| + | +Performance + | +Scalability + | +Flexibility + | +Complexity + | +Functionality + | +
| Key Value + | +high + | +high + | +high + | +none + | +Variable + | +
| Document stores + | +high + | +Variable (high) + | +high + | +low + | +Variable (low) + | +
| Column DB + | +high + | +high + | +moderate + | +low + | +minimal + | +
| Graph + | +Variable + | +Variable + | +high + | +high + | +Graph theory + | +
| + | +SQL Databases + | +NoSQL Databases + | +
| Data Storage Model + | +Tables with fixed rows and columns + | +Document: JSON documents, Key-value: key-value pairs, Wide-column: tables with rows and dynamic columns, Graph: nodes and edges + | +
| Primary Purpose + | +General purpose + | +Document: general purpose, Key-value: large amounts of data with simple lookup queries, Wide-column: large amounts of data with predictable query patterns, Graph: analyzing and traversing relationships between connected data + | +
| Schemas + | +Rigid + | +Flexible + | +
| Scaling + | +Vertical (scale-up with a larger server) + | +Horizontal (scale-out across commodity servers) + | +
| Multi-Record ACID Transactions + | +Supported + | +Most do not support multi-record ACID transactions. However, some—like MongoDB—do. + | +
| Joins + | +Typically required + | +Typically not required + | +
| Data to Object Mapping + | +Requires ORM (object-relational mapping) + | +Many do not require ORMs. Document DB documents map directly to data structures in most popular programming languages. + | +
| Choice + | +Traits + | +Examples + | +
| Consistency + Availability
+ +(Forfeit Partitions) + |
+ 2-phase commits
+ +Cache invalidation protocols + |
+ Single-site databases Cluster databases
+ +LDAP + +xFS file system + |
+
| Consistency + Partition tolerance
+ + (Forfeit Availability) + |
+ Pessimistic locking
+ +Make minority partitions unavailable + |
+ Distributed databases Distributed locking Majority protocols + | +
| Availability + Partition tolerance (Forfeit Consistency) + | +expirations/leases
+ +conflict resolution optimistic + |
+ DNS
+ +Web caching + |
+
+ + + + + + + +
Vector clocks illustration
+ +Vector clocks have the following advantages over other conflict resolution mechanism + + + +1. No dependency on synchronized clocks +2. No total ordering of revision nos required for casual reasoning + +No need to store and maintain multiple versions of the data on different nodes.** ** + + +### Partitioning + +When the amount of data crosses the capacity of a single node, we need to think of splitting data, creating replicas for load balancing & disaster recovery. Depending on how dynamic the infrastructure is, we have a few approaches that we can take. + + + +1. **Memory cached** + + These are partitioned in-memory databases that are primarily used for transient data. These databases are generally used as a front for traditional RDBMS. Most frequently used data is replicated from a rdbms into a memory database to facilitate fast queries and to take the load off from backend DB’s. A very common example is memcached or couchbase. + +2. **Clustering** + + Traditional cluster mechanisms abstract away the cluster topology from clients. A client need not know where the actual data is residing and which node it is talking to. Clustering is very commonly used in traditional RDBMS where it can help scaling the persistent layer to a certain extent. + +3. **Separating reads from writes** + + In this method, you will have multiple replicas hosting the same data. The incoming writes are typically sent to a single node (Leader) or multiple nodes (multi-Leader), while the rest of the replicas (Follower) handle reads requests. The leader replicates writes asynchronously to all followers. However the write lag can’t be completely avoided. Sometimes a leader can crash before it replicates all the data to a follower. When this happens, a follower with the most consistent data can be turned into a leader. As you can realize now, it is hard to enforce full consistency in this model. You also need to consider the ratio of read vs write traffic. This model won’t make sense when writes are higher than reads. The replication methods can also vary widely. Some systems do a complete transfer of state periodically, while others use a delta state transfer approach. You could also transfer the state by transferring the operations in order. The followers can then apply the same operations as the leader to catch up. + +4. **Sharding** + + Sharing refers to dividing data in such a way that data is distributed evenly (both in terms of storage & processing power) across a cluster of nodes. It can also imply data locality, which means similar & related data is stored together to facilitate faster access. A shard in turn can be further replicated to meet load balancing or disaster recovery requirements. A single shard replica might take in all writes (single leader) or multiple replicas can take writes (multi-leader). Reads can be distributed across multiple replicas. Since data is now distributed across multiple nodes, clients should be able to consistently figure out where data is hosted. We will look at some of the common techniques below. The downside of sharding is that joins between shards is not possible. So an upstream/downstream application has to aggregate the results from multiple shards. + ++ + + + + +
Sharding example
+ + +### Hashing + +A hash function is a function that maps one piece of data—typically describing some kind of object, often of arbitrary size—to another piece of data, typically an integer, known as _hash code_, or simply _hash_. In a partitioned database, it is important to consistently map a key to a server/replica. + +For ex: you can use a very simple hash as a modulo function. + + + _p = k mod n_ + +Where + + + p -> partition, + + + k -> primary key + + + n -> no of nodes + +The downside of this simple hash is that, whenever the cluster topology changes, the data distribution also changes. When you are dealing with memory caches, it will be easy to distribute partitions around. Whenever a node joins/leaves a topology, partitions can reorder themselves, a cache miss can be re-populated from backend DB. However when you look at persistent data, it is not possible as the new node doesn’t have the data needed to serve it. This brings us to consistent hashing. + + +#### Consistent Hashing + +Consistent hashing is a distributed hashing scheme that operates independently of the number of servers or objects in a distributed _hash table_ by assigning them a position on an abstract circle, or _hash ring_. This allows servers and objects to scale without affecting the overall system. + +Say that our hash function h() generates a 32-bit integer. Then, to determine to which server we will send a key k, we find the server s whose hash h(s) is the smallest integer that is larger than h(k). To make the process simpler, we assume the table is circular, which means that if we cannot find a server with a hash larger than h(k), we wrap around and start looking from the beginning of the array. + ++ + + + + +
Consistent hashing illustration
+ +In consistent hashing when a server is removed or added then only the keys from that server are relocated. For example, if server S3 is removed then, all keys from server S3 will be moved to server S4 but keys stored on server S4 and S2 are not relocated. But there is one problem, when server S3 is removed then keys from S3 are not equally distributed among remaining servers S4 and S2. They are only assigned to server S4 which increases the load on server S4. + +To evenly distribute the load among servers when a server is added or removed, it creates a fixed number of replicas ( known as virtual nodes) of each server and distributes it along the circle. So instead of server labels S1, S2 and S3, we will have S10 S11…S19, S20 S21…S29 and S30 S31…S39. The factor for a number of replicas is also known as _weight_, depending on the situation. + + + + +All keys which are mapped to replicas Sij are stored on server Si. To find a key we do the same thing, find the position of the key on the circle and then move forward until you find a server replica. If the server replica is Sij then the key is stored in server Si. + +Suppose server S3 is removed, then all S3 replicas with labels S30 S31 … S39 must be removed. Now the objects keys adjacent to S3X labels will be automatically re-assigned to S1X, S2X and S4X. All keys originally assigned to S1, S2 & S4 will not be moved. + +Similar things happen if we add a server. Suppose we want to add a server S5 as a replacement of S3 then we need to add labels S50 S51 … S59. In the ideal case, one-fourth of keys from S1, S2 and S4 will be reassigned to S5. + +When applied to persistent storages, further issues arise: if a node has left the scene, data stored on this node becomes unavailable, unless it has been replicated to other nodes before; in the opposite case of a new node joining the others, adjacent nodes are no longer responsible for some pieces of data which they still store but not get asked for anymore as the corresponding objects are no longer hashed to them by requesting clients. In order to address this issue, a replication factor (r) can be introduced. + +Introducing replicas in a partitioning scheme—besides reliability benefits—also makes it possible to spread workload for read requests that can go to any physical node responsible for a requested piece of data. Scalability doesn’t work if the clients have to decide between multiple versions of the dataset, because they need to read from a quorum of servers which in turn reduces the efficiency of load balancing. + + + + +### Quorum + +Quorum is the minimum number of nodes in a cluster that must be online and be able to communicate with each other. If any additional node failure occurs beyond this threshold, the cluster will stop running. + + + + + +To attain a quorum, you need a majority of the nodes. Commonly it is (N/2 + 1), where N is the total no of nodes in the system. For ex, + +In a 3 node cluster, you need 2 nodes for a majority, + +In a 5 node cluster, you need 3 nodes for a majority, + +In a 6 node cluster, you need 4 nodes for a majority. + ++ + + + +
Quorum example
+ + + +Network problems can cause communication failures among cluster nodes. One set of nodes might be able to communicate together across a functioning part of a network but not be able to communicate with a different set of nodes in another part of the network. This is known as split brain in cluster or cluster partitioning. + +Now the partition which has quorum is allowed to continue running the application. The other partitions are removed from the cluster. + +Eg: In a 5 node cluster, consider what happens if nodes 1, 2, and 3 can communicate with each other but not with nodes 4 and 5. Nodes 1, 2, and 3 constitute a majority, and they continue running as a cluster. Nodes 4 and 5, being a minority, stop running as a cluster. If node 3 loses communication with other nodes, all nodes stop running as a cluster. However, all functioning nodes will continue to listen for communication, so that when the network begins working again, the cluster can form and begin to run. + +Below diagram demonstrates Quorum selection on a cluster partitioned into two sets. + ++ + + + +**
Cluster Quorum example
** + diff --git a/courses/git/branches.md b/courses/git/branches.md index 92e07e8..856db58 100644 --- a/courses/git/branches.md +++ b/courses/git/branches.md @@ -83,7 +83,7 @@ Above tree structure should make things clear. Notice a clear branch/fork on com ## 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 will pull the latest code from upstream (eg: GitHub). Then you need to merge your code from `b1` into master. And 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: @@ -96,7 +96,7 @@ spatel1-mn1:school-of-sre spatel1$ git log --oneline --graph --all * df2fb7a adding file 1 ``` -**Option 1: Directly merge the branch.** Merging the branch b1 into master will result in a new merge commit which 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. ```bash spatel1-mn1:school-of-sre spatel1$ git merge b1 diff --git a/courses/git/git-basics.md b/courses/git/git-basics.md index f89dc33..be8df86 100644 --- a/courses/git/git-basics.md +++ b/courses/git/git-basics.md @@ -1,6 +1,6 @@ -# School Of SRE: Git +# Git -## Pre - Reads +## Prerequisites 1. Have Git installed [https://git-scm.com/downloads](https://git-scm.com/downloads) 2. Have taken any git high level tutorial or following LinkedIn learning courses @@ -8,26 +8,26 @@ - [https://www.linkedin.com/learning/git-branches-merges-and-remotes/](https://www.linkedin.com/learning/git-branches-merges-and-remotes/) - [The Official Git Docs](https://git-scm.com/doc) -## What to expect from this training +## 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, 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 training +## What is not covered under this course Advanced usage and specifics of internal implementation details of Git. -## Training Content +## Course Content ### Table of Contents - 1. Git Basics - 2. Working with Branches - 3. Git with Github - 4. Hooks + 1. [Git Basics](https://linkedin.github.io/school-of-sre/git/git-basics/#git-basics) + 2. [Working with Branches](https://linkedin.github.io/school-of-sre/git/branches/) + 3. [Git with Github](https://linkedin.github.io/school-of-sre/git/github-hooks/#git-with-github) + 4. [Hooks](https://linkedin.github.io/school-of-sre/git/github-hooks/#hooks) ## Git Basics -Though you might be aware already, let's revisit why we need a version control system. As the project grows and multiple developers start working on it, an efficient method for collaboration is warranted. Git helps the team collaborate easily and also maintains history of the changes happened with the codebase. +Though you might be aware already, let's revisit why we need a version control system. As the project grows and multiple developers start working on it, an efficient method for collaboration is warranted. Git helps the team collaborate easily and also maintains the history of the changes happening with the codebase. ### Creating a Git Repo @@ -92,7 +92,7 @@ spatel1-mn1:school-of-sre spatel1$ git commit -m "adding file 1" create mode 100644 file1.txt ``` -Notice how after adding the file, git status says `Changes to be commited:`. 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 messaged via `-m`. ### More About a Commit diff --git a/courses/git/github-hooks.md b/courses/git/github-hooks.md index a16a9e2..db78e76 100644 --- a/courses/git/github-hooks.md +++ b/courses/git/github-hooks.md @@ -1,4 +1,4 @@ -## Git with Github +# 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. diff --git a/courses/img/favicon.ico b/courses/img/favicon.ico new file mode 100644 index 0000000..0090cbb Binary files /dev/null and b/courses/img/favicon.ico differ diff --git a/courses/img/sos.png b/courses/img/sos.png new file mode 100644 index 0000000..584c1b4 Binary files /dev/null and b/courses/img/sos.png differ diff --git a/courses/index.md b/courses/index.md index 5ac001b..28d24aa 100644 --- a/courses/index.md +++ b/courses/index.md @@ -1 +1,25 @@ -Hello, World!!! +# School of SRE + +Early 2019, we started visiting campuses to recruit the brightest minds to ensure LinkedIn and all the services that it is composed of is always available for everyone. This function at Linkedin falls in the purview of the Site Reliability Engineering team and Site Reliability Engineers ( SRE ) who are Software Engineers who specialize in reliability. SREs apply the principles of computer science and engineering to the design and development of computer systems: generally, large distributed ones. + +As we continued on this journey we started getting a lot of questions from these campuses on what exactly site engineering roll entails? and, how could someone learn the skills and the disciplines involved to become a successful site engineer? Fast forward a few months, and a few of these campus students had joined LinkedIn either as Interns or as full time engineers to become a part of the Site Engineering team, we also had a few lateral hires who joined our organization who were not from a traditional SRE background. That's when a few of us got together and started to think about how we can on board new new graduate engineers to the site engineering team. + +There is a vast amount of resources scattered throughout the web on what are the roles and responsibilities of an SREs, how to monitor site health, handling incidents, maintain SLO/SLI etc. But there are very few resources out there guiding someone on what all basic skill sets one has to acquire as a beginner. Because of the lack of these resources we felt that individuals are having a tough time getting into open positions in the industry. We created School Of SRE as a starting point for anyone wanting to build their career in the role of SRE. + +In this course we are focusing on building strong foundational skills. The course is structured in a way to provide more real life examples and how learning each of the topics can play a bigger role in your day to day SRE life. Currently we are covering the following topics under the School Of SRE: + +- Fundamentals Series + - [Linux Basics](https://linkedin.github.io/school-of-sre/linux_basics/intro/) + - [Git](https://linkedin.github.io/school-of-sre/git/git-basics/) + - [Linux Networking](https://linkedin.github.io/school-of-sre/linux_networking/intro/) +- [Python and Web](https://linkedin.github.io/school-of-sre/python_web/intro/) +- Data + - Relational databases (MySQL) + - NoSQL concepts + - [Big Data](https://linkedin.github.io/school-of-sre/big_data/intro/) +- [Systems Design](https://linkedin.github.io/school-of-sre/systems_design/intro/) +- [Security](https://linkedin.github.io/school-of-sre/security/intro/) + +We believe continuous learning will help in acquiring deeper knowledge and competencies in order to expand your skill sets, every module has added reference which could be a guide for further learning. Our hope is that by going through these modules we should be able build the essential skills required for a Site Reliability Engineer. + +At Linkedin we are using this curriculum for onboarding our non-traditional hires and new college grads to the SRE role. We had multiple rounds of successful onboarding experience with the new members and helped them to be productive in a very short period of time. This motivated us to opensource these contents for helping other organisations onboarding new engineers to the role and individuals to get into the role. We realise that the initial content we created is just a starting point and our hope is that the community can help in the journey refining and extending the contents. diff --git a/courses/linux_basics/command_line_basics.md b/courses/linux_basics/command_line_basics.md new file mode 100644 index 0000000..33294b0 --- /dev/null +++ b/courses/linux_basics/command_line_basics.md @@ -0,0 +1,445 @@ +# 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](https://repl.it/languages/bash) 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 +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 +for manual pages. + +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. +These directories in turn can either store system files or application +files or user related files. + + + + bin | The executable program of most commonly used commands reside in bin directory + sbin | This directory contains programs used for system administration. + home | This directory contains user related files and directories. + lib | This directory contains all the library files + etc | This directory contains all the system configuration files + proc | This directory contains files related to the running processes on the system + dev | This directory contains files related to devices on the system + mnt | This directory contains files related to mounted devices on the system + tmp | This directory is used to store temporary files on the system + usr | This 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: + +- ls + +- pwd + +- cd + +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. + +### 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. + + + +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 +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. + + + +### ls (list files and directories)** + +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 +directories present in the current directory. + + + +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 + +There are four basic commands which are used frequently to manipulate +files: + +- touch + +- mkdir + +- cp + +- mv + +- rm + +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. + +### 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 simplest use case of creating a new file. + +General syntax of using touch command + +``` +touch