protocol version but they should be straight forward to understand post the course.
    
-
    HTTP is  called stateless protocol. This section we will try to understand what stateless means. Say we logged in to linkedin.com, each request to linkedin.com from the client will have no context of the user and it makes no sense to prompt user to login for each page/resource. This problem of HTTP is solved by COOKIE. A user is created a session when a user logs in. This session identifier is sent to the browser via SET-COOKIE header. The browser stores the COOKIE till the expiry set by the server and sends the cookie for each request from hereon for linkedin.com. More details on cookies are available here. Cookies are a critical piece of information like password and since HTTP is a plain text protocol, any man in the middle can capture either password or cookies and can breach the privacy of the user. Similarly as discussed during DNS a spoofed IP of linkedin.com can cause a phishing attack on users where an user can give linkedin’s password to login on the malicious site. To solve both problems HTTPs came in place and HTTPs has to be mandated.
    
-
    HTTPS has to provide server identification and encryption of data between client and server. The server administrator has to generate a private public key pair and certificate request. This certificate request has to be signed by a certificate authority which converts the certificate request to a certificate. The server administrator has to update the certificate and private key to the webserver. The certificate has details about the server (like domain name for which it serves, expiry date), public key of the server. The private key is a secret to the server and losing the private key loses the trust the server provides. When clients connect, the client sends a HELLO. The server sends its certificate to the client. The client checks the validity of the cert by seeing if it is within its expiry time, if it is signed by a trusted authority and the hostname in the cert is the same as the server. This validation makes sure the server is the right server and there is no phishing. Once that is validated, the client negotiates a symmetrical key and cipher with the server by encrypting the negotiation with the public key of the server. Nobody else other than the server who has the private key can understand this data. Once negotiation is complete, that symmetric key and algorithm is used for further encryption which can be decrypted only by client and server from thereon as they only know the symmetric key and algorithm. The switch to symmetric algorithm from asymmetric encryption algorithm is to not strain the resources of client devices as symmetric encryption is generally less resource intensive than asymmetric. 
    
-
    #Try the following on your terminal to see the cert details like Subject Name(domain name), Issuer details, Expiry date
+
    This would get server response and waits for next input as the underlying connection to www.linkedin.com can be reused for further queries. While going through TCP, we can understand the benefits of this. But in HTTP/1.0, this connection will be immediately closed after the response meaning new connection has to be opened for each query. HTTP/1.1 can have only one inflight request in an open connection but connection can be reused for multiple requests one after another. One of the benefits of HTTP/2.0 over HTTP/1.1 is we can have multiple inflight requests on the same connection. We are restricting our scope to generic HTTP and not jumping to the intricacies of each protocol version but they should be straight forward to understand post the course.
    
+
    HTTP is called stateless protocol. This section we will try to understand what stateless means. Say we logged in to linkedin.com, each request to linkedin.com from the client will have no context of the user and it makes no sense to prompt user to login for each page/resource. This problem of HTTP is solved by COOKIE. A user is created a session when a user logs in. This session identifier is sent to the browser via SET-COOKIE header. The browser stores the COOKIE till the expiry set by the server and sends the cookie for each request from hereon for linkedin.com. More details on cookies are available here. Cookies are a critical piece of information like password and since HTTP is a plain text protocol, any man-in-the-middle can capture either password or cookies and can breach the privacy of the user. Similarly as discussed during DNS, a spoofed IP of linkedin.com can cause a phishing attack on users where an user can give LinkedIn’s password to login on the malicious site. To solve both problems, HTTPS came in place and HTTPS has to be mandated.
    
+
    HTTPS has to provide server identification and encryption of data between client and server. The server administrator has to generate a private-public key pair and certificate request. This certificate request has to be signed by a certificate authority which converts the certificate request to a certificate. The server administrator has to update the certificate and private key to the webserver. The certificate has details about the server (like domain name for which it serves, expiry date), public key of the server. The private key is a secret to the server and losing the private key loses the trust the server provides. When clients connect, the client sends a HELLO. The server sends its certificate to the client. The client checks the validity of the cert by seeing if it is within its expiry time, if it is signed by a trusted authority and the hostname in the cert is the same as the server. This validation makes sure the server is the right server and there is no phishing. Once that is validated, the client negotiates a symmetrical key and cipher with the server by encrypting the negotiation with the public key of the server. Nobody else other than the server who has the private key can understand this data. Once negotiation is complete, that symmetric key and algorithm is used for further encryption which can be decrypted only by client and server from thereon as they only know the symmetric key and algorithm. The switch to symmetric algorithm from asymmetric encryption algorithm is to not strain the resources of client devices as symmetric encryption is generally less resource intensive than asymmetric. 
    
+
    # Try the following on your terminal to see the cert details like Subject Name (domain name), Issuer details, Expiry date
 curl https://www.linkedin.com -v 
 
    
 
    * Connected to www.linkedin.com (13.107.42.14) port 443 (#0)
@@ -2216,7 +2216,7 @@ date: Mon, 09 Nov 2020 10:50:10 GMT
 
 * Closing connection 0
 
    
-
    Here my system has a list of certificate authorities it trusts in this file  /etc/ssl/cert.pem. Curl validates the certificate is for www.linkedin.com by seeing the CN section of the subject part of the certificate. It also makes sure the certificate is not expired by seeing the expire date. It also validates the signature on the certificate by using the public key of issuer Digicert in  /etc/ssl/cert.pem. Once this is done, using the public key of www.linkedin.com it negotiates cipher TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 with a symmetric key. Subsequent data transfer including first HTTP request uses the same cipher and symmetric key.
    
+
    Here, my system has a list of certificate authorities it trusts in this file /etc/ssl/cert.pem. cURL validates the certificate is for www.linkedin.com by seeing the CN section of the subject part of the certificate. It also makes sure the certificate is not expired by seeing the expire date. It also validates the signature on the certificate by using the public key of issuer Digicert in /etc/ssl/cert.pem. Once this is done, using the public key of www.linkedin.com it negotiates cipher TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 with a symmetric key. Subsequent data transfer including first HTTP request uses the same cipher and symmetric key.
    
                 
               
               
diff --git a/level101/linux_networking/intro/index.html b/level101/linux_networking/intro/index.html
index 9d2bc1b..e371a3e 100644
--- a/level101/linux_networking/intro/index.html
+++ b/level101/linux_networking/intro/index.html
@@ -2209,15 +2209,15 @@
                 
    Linux Networking Fundamentals
    
 
    Prerequisites
    
 
      
-
    • High-level knowledge of commonly used jargon in TCP/IP stack like  DNS, TCP, UDP and HTTP
      • 
+
    • High-level knowledge of commonly used jargon in TCP/IP stack like DNS, TCP, UDP and HTTP
      • 
 
    • Linux Commandline Basics
      • 
 
    
 
    What to expect from this course
    
 
    Throughout the course, we cover how an SRE can optimize the system to improve their web stack performance and troubleshoot if there is an issue in any of the layers of the networking stack. This course tries to dig through each layer of traditional TCP/IP stack and expects an SRE to have a picture beyond the bird’s eye view of the functioning of the Internet.
    
 
    What is not covered under this course
    
-
    This course spends time on the fundamentals. We are not covering concepts like HTTP/2.0, QUIC, TCP congestion control protocols, Anycast, BGP, CDN, Tunnels and Multicast. We expect that this course will provide the relevant basics to understand such concepts
    
+
    This course spends time on the fundamentals. We are not covering concepts like HTTP/2.0, QUIC, TCP congestion control protocols, Anycast, BGP, CDN, Tunnels and Multicast. We expect that this course will provide the relevant basics to understand such concepts.
    
 
    Birds eye view of the course
    
-
    The course covers the question “What happens when you open linkedin.com in your browser?” The course follows the flow of TCP/IP stack.More specifically, the course covers topics of Application layer protocols DNS and HTTP, transport layer protocols UDP and TCP, networking layer protocol IP and Data Link Layer protocol 
    
+
    The course covers the question “What happens when you open linkedin.com in your browser?” The course follows the flow of TCP/IP stack. More specifically, the course covers topics of Application layer protocols (DNS and HTTP), transport layer protocols (UDP and TCP), networking layer protocol (IP) and data link layer protocol.
    
 
    Course Contents
    
 
      
 
    1. DNS
      2. 
diff --git a/level101/linux_networking/ipr/index.html b/level101/linux_networking/ipr/index.html
index f6a4ab7..67d70f2 100644
--- a/level101/linux_networking/ipr/index.html
+++ b/level101/linux_networking/ipr/index.html
@@ -2151,8 +2151,8 @@
                 
                 
                 
      IP Routing and Data Link Layer
      
-
      We will dig how packets that leave the client reach the server and vice versa. When the packet reaches the IP layer, the transport layer populates source port, destination port. IP/Network layer populates destination IP(discovered from DNS) and then looks up the route to the destination IP on the routing table. 
      
-
      #Linux route -n command gives the default routing table
+
      We will dig how packets that leave the client reach the server and vice versa. When the packet reaches the IP layer, the transport layer populates source port, destination port. IP/Network layer populates destination IP (discovered from DNS) and then looks up the route to the destination IP on the routing table. 
      
+
      # Linux `route -n` command gives the default routing table
 route -n
 
      
 
      Kernel IP routing table
@@ -2160,18 +2160,18 @@ Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
 0.0.0.0         172.17.0.1      0.0.0.0         UG    0      0        0 eth0
 172.17.0.0      0.0.0.0         255.255.0.0     U     0      0        0 eth0
 
      
-
      Here the destination IP is bitwise AND’d with the Genmask and if the answer is the destination part of the table then that gateway and interface is picked for routing. Here linkedin.com’s IP 108.174.10.10 is AND’d with 255.255.255.0 and the answer we get is 108.174.10.0 which doesn’t match with any destination in the routing table. Then Linux does an AND of destination IP with 0.0.0.0 and we get 0.0.0.0. This answer matches the default row
      
-
      Routing table is processed in the order of more octets of 1 set in genmask and genmask 0.0.0.0 is the default route if nothing matches. 
-At the end of this operation Linux figured out that the packet has to be sent to next hop 172.17.0.1 via eth0. The source IP of the packet will be set as the IP of interface eth0. 
-Now to send the packet to 172.17.0.1 linux has to figure out the MAC address of 172.17.0.1. MAC address is figured by looking at the internal arp cache which stores translation between IP address and MAC address. If there is a cache miss, Linux broadcasts ARP request within the internal network asking who has 172.17.0.1. The owner of the IP sends an ARP response which is cached by the kernel and the kernel sends the packet to the gateway by setting Source mac address as mac address of eth0 and destination mac address of 172.17.0.1 which we got just now. Similar routing lookup process is followed in each hop till the packet reaches the actual server. Transport layer and layers above it come to play only at end servers. During intermediate hops only till the IP/Network layer is involved.
      
+
      Here, the destination IP is bitwise AND’d with the Genmask and if the answer is the destination part of the table, then that gateway and interface is picked for routing. Here, linkedin.com’s IP 108.174.10.10 is AND’d with 255.255.255.0 and the answer we get is 108.174.10.0 which doesn’t match with any destination in the routing table. Then, Linux does an AND of destination IP with 0.0.0.0 and we get 0.0.0.0. This answer matches the default row.
      
+
      Routing table is processed in the order of more octets of 1 set in Genmask and Genmask 0.0.0.0 is the default route if nothing matches. 
+At the end of this operation, Linux figured out that the packet has to be sent to next hop 172.17.0.1 via eth0. The source IP of the packet will be set as the IP of interface eth0. 
+Now, to send the packet to 172.17.0.1, Linux has to figure out the MAC address of 172.17.0.1. MAC address is figured by looking at the internal ARP cache which stores translation between IP address and MAC address. If there is a cache miss, Linux broadcasts ARP request within the internal network asking who has 172.17.0.1. The owner of the IP sends an ARP response which is cached by the kernel and the kernel sends the packet to the gateway by setting Source MAC address as MAC address of eth0 and destination MAC address of 172.17.0.1 which we got just now. Similar routing lookup process is followed in each hop till the packet reaches the actual server. Transport layer and layers above it come to play only at end servers. During intermediate hops, only till the IP/Network layer is involved.
      
 
      Screengrab for above explanation
      
-
      One weird gateway we saw in the routing table is 0.0.0.0. This gateway means no Layer3(Network layer) hop is needed to send the packet. Both source and destination are in the same network. Kernel has to figure out the mac of the destination and populate source and destination mac appropriately and send the packet out so that it reaches the destination without any Layer3 hop in the middle
      
-
      As we followed in other modules, lets complete this session with SRE usecases
      
+
      One weird gateway we saw in the routing table is 0.0.0.0. This gateway means no Layer3 (Network layer) hop is needed to send the packet. Both source and destination are in the same network. Kernel has to figure out the MAC of the destination and populate source and destination MAC appropriately and send the packet out so that it reaches the destination without any Layer3 hop in the middle.
      
+
      As we followed in other modules, let's complete this session with SRE use cases.
      
 
      Applications in SRE role
      
 
        
-
      1. Generally the routing table is populated by DHCP and playing around is not a good practice. There can be reasons where one has to play around the routing table but take that path only when it's absolutely necessary
        2. 
-
      2. Understanding error messages better like, “No route to host” error can mean mac address of the destination host is not found and it can mean the destination host is down 
        3. 
-
      3. On rare cases looking at the ARP table can help us understand if there is a IP conflict where same IP is assigned to two hosts by mistake and this is causing unexpected behavior
        4. 
+
      4. Generally the routing table is populated by DHCP and playing around is not a good practice. There can be reasons where one has to play around the routing table but take that path only when it's absolutely necessary.
        5. 
+
      5. Understanding error messages better like, “No route to host” error can mean MAC address of the destination host is not found and it can mean the destination host is down. 
        6. 
+
      6. On rare cases, looking at the ARP table can help us understand if there is a IP conflict where same IP is assigned to two hosts by mistake and this is causing unexpected behavior.
        7. 
 
      
                 
               
diff --git a/level101/linux_networking/tcp/index.html b/level101/linux_networking/tcp/index.html
index 2e3051d..adf0e3d 100644
--- a/level101/linux_networking/tcp/index.html
+++ b/level101/linux_networking/tcp/index.html
@@ -2152,27 +2152,27 @@
                 
                 
      TCP
      
 
      TCP is a transport layer protocol like UDP but it guarantees reliability, flow control and congestion control.
-TCP guarantees reliable delivery by using sequence numbers. A TCP connection is established by a three way handshake. In our case, the client sends a SYN packet along with the starting sequence number it plans to use, the server acknowledges the SYN packet and sends a SYN with its sequence number. Once the client acknowledges the syn packet, the connection is established. Each data transferred from here on is considered delivered reliably once acknowledgement for that sequence is received by the concerned party
      
+TCP guarantees reliable delivery by using sequence numbers. A TCP connection is established by a three-way handshake. In our case, the client sends a SYN packet along with the starting sequence number it plans to use, the server acknowledges the SYN packet and sends a SYN with its sequence number. Once the client acknowledges the SYN packet, the connection is established. Each data transferred from here on is considered delivered reliably once acknowledgement for that sequence is received by the concerned party.
      
 
      3-way handshake
      
-
      #To understand handshake run packet capture on one bash session
+
      # To understand handshake run packet capture on one bash session
 tcpdump -S -i any port 80
-#Run curl on one bash session
+# Run curl on one bash session
 curl www.linkedin.com
 
      
 
      tcpdump-3way
      
-
      Here client sends a syn flag shown by [S] flag with a sequence number 1522264672. The server acknowledges receipt of SYN with an ack [.] flag and a Syn flag for its sequence number[S]. The server uses the sequence number 1063230400 and acknowledges the client it’s expecting sequence number 1522264673 (client sequence+1). Client sends a zero length acknowledgement packet to the server(server sequence+1) and connection stands established. This is called three way handshake. The client sends a 76 bytes length packet after this and increments its sequence number by 76. Server sends a 170 byte response and closes the connection. This was the difference we were talking about between HTTP/1.1 and HTTP/1.0. In HTTP/1.1 this same connection can be reused which reduces overhead of 3 way handshake for each HTTP request. If a packet is missed between client and server, server won’t send an ack to the client and client would retry sending the packet till the ACK is received. This guarantees reliability.
-The flow control is established by the win size field in each segment. The win size says available TCP buffer length in the kernel which can be used to buffer received segments. A size 0 means the receiver has a lot of lag to catch from its socket buffer and the sender has to pause sending packets so that receiver can cope up. This flow control protects from slow receiver and fast sender problem
      
-
      TCP also does congestion control which determines how many segments can be in transit without an ack. Linux provides us the ability to configure algorithms for congestion control which we are not covering here.
      
-
      While closing a connection, client/server calls a close syscall. Let's assume client do that. Client’s kernel will send a FIN packet to the server. Server’s kernel can’t close the connection till the close syscall is called by the server application. Once server app calls close, server also sends a FIN packet and client enters into time wait state for 2*MSS(120s) so that this socket can’t be reused for that time period to prevent any TCP state corruptions due to stray stale packets. 
      
+
      Here, client sends a SYN flag shown by [S] flag with a sequence number 1522264672. The server acknowledges receipt of SYN with an ACK [.] flag and a SYN flag for its sequence number [S]. The server uses the sequence number 1063230400 and acknowledges the client it's expecting sequence number 1522264673 (client sequence + 1). Client sends a zero length acknowledgement packet to the server (server sequence + 1) and connection stands established. This is called three way handshake. The client sends a 76 bytes length packet after this and increments its sequence number by 76. Server sends a 170 byte response and closes the connection. This was the difference we were talking about between HTTP/1.1 and HTTP/1.0. In HTTP/1.1, this same connection can be reused which reduces overhead of three-way handshake for each HTTP request. If a packet is missed between client and server, server won’t send an ACK to the client and client would retry sending the packet till the ACK is received. This guarantees reliability.
+The flow control is established by the WIN size field in each segment. The WIN size says available TCP buffer length in the kernel which can be used to buffer received segments. A size 0 means the receiver has a lot of lag to catch from its socket buffer and the sender has to pause sending packets so that receiver can cope up. This flow control protects from slow receiver and fast sender problem.
      
+
      TCP also does congestion control which determines how many segments can be in transit without an ACK. Linux provides us the ability to configure algorithms for congestion control which we are not covering here.
      
+
      While closing a connection, client/server calls a close syscall. Let's assume client do that. Client’s kernel will send a FIN packet to the server. Server’s kernel can’t close the connection till the close syscall is called by the server application. Once server app calls close, server also sends a FIN packet and client enters into TIME_WAIT state for 2*MSS (120s) so that this socket can’t be reused for that time period to prevent any TCP state corruptions due to stray stale packets. 
      
 
      Connection tearing
      
-
      Armed with our TCP and HTTP knowledge lets see how this is used by SREs in their role
      
+
      Armed with our TCP and HTTP knowledge, let's see how this is used by SREs in their role.
      
 
      Applications in SRE role
      
 
        
 
      1. Scaling HTTP performance using load balancers need consistent knowledge about both TCP and HTTP. There are different kinds of load balancing like L4, L7 load balancing, Direct Server Return etc. HTTPs offloading can be done on Load balancer or directly on servers based on the performance and compliance needs.
        2. 
-
      2. Tweaking sysctl variables for rmem and wmem like we did for UDP can improve throughput of sender and receiver.
        3. 
-
      3. Sysctl variable tcp_max_syn_backlog and socket variable somax_conn determines how many connections for which the kernel can complete 3 way handshake before app calling accept syscall. This is much useful in single threaded applications. Once the backlog is full, new connections stay in SYN_RCVD state (when you run netstat) till the application calls accept syscall
        4. 
-
      4. Apps can run out of file descriptors if there are too many short lived connections. Digging through tcp_reuse and tcp_recycle can help reduce time spent in the time wait state(it has its own risk). Making apps reuse a pool of connections instead of creating ad hoc connection can also help
        5. 
-
      5. Understanding performance bottlenecks by seeing metrics and classifying whether its a problem in App or network side. Example too many sockets in Close_wait state is a problem on application whereas retransmissions can be a problem more on network or on OS stack than the application itself. Understanding the fundamentals can help us narrow down where the bottleneck is
        6. 
+
      6. Tweaking sysctl variables for rmem and wmem like we did for UDP can improve throughput of sender and receiver.
        7. 
+
      7. sysctl variable tcp_max_syn_backlog and socket variable somax_conn determines how many connections for which the kernel can complete 3-way handshake before app calling accept syscall. This is much useful in single-threaded applications. Once the backlog is full, new connections stay in SYN_RCVD state (when you run netstat) till the application calls accept syscall.
        8. 
+
      8. Apps can run out of file descriptors if there are too many short-lived connections. Digging through tcp_reuse and tcp_recycle can help reduce time spent in the TIME_WAIT state (it has its own risk). Making apps reuse a pool of connections instead of creating ad hoc connection can also help.
        9. 
+
      9. Understanding performance bottlenecks by seeing metrics and classifying whether it's a problem in App or network side. Example too many sockets in CLOSE_WAIT state is a problem on application whereas retransmissions can be a problem more on network or on OS stack than the application itself. Understanding the fundamentals can help us narrow down where the bottleneck is.
        10. 
 
      
                 
               
diff --git a/level101/linux_networking/udp/index.html b/level101/linux_networking/udp/index.html
index b176a76..3c9287e 100644
--- a/level101/linux_networking/udp/index.html
+++ b/level101/linux_networking/udp/index.html
@@ -2151,13 +2151,13 @@
                 
                 
                 
      UDP
      
-
      UDP is a transport layer protocol. DNS is an application layer protocol that runs on top of UDP(most of the times). Before jumping into UDP, let's try to understand what an application and transport layer is. DNS protocol is used by a DNS client(eg dig) and DNS server(eg named). The transport layer makes sure the DNS request reaches the DNS server process and similarly the response reaches the DNS client process. Multiple processes can run on a system and they can listen on any ports. DNS servers usually listen on port number 53. When a client makes a DNS request, after filling the necessary application payload, it passes the payload to the kernel via sendto system call. The kernel picks a random port number(>1024) as source port number and puts 53 as destination port number and sends the packet to lower layers. When the kernel on server side receives the packet, it checks the port number and queues the packet to the application buffer of the DNS server process which makes a recvfrom system call and reads the packet. This process by the kernel is called multiplexing(combining packets from multiple applications to same lower layers) and demultiplexing(segregating packets from single lower layer to multiple applications). Multiplexing and Demultiplexing is done by the Transport layer.
      
-
      UDP is one of the simplest transport layer protocol and it does only multiplexing and demultiplexing. Another common transport layer protocol TCP does a bunch of other things like reliable communication, flow control and congestion control. UDP is designed to be lightweight and handle communications with little overhead. So it doesn’t do anything beyond multiplexing and demultiplexing. If applications running on top of UDP need any of the features of TCP, they have to implement that in their application
      
-
      This example from python wiki covers a sample UDP client and server where “Hello World” is an application payload sent to server listening on port number 5005. The server receives the packet and prints the “Hello World” string from the client
      
+
      UDP is a transport layer protocol. DNS is an application layer protocol that runs on top of UDP (most of the times). Before jumping into UDP, let's try to understand what an application and transport layer is. DNS protocol is used by a DNS client (eg dig) and DNS server (eg named). The transport layer makes sure the DNS request reaches the DNS server process and similarly the response reaches the DNS client process. Multiple processes can run on a system and they can listen on any ports. DNS servers usually listen on port number 53. When a client makes a DNS request, after filling the necessary application payload, it passes the payload to the kernel via sendto system call. The kernel picks a random port number (>1024) as source port number and puts 53 as destination port number and sends the packet to lower layers. When the kernel on server-side receives the packet, it checks the port number and queues the packet to the application buffer of the DNS server process which makes a recvfrom system call and reads the packet. This process by the kernel is called multiplexing (combining packets from multiple applications to same lower layers) and demultiplexing (segregating packets from single lower layer to multiple applications). Multiplexing and Demultiplexing is done by the Transport layer.
      
+
      UDP is one of the simplest transport layer protocol and it does only multiplexing and demultiplexing. Another common transport layer protocol TCP does a bunch of other things like reliable communication, flow control and congestion control. UDP is designed to be lightweight and handle communications with little overhead. So, it doesn’t do anything beyond multiplexing and demultiplexing. If applications running on top of UDP need any of the features of TCP, they have to implement that in their application.
      
+
      This example from python wiki covers a sample UDP client and server where “Hello World” is an application payload sent to server listening on port number 5005. The server receives the packet and prints the “Hello World” string from the client.
      
 
      Applications in SRE role
      
 
        
-
      1. If the underlying network is slow and the UDP layer is unable to queue packets down to the networking layer, sendto syscall from the application will hang till the kernel finds some of its buffer is freed. This can affect the throughput of the system. Increasing write memory buffer values using sysctl variables net.core.wmem_max and net.core.wmem_default provides some cushion to the application from the slow network
        2. 
-
      2. Similarly if the receiver process is slow in consuming from its buffer, the kernel has to drop packets which it can’t queue due to the buffer being full. Since UDP doesn’t guarantee reliability these dropped packets can cause data loss unless tracked by the application layer. Increasing  sysctl variables rmem_default and rmem_max can provide some cushion to slow applications from fast senders.
        3. 
+
      3. If the underlying network is slow and the UDP layer is unable to queue packets down to the networking layer, sendto syscall from the application will hang till the kernel finds some of its buffer is freed. This can affect the throughput of the system. Increasing write memory buffer values using sysctl variables net.core.wmem_max and net.core.wmem_default provides some cushion to the application from the slow network
        4. 
+
      4. Similarly, if the receiver process is slow in consuming from its buffer, the kernel has to drop packets which it can’t queue due to the buffer being full. Since UDP doesn’t guarantee reliability these dropped packets can cause data loss unless tracked by the application layer. Increasing  sysctl variables rmem_default and rmem_max can provide some cushion to slow applications from fast senders.
        5. 
 
      
                 
               
diff --git a/level101/messagequeue/intro/index.html b/level101/messagequeue/intro/index.html
index 0712060..4f615d9 100644
--- a/level101/messagequeue/intro/index.html
+++ b/level101/messagequeue/intro/index.html
@@ -2234,7 +2234,7 @@
                 
                 
      Messaging services
      
 
      What to expect from this course
      
-
      At the end of training, you will have an understanding of what a Message Services is, learn about different types of Message Service implementation and understand some of the underlying concepts & trade offs.
      
+
      At the end of training, you will have an understanding of what a Message Services is, learn about different types of Message Service implementation and understand some of the underlying concepts & trade-offs.
      
 
      What is not covered under this course
      
 
      We will not be deep diving into any specific Message Service. 
      
 
      Course Contents
      
diff --git a/level101/messagequeue/key_concepts/index.html b/level101/messagequeue/key_concepts/index.html
index 3964999..4caec80 100644
--- a/level101/messagequeue/key_concepts/index.html
+++ b/level101/messagequeue/key_concepts/index.html
@@ -2287,7 +2287,7 @@
                 
                 
                 
      Key Concepts
      
-
      Lets looks at some of the key concepts when we talk about messaging system
      
+
      Let's looks at some of the key concepts when we talk about messaging system
      
 
      Delivery guarantees
      
 
      One of the essential aspects of messaging services is ensuring that messages are delivered to their intended recipients. Different systems offer varying levels of delivery guarantees, and it is crucial to understand these guarantees to choose the right messaging service for your needs.
      
 
        
diff --git a/level101/metrics_and_monitoring/alerts/index.html b/level101/metrics_and_monitoring/alerts/index.html
index 541e6b1..86d7357 100644
--- a/level101/metrics_and_monitoring/alerts/index.html
+++ b/level101/metrics_and_monitoring/alerts/index.html
@@ -2109,11 +2109,11 @@
 
        Earlier we discussed different ways to collect key metric data points
 from a service and its underlying infrastructure. This data gives us a
 better understanding of how the service is performing. One of the main
-objectives of monitoring is to detect any service degradations early
+objectives of monitoring is to detect any service degradations early 
 (reduce Mean Time To Detect) and notify stakeholders so that the issues
 are either avoided or can be fixed early, thus reducing Mean Time To
 Recover (MTTR). For example, if you are notified when resource usage by
-a service exceeds 90 percent, you can take preventive measures to avoid
+a service exceeds 90%, you can take preventive measures to avoid
 any service breakdown due to a shortage of resources. On the other hand,
 when a service goes down due to an issue, early detection and
 notification of such incidents can help you quickly fix the issue.
        
@@ -2124,12 +2124,12 @@ notification of such incidents can help you quickly fix the issue.
        
 set up alerts on one or a combination of metrics to actively monitor the
 service health. These alerts have a set of defined rules or conditions,
 and when the rule is broken, you are notified. These rules can be as
-simple as notifying when the metric value exceeds n to as complex as a
-week over week (WoW) comparison of standard deviation over a period of
+simple as notifying when the metric value exceeds n to as complex as a
+week-over-week (WoW) comparison of standard deviation over a period of
 time. Monitoring tools notify you about an active alert, and most of
 these tools support instant messaging (IM) platforms, SMS, email, or
 phone calls. Figure 8 shows a sample alert notification received on
-Slack for memory usage exceeding 90 percent of total RAM space on the
+Slack for memory usage exceeding 90% of total RAM space on the
 host.
        
                 
               
diff --git a/level101/metrics_and_monitoring/best_practices/index.html b/level101/metrics_and_monitoring/best_practices/index.html
index 01a9856..aa5f671 100644
--- a/level101/metrics_and_monitoring/best_practices/index.html
+++ b/level101/metrics_and_monitoring/best_practices/index.html
@@ -2110,22 +2110,22 @@
 practices in mind.
        
 
          
 
        • 
-
          Use the right metric type -- Most of the libraries available
+
          Use the right metric type—Most of the libraries available
      today offer various metric types. Choose the appropriate metric
      type for monitoring your system. Following are the types of
      metrics and their purposes.
          
 
            
 
          • 
-
            Gauge -- Gauge is a constant type of metric. After the
+
            Gauge—Gauge is a constant type of metric. After the
      metric is initialized, the metric value does not change unless
      you intentionally update it.
            
 
            • 
 
          • 
-
            Timer -- Timer measures the time taken to complete a
+
            Timer—Timer measures the time taken to complete a
      task.
            
 
            • 
 
          • 
-
            Counter -- Counter counts the number of occurrences of a
+
            Counter—Counter counts the number of occurrences of a
      particular event.
            
 
            • 
 
          
@@ -2135,19 +2135,19 @@ practices in mind.
          
  Types.
          
 
            
 
          • 
-
            Avoid over-monitoring -- Monitoring can be a significant
-     engineering endeavor. Therefore, be sure not to spend too
+
            Avoid over-monitoring—Monitoring can be a significant
+     engineering endeavor. Therefore, be sure not to spend too
      much time and resources on monitoring services, yet make sure all
      important metrics are captured.
            
 
            • 
 
          • 
-
            Prevent alert fatigue -- Set alerts for metrics that are
+
            Prevent alert fatigue—Set alerts for metrics that are
      important and actionable. If you receive too many non-critical
      alerts, you might start ignoring alert notifications over time. As
      a result, critical alerts might get overlooked.
            
 
            • 
 
          • 
-
            Have a runbook for alerts -- For every alert, make sure you have
+
            Have a runbook for alerts—For every alert, make sure you have
      a document explaining what actions and checks need to be performed
      when the alert fires. This enables any engineer on the team to
      handle the alert and take necessary actions, without any help from
diff --git a/level101/metrics_and_monitoring/command-line_tools/index.html b/level101/metrics_and_monitoring/command-line_tools/index.html
index c0622f1..f3e6680 100644
--- a/level101/metrics_and_monitoring/command-line_tools/index.html
+++ b/level101/metrics_and_monitoring/command-line_tools/index.html
@@ -2112,28 +2112,28 @@ understand various subsystem statistics (CPU, memory, network, and so
 on). Let's look at some of the tools that are predominantly used.
            
 
              
 
            • 
-
              ps/top-- The process status command (ps) displays information
+
              ps/top: The process status command (ps) displays information
      about all the currently running processes in a Linux system. The
-     top command is similar to the ps command, but it periodically
+     top command is similar to the ps command, but it periodically
      updates the information displayed until the program is terminated.
-     An advanced version of top, called htop, has a more user-friendly
+     An advanced version of top, called htop, has a more user-friendly
      interface and some additional features. These command-line
      utilities come with options to modify the operation and output of
      the command. Following are some important options supported by the
-     ps command.
              
+     ps command.
              
 
                
 
              • 
-
                -p <pid1, pid2,...> -- Displays information about processes
+
                -p <pid1, pid2,...>: Displays information about processes
      that match the specified process IDs. Similarly, you can use
      -u <uid> and -g <gid> to display information about
      processes belonging to a specific user or group.
                
 
                • 
 
              • 
-
                -a -- Displays information about other users' processes, as well
+
                -a: Displays information about other users' processes, as well
      as one's own.
                
 
                • 
 
              • 
-
                -x -- When displaying processes matched by other options,
+
                -x: When displaying processes matched by other options,
      includes processes that do not have a controlling terminal.
                
 
                • 
 
              
@@ -2145,21 +2145,21 @@ on). Let's look at some of the tools that are predominantly used.
              
 
              
 
                
 
              • 
-
                ss -- The socket statistics command (ss) displays information
+
                ss: The socket statistics command (ss) displays information
      about network sockets on the system. This tool is the successor of
      netstat,
      which is deprecated. Following are some command-line options
-     supported by the ss command:
                
+     supported by the ss command:
                
 
                  
 
                • 
-
                  -t -- Displays the TCP socket. Similarly, -u displays UDP
+
                  -t: Displays the TCP socket. Similarly, -u displays UDP
      sockets, -x is for UNIX domain sockets, and so on.
                  
 
                  • 
 
                • 
-
                  -l -- Displays only listening sockets.
                  
+
                  -l: Displays only listening sockets.
                  
 
                  • 
 
                • 
-
                  -n -- Instructs the command to not resolve service names.
+
                  -n: Instructs the command to not resolve service names.
      Instead displays the port numbers.
                  
 
                  • 
 
                
@@ -2168,7 +2168,7 @@ on). Let's look at some of the tools that are predominantly used.
                
 
                List of listening sockets on a system 
                 Figure
 3: List of listening sockets on a system 
                
 
                  
-
                • free -- The free command displays memory usage statistics on the
+
                • free: The free command displays memory usage statistics on the
      host like available memory, used memory, and free memory. Most often,
      this command is used with the -h command-line option, which
      displays the statistics in a human-readable format.
                  • 
@@ -2177,7 +2177,7 @@ on). Let's look at some of the tools that are predominantly used.
                  
 
                   Figure 4: Memory statistics on a host in human-readable form 
                  
 
 
                    
-
                  • df -- The df command displays disk space usage statistics. The
+
                  • df: The df command displays disk space usage statistics. The
      -i command-line option is also often used to display
      inode usage
      statistics. The -h command-line option is used for displaying
@@ -2189,13 +2189,13 @@ on). Let's look at some of the tools that are predominantly used.
                    
 
 
                      
 
                    • 
-
                      sar -- The sar utility monitors various subsystems, such as CPU
+
                      sar: The sar utility monitors various subsystems, such as CPU
      and memory, in real time. This data can be stored in a file
      specified with the -o option. This tool helps to identify
      anomalies.
                      
 
                      • 
 
                    • 
-
                      iftop -- The interface top command (iftop) displays bandwidth
+
                      iftop: The interface top command (iftop) displays bandwidth
      utilization by a host on an interface. This command is often used
      to identify bandwidth usage by active connections. The -i option
      specifies which network interface to watch.
                      
@@ -2209,31 +2209,31 @@ active connection on the host 
                      
 
                      
 
                        
 
                      • 
-
                        tcpdump -- The tcpdump command is a network monitoring tool that
+
                        tcpdump: The tcpdump command is a network monitoring tool that
      captures network packets flowing over the network and displays a
      description of the captured packets. The following options are
      available:
                        
 
                          
 
                        • 
-
                          -i <interface> -- Interface to listen on
                          
+
                          -i <interface>: Interface to listen on
                          
 
                          • 
 
                        • 
-
                          host <IP/hostname> -- Filters traffic going to or from the
+
                          host <IP/hostname>: Filters traffic going to or from the
      specified host
                          
 
                          • 
 
                        • 
-
                          src/dst -- Displays one-way traffic from the source (src) or to
+
                          src/dst: Displays one-way traffic from the source (src) or to
      the destination (dst)
                          
 
                          • 
 
                        • 
-
                          port <port number> -- Filters traffic to or from a particular
+
                          port <port number>: Filters traffic to or from a particular
      port
                          
 
                          • 
 
                        
 
                        • 
 
                      
 
                      tcpdump of packets on an interface 
                      
-
                       Figure 7: *tcpdump* of packets on *docker0*
+
                       Figure 7: tcpdump of packets on docker0
 interface on a host 
                      
                 
               
diff --git a/level101/metrics_and_monitoring/conclusion/index.html b/level101/metrics_and_monitoring/conclusion/index.html
index c3fb268..4e3051a 100644
--- a/level101/metrics_and_monitoring/conclusion/index.html
+++ b/level101/metrics_and_monitoring/conclusion/index.html
@@ -2151,12 +2151,12 @@
                 
                      Conclusion
                      
 
                      A robust monitoring and alerting system is necessary for maintaining and
 troubleshooting a system. A dashboard with key metrics can give you an
-overview of service performance, all in one place. Well-defined alerts
+overview of service performance, all in one place. Well-defined alerts 
 (with realistic thresholds and notifications) further enable you to
 quickly identify any anomalies in the service infrastructure and in
 resource saturation. By taking necessary actions, you can avoid any
 service degradations and decrease MTTD for service breakdowns.
                      
-
                      In addition to in-house monitoring, monitoring real user experience can
+
                      In addition to in-house monitoring, monitoring real-user experience can
 help you to understand service performance as perceived by the users.
 Many modules are involved in serving the user, and most of them are out
 of your control. Therefore, you need to have real-user monitoring in
diff --git a/level101/metrics_and_monitoring/introduction/index.html b/level101/metrics_and_monitoring/introduction/index.html
index ad01f3f..f1fbafe 100644
--- a/level101/metrics_and_monitoring/introduction/index.html
+++ b/level101/metrics_and_monitoring/introduction/index.html
@@ -2208,7 +2208,7 @@ a system, analyzing the data to derive meaningful information, and
 displaying the data to the users. In simple terms, you measure various
 metrics regularly to understand the state of the system, including but
 not limited to, user requests, latency, and error rate. What gets
-measured, gets fixed---if you can measure something, you can reason
+measured, gets fixed—if you can measure something, you can reason
 about it, understand it, discuss it, and act upon it with confidence.
                      
 
                      Four golden signals of monitoring
                      
 
                      When setting up monitoring for a system, you need to decide what to
@@ -2237,7 +2237,7 @@ if you can measure only four metrics of your service, focus on these
 four. Let's look at each of the four golden signals.
                      
 
                        
 
                      • 
-
                        Traffic -- Traffic gives a better understanding of the service
+
                        Traffic—Traffic gives a better understanding of the service
      demand. Often referred to as service QPS (queries per second),
      traffic is a measure of requests served by the service. This
      signal helps you to decide when a service needs to be scaled up to
@@ -2245,7 +2245,7 @@ four. Let's look at each of the four golden signals.
                        
      cost-effective.
                        
 
                        • 
 
                      • 
-
                        Latency -- Latency is the measure of time taken by the service
+
                        Latency—Latency is the measure of time taken by the service
      to process the incoming request and send the response. Measuring
      service latency helps in the early detection of slow degradation
      of the service. Distinguishing between the latency of successful
@@ -2258,7 +2258,7 @@ four. Let's look at each of the four golden signals.
                        
      overall latency might result in misleading calculations.
                        
 
                        • 
 
                      • 
-
                        Error (rate) -- Error is the measure of failed client
+
                        Error (rate)—Error is the measure of failed client
      requests. These failures can be easily identified based on the
      response codes (HTTP 5XX
      error).
@@ -2274,7 +2274,7 @@ four. Let's look at each of the four golden signals.
                        
      in place to capture errors in addition to the response codes.
                        
 
                        • 
 
                      • 
-
                        Saturation -- Saturation is a measure of the resource
+
                        Saturation—Saturation is a measure of the resource
      utilization by a service. This signal tells you the state of
      service resources and how full they are. These resources include
      memory, compute, network I/O, and so on. Service performance
@@ -2306,19 +2306,19 @@ can build intelligent applications to address specific needs. Some of
 the key use cases follow:
                        
 
                          
 
                        • 
-
                          Reduction in time to resolve issues -- With a good monitoring
+
                          Reduction in time to resolve issues—With a good monitoring
      infrastructure in place, you can identify issues quickly and
      resolve them, which reduces the impact caused by the issues.
                          
 
                          • 
 
                        • 
-
                          Business decisions -- Data collected over a period of time can
+
                          Business decisions—Data collected over a period of time can
      help you make business decisions such as determining the product
      release cycle, which features to invest in, and geographical areas
      to focus on. Decisions based on long-term data can improve the
      overall product experience.
                          
 
                          • 
 
                        • 
-
                          Resource planning -- By analyzing historical data, you can
+
                          Resource planning—By analyzing historical data, you can
      forecast service compute-resource demands, and you can properly
      allocate resources. This allows financially effective decisions,
      with no compromise in end-user experience.
                          
@@ -2328,44 +2328,44 @@ the key use cases follow:
                          
 terminologies.
                          
 
                            
 
                          • 
-
                            Metric -- A metric is a quantitative measure of a particular
-     system attribute---for example, memory or CPU
                            
+
                            Metric—A metric is a quantitative measure of a particular
+     system attribute—for example, memory or CPU
                            
 
                            • 
 
                          • 
-
                            Node or host -- A physical server, virtual machine, or container
+
                            Node or host—A physical server, virtual machine, or container
      where an application is running
                            
 
                            • 
 
                          • 
-
                            QPS -- Queries Per Second, a measure of traffic served by the
+
                            QPS—Queries Per Second, a measure of traffic served by the
      service per second
                            
 
                            • 
 
                          • 
-
                            Latency -- The time interval between user action and the
-     response from the server---for example, time spent after sending a
+
                            Latency—The time interval between user action and the
+     response from the server—for example, time spent after sending a
      query to a database before the first response bit is received
                            
 
                            • 
 
                          • 
-
                            Error rate -- Number of errors observed over a particular
+
                            Error rate—Number of errors observed over a particular
      time period (usually a second)
                            
 
                            • 
 
                          • 
-
                            Graph -- In monitoring, a graph is a representation of one or
+
                            Graph—In monitoring, a graph is a representation of one or
      more values of metrics collected over time
                            
 
                            • 
 
                          • 
-
                            Dashboard -- A dashboard is a collection of graphs that provide
+
                            Dashboard—A dashboard is a collection of graphs that provide
      an overview of system health
                            
 
                            • 
 
                          • 
-
                            Incident -- An incident is an event that disrupts the normal
+
                            Incident—An incident is an event that disrupts the normal
      operations of a system
                            
 
                            • 
 
                          • 
-
                            MTTD -- Mean Time To Detect is the time interval between the
+
                            MTTD—Mean Time To Detect is the time interval between the
      beginning of a service failure and the detection of such failure
                            
 
                            • 
 
                          • 
-
                            MTTR -- Mean Time To Resolve is the time spent to fix a service
+
                            MTTR—Mean Time To Resolve is the time spent to fix a service
      failure and bring the service back to its normal state
                            
 
                            • 
 
                          
@@ -2382,7 +2382,7 @@ notifying concerned parties during any abnormal behavior. Let's look at
 each of these infrastructure components:
                          
 
                            
 
                          • 
-
                            Host metrics agent -- A host metrics agent is a process
+
                            Host metrics agent—A host metrics agent is a process
      running on the host that collects performance statistics for host
      subsystems such as memory, CPU, and network. These metrics are
      regularly relayed to a metrics collector for storage and
@@ -2392,7 +2392,7 @@ each of these infrastructure components:
                            
      and metricbeat.
                            
 
                            • 
 
                          • 
-
                            Metric aggregator -- A metric aggregator is a process running
+
                            Metric aggregator—A metric aggregator is a process running
      on the host. Applications running on the host collect service
      metrics using
      instrumentation.
@@ -2403,7 +2403,7 @@ each of these infrastructure components:
                            
      StatsD.
                            
 
                            • 
 
                          • 
-
                            Metrics collector -- A metrics collector process collects all
+
                            Metrics collector—A metrics collector process collects all
      the metrics from the metric aggregators running on multiple hosts.
      The collector takes care of decoding and stores this data on the
      database. Metric collection and storage might be taken care of by
@@ -2413,13 +2413,13 @@ each of these infrastructure components:
                            
      daemons.
                            
 
                            • 
 
                          • 
-
                            Storage -- A time-series database stores all of these metrics.
+
                            Storage—A time-series database stores all of these metrics.
      Examples are OpenTSDB,
      Whisper,
      and InfluxDB.
                            
 
                            • 
 
                          • 
-
                            Metrics server -- A metrics server can be as basic as a web
+
                            Metrics server—A metrics server can be as basic as a web
      server that graphically renders metric data. In addition, the
      metrics server provides aggregation functionalities and APIs for
      fetching metric data programmatically. Some examples are
@@ -2427,7 +2427,7 @@ each of these infrastructure components:
                            
      Graphite-Web.
                            
 
                            • 
 
                          • 
-
                            Alert manager -- The alert manager regularly polls metric data
+
                            Alert manager—The alert manager regularly polls metric data
      available and, if there are any anomalies detected, notifies you.
      Each alert has a set of rules for identifying such anomalies.
      Today many metrics servers such as
diff --git a/level101/metrics_and_monitoring/observability/index.html b/level101/metrics_and_monitoring/observability/index.html
index cdfcf77..c5dd6aa 100644
--- a/level101/metrics_and_monitoring/observability/index.html
+++ b/level101/metrics_and_monitoring/observability/index.html
@@ -2107,7 +2107,7 @@
                 
                            
 
                            Observability
                            
 
                            Engineers often use observability when referring to building reliable
-systems. Observability is a term derived from control theory, It is a
+systems. Observability is a term derived from control theory, it is a
 measure of how well internal states of a system can be inferred from
 knowledge of its external outputs. Service infrastructures used on a
 daily basis are becoming more and more complex; proactive monitoring
@@ -2177,7 +2177,7 @@ value and take further action.
                            
 Logstash, Kibana), which provides centralized log processing. Beats is a
 collection of lightweight data shippers that can ship logs, audit data,
 network data, and so on over the network. In this use case specifically,
-we are using filebeat as a log shipper. Filebeat watches service log
+we are using Filebeat as a log shipper. Filebeat watches service log
 files and ships the log data to Logstash. Logstash parses these logs and
 transforms the data, preparing it to store on Elasticsearch. Transformed
 log data is stored on Elasticsearch and indexed for fast retrieval.
diff --git a/level101/metrics_and_monitoring/third-party_monitoring/index.html b/level101/metrics_and_monitoring/third-party_monitoring/index.html
index bc111bb..3bc0b0b 100644
--- a/level101/metrics_and_monitoring/third-party_monitoring/index.html
+++ b/level101/metrics_and_monitoring/third-party_monitoring/index.html
@@ -2111,13 +2111,13 @@ addition, a number of companies such as
 Datadog offer
 monitoring-as-a-service. In this section, we are not covering
 monitoring-as-a-service in depth.
                            
-
                            In recent years, more and more people have access to the internet. Many
+
                            In recent years, more and more people have access to the Internet. Many
 services are offered online to cater to the increasing user base. As a
 result, web pages are becoming larger, with increased client-side
 scripts. Users want these services to be fast and error-free. From the
 service point of view, when the response body is composed, an HTTP 200
 OK response is sent, and everything looks okay. But there might be
-errors during transmission or on the client side. As previously
+errors during transmission or on the client-side. As previously
 mentioned, monitoring services from within the service infrastructure
 give good visibility into service health, but this is not enough. You
 need to monitor user experience, specifically the availability of
@@ -2131,7 +2131,7 @@ service is globally accessible. Other third-party monitoring solutions
 for real user monitoring (RUM) provide performance statistics such as
 service uptime and response time, from different geographical locations.
 This allows you to monitor the user experience from these locations,
-which might have different internet backbones, different operating
+which might have different Internet backbones, different operating
 systems, and different browsers and browser versions. Catchpoint
 Global Monitoring
 Network is a
diff --git a/level101/python_web/intro/index.html b/level101/python_web/intro/index.html
index 6094c53..7473ddf 100644
--- a/level101/python_web/intro/index.html
+++ b/level101/python_web/intro/index.html
@@ -2261,17 +2261,17 @@
                 
                            Python and The Web
                            
 
                            Prerequisites
                            
 
                              
-
                            • Basic understanding of python language.
                              • 
-
                            • Basic familiarity with flask framework.
                              • 
+
                            • Basic understanding of Python language.
                              • 
+
                            • Basic familiarity with Flask framework.
                              • 
 
                            
 
                            What to expect from this course
                            
-
                            This course is divided into two high level parts. In the first part, assuming familiarity with python language’s basic operations and syntax usage, we will dive a little deeper into understanding python as a language. We will compare python with other programming languages that you might already know like Java and C. We will also explore concepts of Python objects and with help of that, explore python features like decorators.
                            
-
                            In the second part which will revolve around the web, and also assume familiarity with the Flask framework, we will start from the socket module and work with HTTP requests. This will demystify how frameworks like flask work internally.
                            
-
                            And to introduce SRE flavour to the course, we will design, develop and deploy (in theory) a URL shortening application. We will emphasize parts of the whole process that are more important as an SRE of the said app/service.
                            
+
                            This course is divided into two high-level parts. In the first part, assuming familiarity with Python language’s basic operations and syntax usage, we will dive a little deeper into understanding Python as a language. We will compare Python with other programming languages that you might already know like Java and C. We will also explore concepts of Python objects and with help of that, explore Python features like decorators.
                            
+
                            In the second part which will revolve around the web, and also assume familiarity with the Flask framework, we will start from the socket module and work with HTTP requests. This will demystify how frameworks like Flask work internally.
                            
+
                            And to introduce SRE flavour to the course, we will design, develop and deploy (in theory) a URL-shortening application. We will emphasize parts of the whole process that are more important as an SRE of the said app/service.
                            
 
                            What is not covered under this course
                            
-
                            Extensive knowledge of python internals and advanced python.
                            
+
                            Extensive knowledge of Python internals and advanced Python.
                            
 
                            Lab Environment Setup
                            
-
                            Have latest version of python installed
                            
+
                            Have latest version of Python installed
                            
 
                            Course Contents
                            
 
                              
 
                            1. The Python Language
                                
@@ -2284,7 +2284,7 @@
 
                              1. Flask
                                2. 
 
                              
 
                              2. 
-
                            2. The URL Shortening App
                                
+
                              1. The URL-Shortening App
                                  
 
                                1. Design
                                  2. 
 
                                2. Scaling The App
                                  3. 
 
                                3. Monitoring The App
                                  4. 
@@ -2292,11 +2292,11 @@
 
 
                                
 
                                The Python Language
                                
-
                                Assuming you know a little bit of C/C++ and Java, let's try to discuss the following questions in context of those two languages and python. You might have heard that C/C++ is a compiled language while python is an interpreted language. Generally, with compiled language we first compile the program and then run the executable while in case of python we run the source code directly like python hello_world.py. While Java, being an interpreted language, still has a separate compilation step and then its run. So what's really the difference?
                                
+
                                Assuming you know a little bit of C/C++ and Java, let's try to discuss the following questions in context of those two languages and Python. You might have heard that C/C++ is a compiled language while Python is an interpreted language. Generally, with compiled language we first compile the program and then run the executable while in case of Python we run the source code directly like python hello_world.py. While Java, being an interpreted language, still has a separate compilation step and then it's run. So, what's really the difference?
                                
 
                                Compiled vs. Interpreted
                                
-
                                This might sound a little weird to you: python, in a way is a compiled language! Python has a compiler built-in! It is obvious in the case of java since we compile it using a separate command ie: javac helloWorld.java and it will produce a .class file which we know as a bytecode. Well, python is very similar to that. One difference here is that there is no separate compile command/binary needed to run a python program.
                                
-
                                What is the difference then, between java and python?
-Well, Java's compiler is more strict and sophisticated. As you might know Java is a statically typed language. So the compiler is written in a way that it can verify types related errors during compile time. While python being a dynamic language, types are not known until a program is run. So in a way, python compiler is dumb (or, less strict). But there indeed is a compile step involved when a python program is run. You might have seen python bytecode files with .pyc extension. Here is how you can see bytecode for a given python program.
                                
+
                                This might sound a little weird to you: Python, in a way is a compiled language! Python has a compiler built-in! It is obvious in the case of Java since we compile it using a separate command, ie: javac helloWorld.java and it will produce a .class file which we know as a bytecode. Well, Python is very similar to that. One difference here is that there is no separate compile command/binary needed to run a Python program.
                                
+
                                What is the difference then, between Java and Python?
+Well, Java's compiler is more strict and sophisticated. As you might know Java is a statically typed language. So the compiler is written in a way that it can verify types-related errors during compile time. While Python being a dynamic language, types are not known until a program is run. So in a way, Python compiler is dumb (or, less strict). But there indeed is a compile step involved when a Python program is run. You might have seen Python bytecode files with .pyc extension. Here is how you can see bytecode for a given Python program.
                                
 
                                # Create a Hello World
 $ echo "print('hello world')" > hello_world.py
 
@@ -2313,11 +2313,11 @@ $ python -m dis hello_world.py
              8 LOAD_CONST               1 (None)
             10 RETURN_VALUE
 
                                
-
                                Read more about dis module here
                                
-
                                Now coming to C/C++, there of course is a compiler. But the output is different than what java/python compiler would produce. Compiling a C program would produce what we also know as machine code. As opposed to bytecode.
                                
+
                                Read more about dis module here.
                                
+
                                Now coming to C/C++, there of course is a compiler. But the output is different than what Java/Python compiler would produce. Compiling a C program would produce what we also know as machine code, as opposed to bytecode.
                                
 
                                Running The Programs
                                
 
                                We know compilation is involved in all 3 languages we are discussing. Just that the compilers are different in nature and they output different types of content. In case of C/C++, the output is machine code which can be directly read by your operating system. When you execute that program, your OS will know how exactly to run it. But this is not the case with bytecode.
                                
-
                                Those bytecodes are language specific. Python has its own set of bytecode defined (more in dis module) and so does java. So naturally, your operating system will not know how to run it. To run this bytecode, we have something called Virtual Machines. Ie: The JVM or the Python VM (CPython, Jython). These so called Virtual Machines are the programs which can read the bytecode and run it on a given operating system. Python has multiple VMs available. Cpython is a python VM implemented in C language, similarly Jython is a Java implementation of python VM. At the end of the day, what they should be capable of is to understand python language syntax, be able to compile it to bytecode and be able to run that bytecode. You can implement a python VM in any language! (And people do so, just because it can be done)
                                
+
                                Those bytecodes are language specific. Python has its own set of bytecode defined (more in dis module) and so does Java. So naturally, your operating system will not know how to run it. To run this bytecode, we have something called Virtual Machines. Ie: The JVM or the Python VM (CPython, Jython). These so-called Virtual Machines are the programs which can read the bytecode and run it on a given operating system. Python has multiple VMs available. CPython is a Python VM implemented in C language, similarly Jython is a Java implementation of Python VM. At the end of the day, what they should be capable of is to understand Python language syntax, be able to compile it to bytecode and be able to run that bytecode. You can implement a Python VM in any language! (And people do so, just because it can be done)
                                
 
                                                                                              The Operating System
 
                                                               +------------------------------------+
@@ -2351,7 +2351,7 @@ hello_world.c                     OS Specific machinecode     |         A New Pr
 
                                
 
                                Two things to note for above diagram:
                                
 
                                  
-
                                1. Generally, when we run a python program, a python VM process is started which reads the python source code, compiles it to byte code and run it in a single step. Compiling is not a separate step. Shown only for illustration purpose.
                                  2. 
+
                                2. Generally, when we run a Python program, a Python VM process is started which reads the Python source code, compiles it to bytecode and run it in a single step. Compiling is not a separate step. Shown only for illustration purpose.
                                  3. 
 
                                3. Binaries generated for C like languages are not exactly run as is. Since there are multiple types of binaries (eg: ELF), there are more complicated steps involved in order to run a binary but we will not go into that since all that is done at OS level.
                                  4. 
 
                                
                 
diff --git a/level101/python_web/python-concepts/index.html b/level101/python_web/python-concepts/index.html
index 660fe38..61d0d01 100644
--- a/level101/python_web/python-concepts/index.html
+++ b/level101/python_web/python-concepts/index.html
@@ -2217,10 +2217,10 @@
                 
                 
                 
                                Some Python Concepts
                                
-
                                Though you are expected to know python and its syntax at basic level, let us discuss some fundamental concepts that will help you understand the python language better.
                                
+
                                Though you are expected to know python and its syntax at basic level, let us discuss some fundamental concepts that will help you understand the Python language better.
                                
 
                                Everything in Python is an object.
                                
-
                                That includes the functions, lists, dicts, classes, modules, a running function (instance of function definition), everything. In the CPython, it would mean there is an underlying struct variable for each object.
                                
-
                                In python's current execution context, all the variables are stored in a dict. It'd be a string to object mapping. If you have a function and a float variable defined in the current context, here is how it is handled internally.
                                
+
                                That includes the functions, lists, dicts, classes, modules, a running function (instance of function definition), everything. In the CPython, it would mean there is an underlying struct variable for each object.
                                
+
                                In Python's current execution context, all the variables are stored in a dict. It'd be a string to object mapping. If you have a function and a float variable defined in the current context, here is how it is handled internally.
                                
 
                                >>> float_number=42.0
 >>> def foo_func():
 ...     pass
@@ -2242,7 +2242,7 @@
 '__ne__', '__new__', '__qualname__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__',
 '__subclasshook__']
 
                                
-
                                While there are a lot of them, let's look at some interesting ones
                                
+
                                While there are a lot of them, let's look at some interesting ones.
                                
 
                                globals
                                
 
                                This attribute, as the name suggests, has references of global variables. If you ever need to know what all global variables are in the scope of this function, this will tell you. See how the function start seeing the new variable in globals
                                
 
                                >>> hello.__globals__
@@ -2254,7 +2254,7 @@
 {'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, 'hello': <function hello at 0x7fe4e82554c0>, 'GLOBAL': 'g_val'}
 
                                
 
                                code
                                
-
                                This is an interesting one! As everything in python is an object, this includes the bytecode too. The compiled python bytecode is a python code object. Which is accessible via __code__ attribute here. A function has an associated code object which carries some interesting information.
                                
+
                                This is an interesting one! As everything in Python is an object, this includes the bytecode too. The compiled Python bytecode is a Python code object. Which is accessible via __code__ attribute here. A function has an associated code object which carries some interesting information.
                                
 
                                # the file in which function is defined
 # stdin here since this is run in an interpreter
 >>> hello.__code__.co_filename
@@ -2272,9 +2272,9 @@
 >>> hello.__code__.co_code
 b't\x00d\x01|\x00\x9b\x00d\x02\x9d\x03\x83\x01\x01\x00d\x00S\x00'
 
                                
-
                                There are more code attributes which you can enlist by >>> dir(hello.__code__)
                                
+
                                There are more code attributes which you can enlist by >>> dir(hello.__code__).
                                
 
                                Decorators
                                
-
                                Related to functions, python has another feature called decorators. Let's see how that works, keeping  everything is an object in mind.
                                
+
                                Related to functions, Python has another feature called decorators. Let's see how that works, keeping  everything is an object in mind.
                                
 
                                Here is a sample decorator:
                                
 
                                >>> def deco(func):
 ...     def inner():
@@ -2304,7 +2304,7 @@ after
 
                              2. Function hello_world is created
                                3. 
 
                              3. It is passed to deco function
                                4. 
 
                              4. deco create a new function
                                  
-
                                1. This new function is calls hello_world function
                                  2. 
+
                                2. This new function calls hello_world function
                                  3. 
 
                                3. And does a couple other things
                                  4. 
 
                                
 
                                5. 
@@ -2343,10 +2343,10 @@ after
 
                                Note how the hello_world name points to a new function object but that new function object knows the reference (ID) of the original function.
                                
 
                                Some Gotchas
                                
 
                                  
-
                                • While it is very quick to build prototypes in python and there are tons of libraries available, as the codebase complexity increases, type errors become more common and will get hard to deal with. (There are solutions to that problem like type annotations in python. Checkout mypy.)
                                  • 
-
                                • Because python is dynamically typed language, that means all types are determined at runtime. And that makes python run very slow compared to other statically typed languages.
                                  • 
+
                                • While it is very quick to build prototypes in Python and there are tons of libraries available, as the codebase complexity increases, type errors become more common and will get hard to deal with. (There are solutions to that problem like type annotations in Python. Checkout mypy.)
                                  • 
+
                                • Because Python is dynamically typed language, that means all types are determined at runtime. And that makes Python run very slow compared to other statically typed languages.
                                  • 
 
                                • Python has something called GIL (global interpreter lock) which is a limiting factor for utilizing multiple CPU cores for parallel computation.
                                  • 
-
                                • Some weird things that python does: https://github.com/satwikkansal/wtfpython
                                  • 
+
                                • Some weird things that Python does: https://github.com/satwikkansal/wtfpython.
                                  • 
 
                                
                 
               
diff --git a/level101/python_web/python-web-flask/index.html b/level101/python_web/python-web-flask/index.html
index c249b21..4ac53c3 100644
--- a/level101/python_web/python-web-flask/index.html
+++ b/level101/python_web/python-web-flask/index.html
@@ -2163,10 +2163,10 @@
                 
                 
                 
                                Python, Web and Flask
                                
-
                                Back in the old days, websites were simple. They were simple static html contents. A webserver would be listening on a defined port and according to the HTTP request received, it would read files from disk and return them in response. But since then, complexity has evolved and websites are now dynamic. Depending on the request, multiple operations need to be performed like reading from database or calling other API and finally returning some response (HTML data, JSON content etc.)
                                
-
                                Since serving web requests is no longer a simple task like reading files from disk and return contents, we need to process each http request, perform some operations programmatically and construct a response.
                                
+
                                Back in the old days, websites were simple. They were simple static html contents. A webserver would be listening on a defined port and according to the HTTP request received, it would read files from disk and return them in response. But since then, complexity has evolved and websites are now dynamic. Depending on the request, multiple operations need to be performed like reading from database or calling other API and finally returning some response (HTML data, JSON content, etc.)
                                
+
                                Since serving web requests is no longer a simple task like reading files from disk and return contents, we need to process each HTTP request, perform some operations programmatically and construct a response.
                                
 
                                Sockets
                                
-
                                Though we have frameworks like flask, HTTP is still a protocol that works over TCP protocol. So let us setup a TCP server and send an HTTP request and inspect the request's payload. Note that this is not a tutorial on socket programming but what we are doing here is inspecting HTTP protocol at its ground level and look at what its contents look like. (Ref: Socket Programming in Python (Guide) on RealPython)
                                
+
                                Though we have frameworks like Flask, HTTP is still a protocol that works over TCP protocol. So, let us setup a TCP server and send an HTTP request and inspect the request's payload. Note that this is not a tutorial on socket programming but what we are doing here is inspecting HTTP protocol at its ground level and look at what its contents look like. (Ref: Socket Programming in Python (Guide) on RealPython)
                                
 
                                import socket
 
 HOST = '127.0.0.1'  # Standard loopback interface address (localhost)
@@ -2184,7 +2184,7 @@ with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
                break
            print(data)
 
                                
-
                                Then we open localhost:65432 in our web browser and following would be the output:
                                
+
                                Then, we open localhost:65432 in our web browser and following would be the output:
                                
 
                                Connected by ('127.0.0.1', 54719)
 b'GET / HTTP/1.1\r\nHost: localhost:65432\r\nConnection: keep-alive\r\nDNT: 1\r\nUpgrade-Insecure-Requests: 1\r\nUser-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.83 Safari/537.36 Edg/85.0.564.44\r\nAccept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9\r\nSec-Fetch-Site: none\r\nSec-Fetch-Mode: navigate\r\nSec-Fetch-User: ?1\r\nSec-Fetch-Dest: document\r\nAccept-Encoding: gzip, deflate, br\r\nAccept-Language: en-US,en;q=0.9\r\n\r\n'
 
                                
@@ -2195,10 +2195,10 @@ HEADERS_SEPARATED_BY_SEPARATOR
 
                                So though it's a blob of bytes, knowing http protocol specification, you can parse that string (ie: split by \r\n) and get meaningful information out of it.
                                
 
                                Flask
                                
 
                                Flask, and other such frameworks does pretty much what we just discussed in the last section (with added more sophistication). They listen on a port on a TCP socket, receive an HTTP request, parse the data according to protocol format and make it available to you in a convenient manner.
                                
-
                                ie: you can access headers in flask by request.headers which is made available to you by splitting above payload by /r/n, as defined in http protocol.
                                
-
                                Another example: we register routes in flask by @app.route("/hello"). What flask will do is maintain a registry internally which will map /hello with the function you decorated with. Now whenever a request comes with the /hello route (second component in the first line, split by space), flask calls the registered function and returns whatever the function returned.
                                
+
                                That is you can access headers in Flask by request.headers which is made available to you by splitting above payload by /r/n, as defined in HTTP protocol.
                                
+
                                Another example: we register routes in Flask by @app.route("/hello"). What Flask will do is maintain a registry internally which will map /hello with the function you decorated with. Now, whenever a request comes with the /hello route (second component in the first line, split by space), Flask calls the registered function and returns whatever the function returned.
                                
 
                                Same with all other web frameworks in other languages too. They all work on similar principles. What they basically do is understand the HTTP protocol, parses the HTTP request data and gives us programmers a nice interface to work with HTTP requests.
                                
-
                                Not so much of magic, innit?
                                
+
                                Not so much of magic in it?
                                
                 
               
               
diff --git a/level101/python_web/sre-conclusion/index.html b/level101/python_web/sre-conclusion/index.html
index 93e693c..c18d619 100644
--- a/level101/python_web/sre-conclusion/index.html
+++ b/level101/python_web/sre-conclusion/index.html
@@ -2207,33 +2207,33 @@
                 
                                Conclusion
                                
 
                                Scaling The App
                                
 
                                The design and development is just a part of the journey. We will need to setup continuous integration and continuous delivery pipelines sooner or later. And we have to deploy this app somewhere.
                                
-
                                Initially we can start with deploying this app on one virtual machine on any cloud provider. But this is a Single point of failure which is something we never allow as an SRE (or even as an engineer). So an improvement here can be having multiple instances of applications deployed behind a load balancer. This certainly prevents problems of one machine going down.
                                
+
                                Initially, we can start with deploying this app on one virtual machine on any cloud provider. But this is a Single point of failure which is something we never allow as an SRE (or even as an engineer). So an improvement here can be having multiple instances of applications deployed behind a load balancer. This certainly prevents problems of one machine going down.
                                
 
                                Scaling here would mean adding more instances behind the load balancer. But this is scalable upto only a certain point. After that, other bottlenecks in the system will start appearing. ie: DB will become the bottleneck, or perhaps the load balancer itself. How do you know what is the bottleneck? You need to have observability into each aspects of the application architecture.
                                
 
                                Only after you have metrics, you will be able to know what is going wrong where. What gets measured, gets fixed!
                                
 
                                Get deeper insights into scaling from School Of SRE's Scalability module and post going through it, apply your learnings and takeaways to this app. Think how will we make this app geographically distributed and highly available and scalable.
                                
 
                                Monitoring Strategy
                                
-
                                Once we have our application deployed. It will be working ok. But not forever. Reliability is in the title of our job and we make systems reliable by making the design in a certain way. But things still will go down. Machines will fail. Disks will behave weirdly. Buggy code will get pushed to production. And all these possible scenarios will make the system less reliable. So what do we do? We monitor!
                                
+
                                Once we have our application deployed. It will be working okay. But not forever. Reliability is in the title of our job and we make systems reliable by making the design in a certain way. But things still will go down. Machines will fail. Disks will behave weirdly. Buggy code will get pushed to production. And all these possible scenarios will make the system less reliable. So what do we do? We monitor!
                                
 
                                We keep an eye on the system's health and if anything is not going as expected, we want ourselves to get alerted.
                                
-
                                Now let's think in terms of the given url shortening app. We need to monitor it. And we would want to get notified in case something goes wrong. But we first need to decide what is that something that we want to keep an eye on.
                                
+
                                Now let's think in terms of the given URL-shortening app. We need to monitor it. And we would want to get notified in case something goes wrong. But we first need to decide what is that something that we want to keep an eye on.
                                
 
                                  
 
                                1. Since it's a web app serving HTTP requests, we want to keep an eye on HTTP Status codes and latencies
                                  2. 
 
                                2. Request volume again is a good candidate, if the app is receiving an unusual amount of traffic, something might be off.
                                  3. 
-
                                3. We also want to keep an eye on the database so depending on the database solution chosen. Query times, volumes, disk usage etc.
                                  4. 
+
                                4. We also want to keep an eye on the database so depending on the database solution chosen. Query times, volumes, disk usage, etc.
                                  5. 
 
                                5. Finally, there also needs to be some external monitoring which runs periodic tests from devices outside of your data centers. This emulates customers and ensures that from customer point of view, the system is working as expected.
                                  6. 
 
                                
 
                                Applications in SRE role
                                
-
                                In the world of SRE, python is a widely used language. For small scripts and tooling developed for various purposes. Since tooling developed by SRE works with critical pieces of infrastructure and has great power (to bring things down), it is important to know what you are doing while using a programming language and its features. Also it is equally important to know the language and its characteristics while debugging the issues. As an SRE having a deeper understanding of python language, it has helped me a lot to debug very sneaky bugs and be generally more aware and informed while making certain design decisions.
                                
+
                                In the world of SRE, Python is a widely used language for small scripts and tooling developed for various purposes. Since tooling developed by SRE works with critical pieces of infrastructure and has great power (to bring things down), it is important to know what you are doing while using a programming language and its features. Also it is equally important to know the language and its characteristics while debugging the issues. As an SRE having a deeper understanding of Python language, it has helped me a lot to debug very sneaky bugs and be generally more aware and informed while making certain design decisions.
                                
 
                                While developing tools may or may not be part of SRE job, supporting tools or services is more likely to be a daily duty. Building an application or tool is just a small part of productionization. While there is certainly that goes in the design of the application itself to make it more robust, as an SRE you are responsible for its reliability and stability once it is deployed and running. And to ensure that, you’d need to understand the application first and then come up with a strategy to monitor it properly and be prepared for various failure scenarios.
                                
 
                                Optional Exercises
                                
 
                                  
 
                                1. Make a decorator that will cache function return values depending on input parameters.
                                  2. 
-
                                2. Host the URL shortening app on any cloud provider.
                                  3. 
-
                                3. Setup monitoring using many of the tools available like catchpoint, datadog etc.
                                  4. 
-
                                4. Create a minimal flask-like framework on top of TCP sockets.
                                  5. 
+
                                5. Host the URL-shortening app on any cloud provider.
                                  6. 
+
                                6. Setup monitoring using many of the tools available like Catchpoint, Datadog, etc.
                                  7. 
+
                                7. Create a minimal Flask-like framework on top of TCP sockets.
                                  8. 
 
                                
 
                                Conclusion
                                
-
                                This module, in the first part, aims to make you more aware of the things that will happen when you choose python as your programming language and what happens when you run a python program. With the knowledge of how python handles things internally as objects, lot of seemingly magic things in python will start to make more sense.
                                
-
                                The second part will first explain how a framework like flask works using the existing knowledge of protocols like TCP and HTTP. It then touches the whole lifecycle of an application development lifecycle including the SRE parts of it. While the design and areas in architecture considered will not be exhaustive, it will give a good overview of things that are also important being an SRE and why they are important.
                                
+
                                This module, in the first part, aims to make you more aware of the things that will happen when you choose Python as your programming language and what happens when you run a Python program. With the knowledge of how Python handles things internally as objects, lot of seemingly magic things in Python will start to make more sense.
                                
+
                                The second part will first explain how a framework like Flask works using the existing knowledge of protocols like TCP and HTTP. It then touches the whole lifecycle of an application development lifecycle including the SRE parts of it. While the design and areas in architecture considered will not be exhaustive, it will give a good overview of things that are also important being an SRE and why they are important.
                                
                 
               
               
diff --git a/level101/python_web/url-shorten-app/index.html b/level101/python_web/url-shorten-app/index.html
index b7c230b..16b0020 100644
--- a/level101/python_web/url-shorten-app/index.html
+++ b/level101/python_web/url-shorten-app/index.html
@@ -2231,17 +2231,17 @@
                 
                 
                 
                                The URL Shortening App
                                
-
                                Let's build a very simple URL shortening app using flask and try to incorporate all aspects of the development process including the reliability aspects. We will not be building the UI and we will come up with a minimal set of API that will be enough for the app to function well.
                                
+
                                Let's build a very simple URL-shortening app using Flask and try to incorporate all aspects of the development process including the reliability aspects. We will not be building the UI and we will come up with a minimal set of API that will be enough for the app to function well.
                                
 
                                Design
                                
 
                                We don't jump directly to coding. First thing we do is gather requirements. Come up with an approach. Have the approach/design reviewed by peers. Evolve, iterate, document the decisions and tradeoffs. And then finally implement. While we will not do the full blown design document here, we will raise certain questions here that are important to the design.
                                
 
                                1. High Level Operations and API Endpoints
                                
-
                                Since it's a URL shortening app, we will need an API for generating the shorten link given an original link. And an API/Endpoint which will accept the shorten link and redirect to original URL. We are not including the user aspect of the app to keep things minimal. These two API should make app functional and usable by anyone.
                                
+
                                Since it's a URL-shortening app, we will need an API for generating the shorten link given an original link. And an API/Endpoint which will accept the shorten link and redirect to original URL. We are not including the user aspect of the app to keep things minimal. These two API should make app functional and usable by anyone.
                                
 
                                2. How to shorten?
                                
-
                                Given a url, we will need to generate a shortened version of it. One approach could be using random characters for each link. Another thing that can be done is to use some sort of hashing algorithm. The benefit here is we will reuse the same hash for the same link. ie: if lot of people are shortening https://www.linkedin.com they all will have the same value, compared to multiple entries in DB if chosen random characters.
                                
-
                                What about hash collisions? Even in random characters approach, though there is a less probability, hash collisions can happen. And we need to be mindful of them. In that case we might want to prepend/append the string with some random value to avoid conflict.
                                
+
                                Given a URL, we will need to generate a shortened version of it. One approach could be using random characters for each link. Another thing that can be done is to use some sort of hashing algorithm. The benefit here is we will reuse the same hash for the same link. Ie: if lot of people are shortening https://www.linkedin.com, they all will have the same value, compared to multiple entries in DB if chosen random characters.
                                
+
                                What about hash collisions? Even in random characters approach, though there is a less probability, hash collisions can happen. And we need to be mindful of them. In that case, we might want to prepend/append the string with some random value to avoid conflict.
                                
 
                                Also, choice of hash algorithm matters. We will need to analyze algorithms. Their CPU requirements and their characteristics. Choose one that suits the most.
                                
 
                                3. Is URL Valid?
                                
-
                                Given a URL to shorten, how do we verify if the URL is valid? Do we even verify or validate? One basic check that can be done is see if the URL matches a regex of a URL. To go even further we can try opening/visiting the URL. But there are certain gotchas here.
                                
+
                                Given a URL to shorten, how do we verify if the URL is valid? Do we even verify or validate? One basic check that can be done is see if the URL matches a regex of a URL. To go even further, we can try opening/visiting the URL. But there are certain gotchas here.
                                
 
                                  
 
                                1. We need to define success criteria. ie: HTTP 200 means it is valid.
                                  2. 
 
                                2. What if the URL is in private network?
                                  3. 
@@ -2250,10 +2250,9 @@
 
                                  4. Storage
                                  
 
                                  Finally, storage. Where will we store the data that we will generate over time? There are multiple database solutions available and we will need to choose the one that suits this app the most. Relational database like MySQL would be a fair choice but be sure to checkout School of SRE's SQL database section and NoSQL databases section for deeper insights into making a more informed decision.
                                  
 
                                  5. Other
                                  
-
                                  We are not accounting for users into our app and other possible features like rate limiting, customized links etc but it will eventually come up with time. Depending on the requirements, they too might need to get incorporated.
                                  
-
                                  The minimal working code is given below for reference but I'd encourage you to come up with your own.
                                  
+
                                  We are not accounting for users into our app and other possible features like rate limiting, customized links, etc. but it will eventually come up with time. Depending on the requirements, they too might need to get incorporated.
                                  
+
                                  The minimal working code is given below for reference, but I'd encourage you to come up with your own.
                                  
 
                                  from flask import Flask, redirect, request
-
 from hashlib import md5
 
 app = Flask("url_shortener")
diff --git a/level101/security/conclusion/index.html b/level101/security/conclusion/index.html
index 26e5690..4fe6620 100644
--- a/level101/security/conclusion/index.html
+++ b/level101/security/conclusion/index.html
@@ -2168,21 +2168,33 @@
 
                                  Other Resources
                                  
 
                                  Some books that would be a great resource
                                  
 
 
                                  Post Training asks/ Further Reading
                                  
 
                 
               
diff --git a/level101/security/fundamentals/index.html b/level101/security/fundamentals/index.html
index ffc6140..22705db 100644
--- a/level101/security/fundamentals/index.html
+++ b/level101/security/fundamentals/index.html
@@ -2352,33 +2352,35 @@
 
                                3. They have quite a big role in System design & hence are quite sometimes the first line of defence.
                                  4. 
 
                                4. SRE’s help in preventing bad design & implementations which can affect the overall security of the infrastructure.  
                                  5. 
 
                                5. Successfully designing, implementing, and maintaining systems requires a commitment to the full system lifecycle. This commitment is possible only when security and reliability are central elements in the architecture of systems.
                                  6. 
-
                                6. Core Pillars of Information Security :
                                  7. 
-
                                7. Confidentiality – only allow access to data for which the user is permitted
                                  8. 
-
                                8. Integrity – ensure data is not tampered or altered by unauthorized users
                                  9. 
 
                                9. 
-
                                  Availability – ensure systems and data are available to authorized users when they need it
                                  
-
                                  10. 
-
                                10. 
-
                                  Thinking like a Security Engineer
                                  
-
                                  11. 
-
                                11. 
-
                                  When starting a new application or re-factoring an existing application, you should consider each functional feature, and consider:
                                  
+
                                  Core Pillars of Information Security:
                                  
 
                                    
+
                                  • Confidentiality—only allow access to data for which the user is permitted
                                    • 
+
                                  • Integrity—ensure data is not tampered or altered by unauthorized users
                                    • 
+
                                  • Availability—ensure systems and data are available to authorized users when they need it
                                    • 
+
                                  
+
                                  12. 
+
                                12. 
+
                                  Thinking like a Security Engineer:
                                  
+
                                    
+
                                  • When starting a new application or re-factoring an existing application, you should consider each functional feature, and consider:
                                      
 
                                    • Is the process surrounding this feature as safe as possible? In other words, is this a flawed process?
                                      • 
 
                                    • If I were evil, how would I abuse this feature? Or more specifically failing to address how a feature can be abused can cause design flaws.
                                      • 
 
                                    • Is the feature required to be on by default? If so, are there limits or options that could help reduce the risk from this feature?
                                      • 
 
                                    
 
                                    • 
+
                                  
+
                                  13. 
 
                                13. 
 
                                  Security Principles By OWASP (Open Web Application Security Project)
                                  
 
                                  14. 
-
                                14. Minimize attack surface area :
                                    
+
                                  • Minimize attack surface area:
                                      
 
                                    • Every feature that is added to an application adds a certain amount of risk to the overall application. The aim of secure development is to reduce the overall risk by reducing the attack surface area.
                                      • 
 
                                    • For example, a web application implements online help with a search function. The search function may be vulnerable to SQL injection attacks. If the help feature was limited to authorized users, the attack likelihood is reduced. If the help feature’s search function was gated through centralized data validation routines, the ability to perform SQL injection is dramatically reduced. However, if the help feature was re-written to eliminate the search function (through a better user interface, for example), this almost eliminates the attack surface area, even if the help feature was available to the Internet at large.
                                      • 
 
                                    
 
                                    • 
 
                                  • Establish secure defaults:
                                      
-
                                    • There are many ways to deliver an “out of the box” experience for users. However, by default, the experience should be secure, and it should be up to the user to reduce their security – if they are allowed.
                                      • 
+
                                    • There are many ways to deliver an “out of the box” experience for users. However, by default, the experience should be secure, and it should be up to the user to reduce their security—if they are allowed.
                                      • 
 
                                    • For example, by default, password ageing and complexity should be enabled. Users might be allowed to turn these two features off to simplify their use of the application and increase their risk.
                                      • 
 
                                    • Default Passwords of routers, IoT devices should be changed
                                      • 
 
                                    
@@ -2397,19 +2399,19 @@
 
                                  • 
 
                                    Fail securely
                                    
 
                                      
-
                                    • Applications regularly fail to process transactions for many reasons. How they fail can determine if an application is secure or not.
                                      • 
+
                                    • Applications regularly fail to process transactions for many reasons. How they fail can determine if an application is secure or not.
+  
                                      
+  is_admin = true;
+  try {
+    code_which_may_fail();
+    is_admin = is_user_assigned_role("Adminstrator");
+  }
+  catch (Exception err) {
+    log.error(err.toString());
+  }
+  
                                      • 
+
                                    • If either codeWhichMayFail() or isUserInRole fails or throws an exception, the user is an admin by default. This is obviously a security risk.
                                      • 
 
                                    
-
                                    ```
                                    
-
                                    is_admin =  true;
-try {
-    code_which_may_faile();
-     is_admin = is_user_assigned_role("Adminstrator");
-}
-catch (Exception err) {
-log.error(err.toString());
-}
                                    
-
                                    ```
-- If either codeWhichMayFail() or isUserInRole fails or throws an exception, the user is an admin by default. This is obviously a security risk.
                                    
 
                                    • 
 
                                  • 
 
                                    Don’t trust services
                                    
@@ -2422,7 +2424,7 @@ log.error(err.toString());
 
                                  • Separation of duties
                                      
 
                                    • The key to fraud control is the separation of duties. For example, someone who requests a computer cannot also sign for it, nor should they directly receive the computer. This prevents the user from requesting many computers and claiming they never arrived.
                                      • 
 
                                    • Certain roles have different levels of trust than normal users. In particular, administrators are different from normal users. In general, administrators should not be users of the application.
                                      • 
-
                                    • For example, an administrator should be able to turn the system on or off, set password policy but shouldn’t be able to log on to the storefront as a super privileged user, such as being able to “buy” goods on behalf of other users.
                                      • 
+
                                    • For example, an administrator should be able to turn the system on or off, set password policy but shouldn't be able to log on to the storefront as a super privileged user, such as being able to "buy" goods on behalf of other users.
                                      • 
 
                                    
 
                                    • 
 
                                  • Avoid security by obscurity
                                      
@@ -2444,7 +2446,7 @@ log.error(err.toString());
 
 
                                    • Reliability & Security
                                        
 
                                      • Reliability and security are both crucial components of a truly trustworthy system, but building systems that are both reliable and secure is difficult. While the requirements for reliability and security share many common properties, they also require different design considerations. It is easy to miss the subtle interplay between reliability and security that can cause unexpected outcomes
                                        • 
-
                                      • Ex: A password management application failure was triggered by a reliability problem i.e poor load-balancing and load-shedding strategies and its recovery were later complicated by multiple measures (HSM mechanism which needs to be plugged into server racks, which works as an authentication & the HSM token supposedly locked inside a case.. & the problem can be further elongated ) designed to increase the security of the system.
                                        • 
+
                                      • Ex: A password management application failure was triggered by a reliability problem i.e poor load-balancing and load-shedding strategies and its recovery were later complicated by multiple measures (HSM mechanism which needs to be plugged into server racks, which works as an authentication & the HSM token supposedly locked inside a case.. & the problem can be further elongated) designed to increase the security of the system.
                                        • 
 
                                      
 
                                      • 
 
                                    
@@ -2465,7 +2467,7 @@ log.error(err.toString());
 
                                  
 
                                  OpenID/OAuth
                                  
 
                                  OpenID is an authentication protocol that allows us to authenticate users without using a local auth system. In such a scenario, a user has to be registered with an OpenID Provider and the same provider should be integrated with the authentication flow of your application. To verify the details, we have to forward the authentication requests to the provider. On successful authentication, we receive a success message and/or profile details with which we can execute the necessary flow.
                                  
-
                                  OAuth is an authorization mechanism that allows your application user access to a provider(Gmail/Facebook/Instagram/etc). On successful response, we (your application) receive a token with which the application can access certain APIs on behalf of a user. OAuth is convenient in case your business use case requires some certain user-facing APIs like access to Google Drive or sending tweets on your behalf. Most OAuth 2.0 providers can be used for pseudo authentication. Having said that, it can get pretty complicated if you are using multiple OAuth providers to authenticate users on top of the local authentication system.
                                  
+
                                  OAuth is an authorization mechanism that allows your application user access to a provider (Gmail/Facebook/Instagram/etc). On successful response, we (your application) receive a token with which the application can access certain APIs on behalf of a user. OAuth is convenient in case your business use case requires some certain user-facing APIs like access to Google Drive or sending tweets on your behalf. Most OAuth 2.0 providers can be used for pseudo authentication. Having said that, it can get pretty complicated if you are using multiple OAuth providers to authenticate users on top of the local authentication system.
                                  
 
                                  
 
                                  Cryptography
                                  
 
                                    
@@ -2493,14 +2495,14 @@ D(k,E(k,m)) = m
 
                                  15. 
 
                                  Stream Ciphers:
                                  
 
                                    
-
                                  • The message is broken into characters or bits and enciphered with a key or keystream(should be random and generated independently of the message stream) that is as long as the plaintext bitstream.
                                    • 
+
                                  • The message is broken into characters or bits and enciphered with a key or keystream (should be random and generated independently of the message stream) that is as long as the plaintext bitstream.
                                    • 
 
                                  • If the keystream is random, this scheme would be unbreakable unless the keystream was acquired, making it unconditionally secure. The keystream must be provided to both parties in a secure way to prevent its release.
                                    • 
 
                                  
 
                                  Block Ciphers:
                                  
 
                                    
-
                                  • Block ciphers — process messages in blocks, each of which is then encrypted or decrypted.
                                    • 
+
                                  • Block ciphers—process messages in blocks, each of which is then encrypted or decrypted.
                                    • 
 
                                  • 
-
                                    A block cipher is a symmetric cipher in which blocks of plaintext are treated as a whole and used to produce ciphertext blocks. The block cipher takes blocks that are b bits long and encrypts them to blocks that are also b bits long. Block sizes are typically 64 or 128 bits long. 
                                    
+
                                    A block cipher is a symmetric cipher in which blocks of plaintext are treated as a whole and used to produce ciphertext blocks. The block cipher takes blocks that are b bits long and encrypts them to blocks that are also b bits long. Block sizes are typically 64 or 128 bits long. 
                                    
 
                                    image5
 image6
                                    
 
                                    • 
@@ -2508,7 +2510,7 @@ D(k,E(k,m)) = m
 
                                    Encryption
                                    
 
                                      
 
                                    • Secret Key (Symmetric Key): the same key is used for encryption and decryption
                                      • 
-
                                    • Public Key (Asymmetric Key) in an asymmetric, the encryption and decryption keys are different but related. The encryption key is known as the public key and the decryption key is known as the private key. The public and private keys are known as a key pair.
                                      • 
+
                                    • Public Key (Asymmetric Key): in an asymmetric, the encryption and decryption keys are different but related. The encryption key is known as the public key and the decryption key is known as the private key. The public and private keys are known as a key pair.
                                      • 
 
                                    
 
                                    Symmetric Key Encryption
                                    
 
                                    DES
                                    
@@ -2556,7 +2558,7 @@ D(k,E(k,m)) = m
 
                                    NOTE: In terms of TLS key exchange, this is the common approach.
                                    
 
                                    Diffie-Hellman
                                    
 
                                      
-
                                    • The protocol has two system parameters, p and g. They are both public and may be used by everybody. Parameter p is a prime number, and parameter g (usually called a generator) is an integer that is smaller than p, but with the following property: For every number n between 1 and p – 1 inclusive, there is a power k of g such that n = gk mod p.
                                      • 
+
                                    • The protocol has two system parameters, p and g. They are both public and may be used by everybody. Parameter p is a prime number, and parameter g (usually called a generator) is an integer that is smaller than p, but with the following property: For every number, n between 1 and p – 1 inclusive, there is a power k of g such that n = gk mod p.
                                      • 
 
                                    • Diffie Hellman algorithm is an asymmetric algorithm used to establish a shared secret for a symmetric key algorithm. Nowadays most of the people use hybrid cryptosystem i.e, a combination of symmetric and asymmetric encryption. Asymmetric Encryption is used as a technique in key exchange mechanism to share a secret key and after the key is shared between sender and receiver, the communication will take place using symmetric encryption. The shared secret key will be used to encrypt the communication.
                                      • 
 
                                    • Refer: https://medium.com/@akhigbemmanuel/what-is-the-diffie-hellman-key-exchange-algorithm-84d60025a30d
                                      • 
 
                                    
@@ -2578,10 +2580,12 @@ D(k,E(k,m)) = m
 
 
                                  • 
 
                                    More:
                                    
-
                                    • 
+
+
+
                                  
 
                                  MD5
                                  
 
                                    
 
                                  • MD5 is a one-way function with which it is easy to compute the hash from the given input data, but it is unfeasible to compute input data given only a hash.
                                    • 
@@ -2651,8 +2655,10 @@ D(k,E(k,m)) = m
 
                                  • a client: A user/ a service
                                    • 
 
                                  • 
 
                                    a server: Kerberos protected hosts reside
                                    
-
                                    image10
-  - a Key Distribution Center (KDC), which acts as the trusted third-party authentication service.
                                    
+
                                    image10
                                    
+
                                    • 
+
                                  • 
+
                                    a Key Distribution Center (KDC), which acts as the trusted third-party authentication service.
                                    
 
                                    • 
 
                                  
 
                                  The KDC includes the following two servers:
                                  
@@ -2664,23 +2670,24 @@ D(k,E(k,m)) = m
 
 
                          
 
                          Certificate Chain
                          
-
                          The first part of the output of the OpenSSL command shows three certificates numbered 0, 1, and 2(not 2 anymore). Each certificate has a subject, s, and an issuer, i. The first certificate, number 0, is called the end-entity certificate. The subject line tells us it’s valid for any subdomain of google.com because its subject is set to *.google.com. 
                          
-
                          $ openssl s_client -connect www.google.com:443 -CApath /etc/ssl/certs
+
                          The first part of the output of the OpenSSL command shows three certificates numbered 0, 1, and 2 (not 2 anymore). Each certificate has a subject, s, and an issuer, i. The first certificate, number 0, is called the end-entity certificate. The subject line tells us it’s valid for any subdomain of google.com because its subject is set to *.google.com. 
                          
+
                          $ openssl s_client -connect www.google.com:443 -CApath /etc/ssl/certs
 CONNECTED(00000005)
 depth=2 OU = GlobalSign Root CA - R2, O = GlobalSign, CN = GlobalSign
 verify return:1
 depth=1 C = US, O = Google Trust Services, CN = GTS CA 1O1
 verify return:1
 depth=0 C = US, ST = California, L = Mountain View, O = Google LLC, CN = www.google.com
-verify return:1
----
+verify return:1`
+`---
 Certificate chain
  0 s:/C=US/ST=California/L=Mountain View/O=Google LLC/CN=www.google.com
    i:/C=US/O=Google Trust Services/CN=GTS CA 1O1
  1 s:/C=US/O=Google Trust Services/CN=GTS CA 1O1
    i:/OU=GlobalSign Root CA - R2/O=GlobalSign/CN=GlobalSign
----
-Server certificate
                          
+---
+
                          
+
                          Server certificate
                          
 
                            
 
                          • The issuer line indicates it’s issued by Google Internet Authority G2, which also happens to be the subject of the second certificate, number 1
                            • 
 
                          • What the OpenSSL command line doesn’t show here is the trust store that contains the list of CA certificates trusted by the system OpenSSL runs on.
                            • 
@@ -2696,14 +2703,14 @@ Certificate chain
 
                              
 
                            1. The client sends a HELLO message to the server with a list of protocols and algorithms it supports.
                              2. 
 
                            2. The server says HELLO back and sends its chain of certificates. Based on the capabilities of the client, the server picks a cipher suite.
                              3. 
-
                            3. If the cipher suite supports ephemeral key exchange, like ECDHE does(ECDHE is an algorithm known as the Elliptic Curve Diffie-Hellman Exchange), the server and the client negotiate a pre-master key with the Diffie-Hellman algorithm. The pre-master key is never sent over the wire.
                              4. 
+
                            4. If the cipher suite supports ephemeral key exchange, like ECDHE does (ECDHE is an algorithm known as the Elliptic Curve Diffie-Hellman Exchange), the server and the client negotiate a pre-master key with the Diffie-Hellman algorithm. The pre-master key is never sent over the wire.
                              5. 
 
                            5. The client and server create a session key that will be used to encrypt the data transiting through the connection.
                              6. 
 
                            
-
                            At the end of the handshake, both parties possess a secret session key used to encrypt data for the rest of the connection. This is what OpenSSL refers to as Master-Key
                            
+
                            At the end of the handshake, both parties possess a secret session key used to encrypt data for the rest of the connection. This is what OpenSSL refers to as Master-Key.
                            
 
                            NOTE
                            
 
                              
-
                            • There are 3 versions of TLS , TLS 1.0, 1.1 & 1.2
                              • 
-
                            • TLS 1.0 was released in 1999, making it a nearly two-decade-old protocol. It has been known to be vulnerable to attacks—such as BEAST and POODLE—for years, in addition to supporting weak cryptography, which doesn’t keep modern-day connections sufficiently secure.
                              • 
+
                            • There are 3 versions of TLS, TLS 1.0, 1.1 & 1.2
                              • 
+
                            • TLS 1.0 was released in 1999, making it a nearly two-decade-old protocol. It has been known to be vulnerable to attacks—such as BEAST and POODLE—for years, in addition to supporting weak cryptography, which doesn’t keep modern-day connections sufficiently secure.
                              • 
 
                            • TLS 1.1 is the forgotten “middle child.” It also has bad cryptography like its younger sibling. In most software, it was leapfrogged by TLS 1.2 and it’s rare to see TLS 1.1 used.
                              • 
 
                            
 
                            “Perfect” Forward Secrecy
                            
@@ -2712,9 +2719,11 @@ Certificate chain
 
                          • In a non-ephemeral key exchange, the client sends the pre-master key to the server by encrypting it with the server’s public key. The server then decrypts the pre-master key with its private key. If at a later point in time, the private key of the server is compromised, an attacker can go back to this handshake, decrypt the pre-master key, obtain the session key, and decrypt the entire traffic. Non-ephemeral key exchanges are vulnerable to attacks that may happen in the future on recorded traffic. And because people seldom change their password, decrypting data from the past may still be valuable for an attacker.
                            • 
 
                          • An ephemeral key exchange like DHE, or its variant on elliptic curve, ECDHE, solves this problem by not transmitting the pre-master key over the wire. Instead, the pre-master key is computed by both the client and the server in isolation, using nonsensitive information exchanged publicly. Because the pre-master key can’t be decrypted later by an attacker, the session key is safe from future attacks: hence, the term perfect forward secrecy.
                            • 
 
                          • Keys are changed every X blocks along the stream. That prevents an attacker from simply sniffing the stream and applying brute force to crack the whole thing. "Forward secrecy" means that just because I can decrypt block M, does not mean that I can decrypt block Q
                            • 
-
                          • Downside:
                            • 
+
                          • Downside:
                              
 
                            • The downside to PFS is that all those extra computational steps induce latency on the handshake and slow the user down. To avoid repeating this expensive work at every connection, both sides cache the session key for future use via a technique called session resumption. This is what the session-ID and TLS ticket are for: they allow a client and server that share a session ID to skip over the negotiation of a session key, because they already agreed on one previously, and go directly to exchanging data securely.
                              • 
 
                            
+
                            • 
+
                          
                 
               
               
diff --git a/level101/security/intro/index.html b/level101/security/intro/index.html
index 8766725..48454e4 100644
--- a/level101/security/intro/index.html
+++ b/level101/security/intro/index.html
@@ -2201,7 +2201,7 @@
 
                          • 
 
    
 
    What to expect from this course
    
-
    The course covers fundamentals of information security along with touching on subjects of system security, network & web security. This course aims to get you familiar with the basics of information security in day to day operations & then as an SRE develop the mindset of ensuring that security takes a  front-seat while developing solutions. The course also serves as an introduction to common risks and best practices along with practical ways to find out vulnerable systems and loopholes which might become compromised if not secured.
    
+
    The course covers fundamentals of information security along with touching on subjects of system security, network & web security. This course aims to get you familiar with the basics of information security in day-to-day operations and then as an SRE develop the mindset of ensuring that security takes a front-seat while developing solutions. The course also serves as an introduction to common risks and best practices along with practical ways to find out vulnerable systems and loopholes which might become compromised if not secured.
    
 
    What is not covered under this course
    
 
    The courseware is not an ethical hacking workshop or a very deep dive into the fundamentals of the problems. The course does not deal with hacking or breaking into systems but rather an approach on how to ensure you don’t get into those situations and also to make you aware of different ways a system can be compromised.
    
 
    Course Contents
    
diff --git a/level101/security/network_security/index.html b/level101/security/network_security/index.html
index 76ed874..a0606ea 100644
--- a/level101/security/network_security/index.html
+++ b/level101/security/network_security/index.html
@@ -1285,10 +1285,10 @@
       
         
  • 
   
-    Network  Perimeter Security
+    Network Perimeter Security
   
   
-    
    • 
-
+
-
+
-
+
-
+
-
+
-
+
-
+
@@ -2376,38 +2376,38 @@
 
    
 
 
    
 
 
 
 
 
 
 
 
 
 
 
    
 
 
    99%(Two Nines)99% (Two Nines)
 3.65 days
 7.31 hours
 1.68 hours
 14.40 minutes
 
    
 
    99.5%(Two and a half Nines)99.5% (Two and a half Nines)
 1.83 days
 3.65 hours
 50.40 minutes
 7.20 minutes
 
    
 
    99.9%(Three Nines)99.9% (Three Nines)
 8.77 hours
 43.83 minutes
 10.08 minutes
 1.44 minutes
 
    
 
    99.95%(Three and a half Nines)99.95% (Three and a half Nines)
 4.38 hours
 21.92 minutes
 5.04 minutes
 43.20 seconds
 
    
 
    99.99%(Four Nines)99.99% (Four Nines)
 52.60 minutes
 4.38 minutes
 1.01 minutes
 8.64 seconds
 
    
 
    99.995%(Four and a half Nines)99.995% (Four and a half Nines)
 26.30 minutes
 2.19 minutes
 30.24 seconds
 4.32 seconds
 
    
 
    99.999%(Five Nines)99.999% (Five Nines)
 5.26 minutes
 26.30 seconds
 6.05 seconds
    
 
    Refer
    
 
 
    HA - Availability Serial Components
    
-
    A System with components is operating in the series If the failure of a part leads to the combination becoming inoperable.
    
+
    A System with components is operating in the series if the failure of a part leads to the combination becoming inoperable.
    
 
    For example, if LB in our architecture fails, all access to app tiers will fail. LB and app tiers are connected serially.
    
-
    The combined availability of the system is the product of individual components availability
    
+
    The combined availability of the system is the product of individual components availability:
    
 
    A = Ax x Ay x …..
    
 
    Refer
    
 
 
    HA - Availability Parallel Components
    
-
    A System with components is operating in parallel If the failure of a part leads to the other part taking over the operations of the failed part.
    
-
    If we have more than one LB and if the rest of the LBs can take over the traffic during one LB failure then LBs are operating in parallel
    
+
    A System with components is operating in parallel if the failure of a part leads to the other part taking over the operations of the failed part.
    
+
    If we have more than one LB and if the rest of the LBs can take over the traffic during one LB failure, then LBs are operating in parallel.
    
 
    The combined availability of the system is
    
 
    A = 1 - ( (1-Ax) x (1-Ax) x ….. )
    
 
    Refer
    
 
 
    HA - Core Principles
    
 
    Elimination of single points of failure (SPOF) This means adding redundancy to the system so that the failure of a component does not mean failure of the entire system.
    
 
    Reliable crossover In redundant systems, the crossover point itself tends to become a single point of failure. Reliable systems must provide for reliable crossover.
    
-
    Detection of failures as they occur If the two principles above are observed, then a user may never see a failure 
    
+
    Detection of failures as they occur If the two principles above are observed, then a user may never see a failure. 
    
 
    Refer
    
 
 
    HA - SPOF
    
 
    WHAT: Never implement and always eliminate single points of failure.
    
 
    WHEN TO USE: During architecture reviews and new designs.
    
-
    HOW TO USE: Identify single instances on architectural diagrams. Strive for active/active configurations. At the very least we should have a standby to take control when active instances fail.
    
+
    HOW TO USE: Identify single instances on architectural diagrams. Strive for active/active configurations. At the very least, we should have a standby to take control when active instances fail.
    
 
    WHY: Maximize availability through multiple instances.
    
 
    KEY TAKEAWAYS: Strive for active/active rather than active/passive solutions. Use load balancers to balance traffic across instances of a service. Use control services with active/passive instances for patterns that require singletons.
    
 
    HA - Reliable Crossover
    
@@ -2415,16 +2415,16 @@
 
    WHEN TO USE: During architecture reviews, failure modeling, and designs.
    
 
    HOW TO USE: Identify how available a system is during the crossover and ensure it is within acceptable limits. 
    
 
    WHY: Maximize availability and ensure data handling semantics are preserved.  
    
-
    KEY TAKEAWAYS: Strive for active/active rather than active/passive solutions, they have a lesser risk of cross over being unreliable. Use LB and the right load balancing methods to ensure reliable failover. Model and build your data systems to ensure data is correctly handled when crossover happens. Generally, DB systems follow active/passive semantics for writes. Masters accept writes and when the master goes down, the follower is promoted to master(active from being passive) to accept writes. We have to be careful here that the cutover never introduces more than one master. This problem is called a split brain.
    
+
    KEY TAKEAWAYS: Strive for active/active rather than active/passive solutions, they have a lesser risk of cross over being unreliable. Use LB and the right load-balancing methods to ensure reliable failover. Model and build your data systems to ensure data is correctly handled when crossover happens. Generally, DB systems follow active/passive semantics for writes. Masters accept writes and when the master goes down, the follower is promoted to master (active from being passive) to accept writes. We have to be careful here that the cutover never introduces more than one master. This problem is called a split brain.
    
 
    Applications in SRE role
    
 
      
 
    1. SRE works on deciding an acceptable SLA and make sure the system is available to achieve the SLA
      2. 
 
    2. SRE is involved in architecture design right from building the data center to make sure the site is not affected by a network switch, hardware, power, or software failures
      3. 
 
    3. SRE also run mock drills of failures to see how the system behaves in uncharted territory and comes up with a plan to improve availability if there are misses. 
-https://engineering.linkedin.com/blog/2017/11/resilience-engineering-at-linkedin-with-project-waterbear
      4. 
+https://engineering.linkedin.com/blog/2017/11/resilience-engineering-at-linkedin-with-project-waterbear
 
    
-
    Post our understanding about HA, our architecture diagram looks something like this below
-HA Block Diagram
    
+
    Post our understanding about HA, our architecture diagram looks something like this below:
    
+
    HA Block Diagram
    
                 
               
               
diff --git a/level101/systems_design/conclusion/index.html b/level101/systems_design/conclusion/index.html
index bb74b69..6d9c846 100644
--- a/level101/systems_design/conclusion/index.html
+++ b/level101/systems_design/conclusion/index.html
@@ -2105,7 +2105,7 @@
                 
                 
                 
    Conclusion
    
-
    Armed with these principles, we hope the course will give a fresh perspective to design software systems. It might be over-engineering to get all this on day zero. But some are really important from day 0 like eliminating single points of failure, making scalable services by just increasing replicas. As a bottleneck is reached, we can split code by services, shard data to scale. As the organization matures, bringing in chaos engineering to measure how systems react to failure will help in designing robust software systems.
    
+
    Armed with these principles, we hope the course will give a fresh perspective to design software systems. It might be over-engineering to get all this on day zero. But some are really important from day 0 like eliminating single points of failure, making scalable services by just increasing replicas. As a bottleneck is reached, we can split code by services, shard data to scale. As the organization matures, bringing in chaos engineering to measure how systems react to failure will help in designing robust software systems.
    
                 
               
               
diff --git a/level101/systems_design/fault-tolerance/index.html b/level101/systems_design/fault-tolerance/index.html
index 00bdf79..c53485a 100644
--- a/level101/systems_design/fault-tolerance/index.html
+++ b/level101/systems_design/fault-tolerance/index.html
@@ -1063,10 +1063,10 @@
       
         
  • 
   
-    Fault Tolerance - Failure Metrics
+    Fault Tolerance: Failure Metrics
   
   
-