diff --git a/404.html b/404.html
index 848a37c..ec88030 100644
--- a/404.html
+++ b/404.html
@@ -407,96 +407,11 @@
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
     <label class="md-nav__link" for="nav-4">
-      Systems Design
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-    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
-      <label class="md-nav__title" for="nav-4">
-        <span class="md-nav__icon md-icon"></span>
-        Systems Design
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-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="/systems_design/intro/" class="md-nav__link">
-      Introduction
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="/systems_design/scalability/" class="md-nav__link">
-      Scalability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="/systems_design/availability/" class="md-nav__link">
-      Availability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="/systems_design/fault-tolerance/" class="md-nav__link">
-      Fault Tolerance
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="/systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-    
-      
-      
-      
-
-
-  <li class="md-nav__item md-nav__item--nested">
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-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
-    <label class="md-nav__link" for="nav-5">
       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -510,13 +425,13 @@
 
   <li class="md-nav__item md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" >
-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -610,6 +525,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="/systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="/systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="/systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="/systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="/systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
+
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
diff --git a/big_data/architecture/index.html b/big_data/architecture/index.html
index 6318606..33f3596 100644
--- a/big_data/architecture/index.html
+++ b/big_data/architecture/index.html
@@ -407,103 +407,18 @@
       
       
 
-
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
-    <label class="md-nav__link" for="nav-4">
-      Systems Design
-      <span class="md-nav__icon md-icon"></span>
-    </label>
-    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
-      <label class="md-nav__title" for="nav-4">
-        <span class="md-nav__icon md-icon"></span>
-        Systems Design
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/intro/" class="md-nav__link">
-      Introduction
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/scalability/" class="md-nav__link">
-      Scalability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/availability/" class="md-nav__link">
-      Availability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
-      Fault Tolerance
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-    
-      
-      
-      
-
   
 
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" checked>
-    <label class="md-nav__link" for="nav-5">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" checked>
+    <label class="md-nav__link" for="nav-4">
       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -519,13 +434,13 @@
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" checked>
-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" checked>
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -628,6 +543,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
+
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
diff --git a/big_data/evolution/index.html b/big_data/evolution/index.html
index 816212f..730be8b 100644
--- a/big_data/evolution/index.html
+++ b/big_data/evolution/index.html
@@ -407,103 +407,18 @@
       
       
 
-
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
-    <label class="md-nav__link" for="nav-4">
-      Systems Design
-      <span class="md-nav__icon md-icon"></span>
-    </label>
-    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
-      <label class="md-nav__title" for="nav-4">
-        <span class="md-nav__icon md-icon"></span>
-        Systems Design
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/intro/" class="md-nav__link">
-      Introduction
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/scalability/" class="md-nav__link">
-      Scalability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/availability/" class="md-nav__link">
-      Availability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
-      Fault Tolerance
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-    
-      
-      
-      
-
   
 
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" checked>
-    <label class="md-nav__link" for="nav-5">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" checked>
+    <label class="md-nav__link" for="nav-4">
       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -519,13 +434,13 @@
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" checked>
-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" checked>
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -628,6 +543,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
+
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
diff --git a/big_data/intro/index.html b/big_data/intro/index.html
index 15f7129..3926d7b 100644
--- a/big_data/intro/index.html
+++ b/big_data/intro/index.html
@@ -407,103 +407,18 @@
       
       
 
-
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
-    <label class="md-nav__link" for="nav-4">
-      Systems Design
-      <span class="md-nav__icon md-icon"></span>
-    </label>
-    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
-      <label class="md-nav__title" for="nav-4">
-        <span class="md-nav__icon md-icon"></span>
-        Systems Design
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/intro/" class="md-nav__link">
-      Introduction
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/scalability/" class="md-nav__link">
-      Scalability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/availability/" class="md-nav__link">
-      Availability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
-      Fault Tolerance
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-    
-      
-      
-      
-
   
 
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" checked>
-    <label class="md-nav__link" for="nav-5">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" checked>
+    <label class="md-nav__link" for="nav-4">
       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -519,13 +434,13 @@
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" checked>
-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" checked>
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -691,6 +606,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
+
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
@@ -897,7 +897,7 @@
     <div class="md-footer-nav">
       <nav class="md-footer-nav__inner md-grid" aria-label="Footer">
         
-          <a href="../../systems_design/conclusion/" class="md-footer-nav__link md-footer-nav__link--prev" rel="prev">
+          <a href="../../python_web/sre-conclusion/" class="md-footer-nav__link md-footer-nav__link--prev" rel="prev">
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               <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12z"/></svg>
             </div>
@@ -906,7 +906,7 @@
                 <span class="md-footer-nav__direction">
                   Previous
                 </span>
-                Conclusion
+                SRE Aspects of The App and Conclusion
               </div>
             </div>
           </a>
diff --git a/big_data/overview/index.html b/big_data/overview/index.html
index 9780438..68ac050 100644
--- a/big_data/overview/index.html
+++ b/big_data/overview/index.html
@@ -407,103 +407,18 @@
       
       
 
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-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
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-      Systems Design
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-        
-          
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-          
-
-
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-    <a href="../../systems_design/intro/" class="md-nav__link">
-      Introduction
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-
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-          
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-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/scalability/" class="md-nav__link">
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-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/availability/" class="md-nav__link">
-      Availability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
-      Fault Tolerance
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
-
-        
-      </ul>
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-  </li>
-
-    
-      
-      
-      
-
   
 
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" checked>
-    <label class="md-nav__link" for="nav-5">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" checked>
+    <label class="md-nav__link" for="nav-4">
       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -519,13 +434,13 @@
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" checked>
-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" checked>
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -628,6 +543,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
+
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
diff --git a/big_data/tasks/index.html b/big_data/tasks/index.html
index c6f97a1..480f0e6 100644
--- a/big_data/tasks/index.html
+++ b/big_data/tasks/index.html
@@ -407,103 +407,18 @@
       
       
 
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-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
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-      Systems Design
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-    </label>
-    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
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-        Systems Design
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-          
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-
-  <li class="md-nav__item">
-    <a href="../../systems_design/intro/" class="md-nav__link">
-      Introduction
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-          
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-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/scalability/" class="md-nav__link">
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-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/availability/" class="md-nav__link">
-      Availability
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-  </li>
-
-        
-          
-          
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-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
-      Fault Tolerance
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-    
-      
-      
-      
-
   
 
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" checked>
-    <label class="md-nav__link" for="nav-5">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" checked>
+    <label class="md-nav__link" for="nav-4">
       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -519,13 +434,13 @@
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" checked>
-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" checked>
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -664,6 +579,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
+
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
@@ -846,7 +846,7 @@
           </a>
         
         
-          <a href="../../security/intro/" class="md-footer-nav__link md-footer-nav__link--next" rel="next">
+          <a href="../../systems_design/intro/" class="md-footer-nav__link md-footer-nav__link--next" rel="next">
             <div class="md-footer-nav__title">
               <div class="md-ellipsis">
                 <span class="md-footer-nav__direction">
diff --git a/big_data/usage/index.html b/big_data/usage/index.html
index 9552cdc..06f62e1 100644
--- a/big_data/usage/index.html
+++ b/big_data/usage/index.html
@@ -407,103 +407,18 @@
       
       
 
-
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-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
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-      Systems Design
-      <span class="md-nav__icon md-icon"></span>
-    </label>
-    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
-      <label class="md-nav__title" for="nav-4">
-        <span class="md-nav__icon md-icon"></span>
-        Systems Design
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/intro/" class="md-nav__link">
-      Introduction
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-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/scalability/" class="md-nav__link">
-      Scalability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/availability/" class="md-nav__link">
-      Availability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
-      Fault Tolerance
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-    
-      
-      
-      
-
   
 
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" checked>
-    <label class="md-nav__link" for="nav-5">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" checked>
+    <label class="md-nav__link" for="nav-4">
       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -519,13 +434,13 @@
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" checked>
-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" checked>
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -628,6 +543,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
+
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
diff --git a/git/branches/index.html b/git/branches/index.html
index 68cfe98..2204744 100644
--- a/git/branches/index.html
+++ b/git/branches/index.html
@@ -454,96 +454,11 @@
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
     <label class="md-nav__link" for="nav-4">
-      Systems Design
-      <span class="md-nav__icon md-icon"></span>
-    </label>
-    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
-      <label class="md-nav__title" for="nav-4">
-        <span class="md-nav__icon md-icon"></span>
-        Systems Design
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/intro/" class="md-nav__link">
-      Introduction
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/scalability/" class="md-nav__link">
-      Scalability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/availability/" class="md-nav__link">
-      Availability
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
-      Fault Tolerance
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-    
-      
-      
-      
-
-
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
-    <label class="md-nav__link" for="nav-5">
       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -557,13 +472,13 @@
 
   <li class="md-nav__item md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" >
-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -657,6 +572,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
+
   <li class="md-nav__item md-nav__item--nested">
     
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diff --git a/git/git-basics/index.html b/git/git-basics/index.html
index 117ec5e..e1ba514 100644
--- a/git/git-basics/index.html
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-      Fault Tolerance
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-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
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-    
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     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
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         Data
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@@ -667,13 +582,13 @@
 
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-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
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     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -767,6 +682,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
+
   <li class="md-nav__item md-nav__item--nested">
     
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diff --git a/git/github-hooks/index.html b/git/github-hooks/index.html
index 1146670..973e879 100644
--- a/git/github-hooks/index.html
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-      Fault Tolerance
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-        
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-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
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-      </ul>
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-      
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-
-
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     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
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@@ -567,13 +482,13 @@
 
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-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -667,6 +582,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
+
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
diff --git a/index.html b/index.html
index 3b94a3e..811b392 100644
--- a/index.html
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-
-  <li class="md-nav__item">
-    <a href="systems_design/fault-tolerance/" class="md-nav__link">
-      Fault Tolerance
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
-
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-      </ul>
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-    
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       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -524,13 +439,13 @@
 
   <li class="md-nav__item md-nav__item--nested">
     
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-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -624,6 +539,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
+
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
@@ -740,8 +740,8 @@
 <ul>
 <li>Linux basics</li>
 <li><a href="https://linkedin.github.io/school-of-sre/git/git-basics/">Git</a>  </li>
-<li><a href="https://linkedin.github.io/school-of-sre/linux_networking/intro/">Linux networking</a></li>
-<li><a href="https://linkedin.github.io/school-of-sre/python_web/intro/">Python</a></li>
+<li><a href="https://linkedin.github.io/school-of-sre/linux_networking/intro/">Linux Networking</a></li>
+<li><a href="https://linkedin.github.io/school-of-sre/python_web/intro/">Python and Web</a></li>
 <li>Relational databases (MySQL)</li>
 <li>NoSQL concepts </li>
 <li><a href="https://linkedin.github.io/school-of-sre/big_data/intro/">Big Data</a></li>
diff --git a/linux_networking/dns/index.html b/linux_networking/dns/index.html
index 0b5b8db..5f4a540 100644
--- a/linux_networking/dns/index.html
+++ b/linux_networking/dns/index.html
@@ -454,96 +454,11 @@
     
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-        Systems Design
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-        
-        
-          
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-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/intro/" class="md-nav__link">
-      Introduction
-    </a>
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-        
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-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/scalability/" class="md-nav__link">
-      Scalability
-    </a>
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-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/availability/" class="md-nav__link">
-      Availability
-    </a>
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-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
-      Fault Tolerance
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-    
-      
-      
-      
-
-
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
-    <label class="md-nav__link" for="nav-5">
       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -557,13 +472,13 @@
 
   <li class="md-nav__item md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" >
-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -657,6 +572,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
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+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
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+        
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+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
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@@ -657,6 +572,91 @@
       
 
 
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+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
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+
+  <li class="md-nav__item">
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+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
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+        
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+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
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+  </li>
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+        
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diff --git a/python_web/intro/index.html b/python_web/intro/index.html
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-      Conclusion
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     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
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+      <label class="md-nav__title" for="nav-4">
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         Data
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@@ -623,13 +538,13 @@
 
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       Big Data
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+      <label class="md-nav__title" for="nav-4-1">
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         Big Data
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@@ -723,6 +638,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
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+        Systems Design
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+
+  <li class="md-nav__item">
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+      Introduction
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+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
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+
+  <li class="md-nav__item">
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+        
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+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
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+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
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+    
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diff --git a/python_web/python-concepts/index.html b/python_web/python-concepts/index.html
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-      Conclusion
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       Data
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     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
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+      <label class="md-nav__title" for="nav-4">
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         Data
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@@ -589,13 +504,13 @@
 
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-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
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       Big Data
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     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
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@@ -689,6 +604,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
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+    <label class="md-nav__link" for="nav-5">
+      Systems Design
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+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
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+        
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+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
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+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
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+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
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+  </li>
+
+    
+      
+      
+      
+
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   <li class="md-nav__item md-nav__item--nested">
     
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diff --git a/python_web/python-web-flask/index.html b/python_web/python-web-flask/index.html
index ba411eb..eddc2e6 100644
--- a/python_web/python-web-flask/index.html
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-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
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       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
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@@ -562,13 +477,13 @@
 
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-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -662,6 +577,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
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+        
+          
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+
+
+  <li class="md-nav__item">
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+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
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+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
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+
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+    
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     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
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+      <label class="md-nav__title" for="nav-4-1">
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         Big Data
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@@ -683,6 +598,91 @@
       
 
 
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+  <li class="md-nav__item">
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+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
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+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
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+        
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+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
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+        
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+
+    
+      
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+
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   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
@@ -897,7 +897,7 @@
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     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
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+      <label class="md-nav__title" for="nav-4">
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       Big Data
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     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
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+      <label class="md-nav__title" for="nav-4-1">
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         Big Data
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@@ -696,6 +611,91 @@
       
 
 
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
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+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
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+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
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+        
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+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
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+          
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+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
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+          
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+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
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+        
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+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
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+  </li>
+
+        
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+
+    
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   <li class="md-nav__item md-nav__item--nested">
     
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diff --git a/security/fundamentals/index.html b/security/fundamentals/index.html
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     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
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+      <label class="md-nav__title" for="nav-4">
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         Data
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@@ -515,13 +430,13 @@
 
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-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
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       Big Data
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     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -614,6 +529,91 @@
       
       
 
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
   
 
 
diff --git a/security/intro/index.html b/security/intro/index.html
index b787e48..d1719b8 100644
--- a/security/intro/index.html
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-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
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       Data
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     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
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@@ -515,13 +430,13 @@
 
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+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
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       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -614,6 +529,91 @@
       
       
 
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
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+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
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+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
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+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
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+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
   
 
 
@@ -909,7 +909,7 @@
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-          <a href="../../big_data/tasks/" class="md-footer-nav__link md-footer-nav__link--prev" rel="prev">
+          <a href="../../systems_design/conclusion/" class="md-footer-nav__link md-footer-nav__link--prev" rel="prev">
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@@ -918,7 +918,7 @@
                 <span class="md-footer-nav__direction">
                   Previous
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-                Tasks and conclusion
+                Conclusion
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diff --git a/security/network_security/index.html b/security/network_security/index.html
index 32b825f..49edc5e 100644
--- a/security/network_security/index.html
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-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
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-
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-    
-      
-      
-      
-
-
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-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
-    <label class="md-nav__link" for="nav-5">
       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -515,13 +430,13 @@
 
   <li class="md-nav__item md-nav__item--nested">
     
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-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
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@@ -614,6 +529,91 @@
       
       
 
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
   
 
 
diff --git a/security/threats_attacks_defences/index.html b/security/threats_attacks_defences/index.html
index 338cd15..083b53f 100644
--- a/security/threats_attacks_defences/index.html
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-
-
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-      Fault Tolerance
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-        
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-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
-      Conclusion
-    </a>
-  </li>
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-      </ul>
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-    
-      
-      
-      
-
-
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
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       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
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@@ -515,13 +430,13 @@
 
   <li class="md-nav__item md-nav__item--nested">
     
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-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -614,6 +529,91 @@
       
       
 
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
   
 
 
diff --git a/security/writing_secure_code/index.html b/security/writing_secure_code/index.html
index 51ed02d..47f1912 100644
--- a/security/writing_secure_code/index.html
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-      Fault Tolerance
-    </a>
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-        
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-          
-
-
-  <li class="md-nav__item">
-    <a href="../../systems_design/conclusion/" class="md-nav__link">
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-
-
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       Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
+      <label class="md-nav__title" for="nav-4">
         <span class="md-nav__icon md-icon"></span>
         Data
       </label>
@@ -515,13 +430,13 @@
 
   <li class="md-nav__item md-nav__item--nested">
     
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-    <label class="md-nav__link" for="nav-5-1">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
       Big Data
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
+      <label class="md-nav__title" for="nav-4-1">
         <span class="md-nav__icon md-icon"></span>
         Big Data
       </label>
@@ -614,6 +529,91 @@
       
       
 
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
+    <label class="md-nav__link" for="nav-5">
+      Systems Design
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
+      <label class="md-nav__title" for="nav-5">
+        <span class="md-nav__icon md-icon"></span>
+        Systems Design
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/scalability/" class="md-nav__link">
+      Scalability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/availability/" class="md-nav__link">
+      Availability
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/fault-tolerance/" class="md-nav__link">
+      Fault Tolerance
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../systems_design/conclusion/" class="md-nav__link">
+      Conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
   
 
 
diff --git a/sitemap.xml.gz b/sitemap.xml.gz
index d3087f4..b06caf8 100644
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index ef5a4ef..7e27d6a 100644
--- a/systems_design/availability/index.html
+++ b/systems_design/availability/index.html
@@ -407,18 +407,140 @@
       
       
 
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
+    <label class="md-nav__link" for="nav-4">
+      Data
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Data" data-md-level="1">
+      <label class="md-nav__title" for="nav-4">
+        <span class="md-nav__icon md-icon"></span>
+        Data
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
+      Big Data
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Big Data" data-md-level="2">
+      <label class="md-nav__title" for="nav-4-1">
+        <span class="md-nav__icon md-icon"></span>
+        Big Data
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/overview/" class="md-nav__link">
+      Overview of Big Data
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/usage/" class="md-nav__link">
+      Usage of Big Data techniques
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/evolution/" class="md-nav__link">
+      Evolution of Hadoop
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/architecture/" class="md-nav__link">
+      Architecture of Hadoop
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/tasks/" class="md-nav__link">
+      Tasks and conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
   
 
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" checked>
-    <label class="md-nav__link" for="nav-4">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" checked>
+    <label class="md-nav__link" for="nav-5">
       Systems Design
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
-      <label class="md-nav__title" for="nav-4">
+      <label class="md-nav__title" for="nav-5">
         <span class="md-nav__icon md-icon"></span>
         Systems Design
       </label>
@@ -623,128 +745,6 @@
       
 
 
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
-    <label class="md-nav__link" for="nav-5">
-      Data
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-    </label>
-    <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
-        <span class="md-nav__icon md-icon"></span>
-        Data
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item md-nav__item--nested">
-    
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-    <label class="md-nav__link" for="nav-5-1">
-      Big Data
-      <span class="md-nav__icon md-icon"></span>
-    </label>
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-      <label class="md-nav__title" for="nav-5-1">
-        <span class="md-nav__icon md-icon"></span>
-        Big Data
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/intro/" class="md-nav__link">
-      Introduction
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/overview/" class="md-nav__link">
-      Overview of Big Data
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/usage/" class="md-nav__link">
-      Usage of Big Data techniques
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/evolution/" class="md-nav__link">
-      Evolution of Hadoop
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/architecture/" class="md-nav__link">
-      Architecture of Hadoop
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/tasks/" class="md-nav__link">
-      Tasks and conclusion
-    </a>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-    
-      
-      
-      
-
-
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
diff --git a/systems_design/conclusion/index.html b/systems_design/conclusion/index.html
index febee2f..5ca35b4 100644
--- a/systems_design/conclusion/index.html
+++ b/systems_design/conclusion/index.html
@@ -407,18 +407,140 @@
       
       
 
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
+    <label class="md-nav__link" for="nav-4">
+      Data
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Data" data-md-level="1">
+      <label class="md-nav__title" for="nav-4">
+        <span class="md-nav__icon md-icon"></span>
+        Data
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
+      Big Data
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Big Data" data-md-level="2">
+      <label class="md-nav__title" for="nav-4-1">
+        <span class="md-nav__icon md-icon"></span>
+        Big Data
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/overview/" class="md-nav__link">
+      Overview of Big Data
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/usage/" class="md-nav__link">
+      Usage of Big Data techniques
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/evolution/" class="md-nav__link">
+      Evolution of Hadoop
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/architecture/" class="md-nav__link">
+      Architecture of Hadoop
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/tasks/" class="md-nav__link">
+      Tasks and conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
   
 
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" checked>
-    <label class="md-nav__link" for="nav-4">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" checked>
+    <label class="md-nav__link" for="nav-5">
       Systems Design
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
-      <label class="md-nav__title" for="nav-4">
+      <label class="md-nav__title" for="nav-5">
         <span class="md-nav__icon md-icon"></span>
         Systems Design
       </label>
@@ -529,128 +651,6 @@
       
 
 
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
-    <label class="md-nav__link" for="nav-5">
-      Data
-      <span class="md-nav__icon md-icon"></span>
-    </label>
-    <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
-        <span class="md-nav__icon md-icon"></span>
-        Data
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" >
-    <label class="md-nav__link" for="nav-5-1">
-      Big Data
-      <span class="md-nav__icon md-icon"></span>
-    </label>
-    <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
-        <span class="md-nav__icon md-icon"></span>
-        Big Data
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/intro/" class="md-nav__link">
-      Introduction
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/overview/" class="md-nav__link">
-      Overview of Big Data
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/usage/" class="md-nav__link">
-      Usage of Big Data techniques
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/evolution/" class="md-nav__link">
-      Evolution of Hadoop
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/architecture/" class="md-nav__link">
-      Architecture of Hadoop
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/tasks/" class="md-nav__link">
-      Tasks and conclusion
-    </a>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-    
-      
-      
-      
-
-
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
@@ -809,7 +809,7 @@
           </a>
         
         
-          <a href="../../big_data/intro/" class="md-footer-nav__link md-footer-nav__link--next" rel="next">
+          <a href="../../security/intro/" class="md-footer-nav__link md-footer-nav__link--next" rel="next">
             <div class="md-footer-nav__title">
               <div class="md-ellipsis">
                 <span class="md-footer-nav__direction">
diff --git a/systems_design/fault-tolerance/index.html b/systems_design/fault-tolerance/index.html
index 207b935..777910e 100644
--- a/systems_design/fault-tolerance/index.html
+++ b/systems_design/fault-tolerance/index.html
@@ -407,18 +407,140 @@
       
       
 
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
+    <label class="md-nav__link" for="nav-4">
+      Data
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Data" data-md-level="1">
+      <label class="md-nav__title" for="nav-4">
+        <span class="md-nav__icon md-icon"></span>
+        Data
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
+      Big Data
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Big Data" data-md-level="2">
+      <label class="md-nav__title" for="nav-4-1">
+        <span class="md-nav__icon md-icon"></span>
+        Big Data
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/overview/" class="md-nav__link">
+      Overview of Big Data
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/usage/" class="md-nav__link">
+      Usage of Big Data techniques
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/evolution/" class="md-nav__link">
+      Evolution of Hadoop
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/architecture/" class="md-nav__link">
+      Architecture of Hadoop
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/tasks/" class="md-nav__link">
+      Tasks and conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
   
 
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" checked>
-    <label class="md-nav__link" for="nav-4">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" checked>
+    <label class="md-nav__link" for="nav-5">
       Systems Design
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
-      <label class="md-nav__title" for="nav-4">
+      <label class="md-nav__title" for="nav-5">
         <span class="md-nav__icon md-icon"></span>
         Systems Design
       </label>
@@ -596,128 +718,6 @@
       
 
 
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
-    <label class="md-nav__link" for="nav-5">
-      Data
-      <span class="md-nav__icon md-icon"></span>
-    </label>
-    <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
-        <span class="md-nav__icon md-icon"></span>
-        Data
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" >
-    <label class="md-nav__link" for="nav-5-1">
-      Big Data
-      <span class="md-nav__icon md-icon"></span>
-    </label>
-    <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
-        <span class="md-nav__icon md-icon"></span>
-        Big Data
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/intro/" class="md-nav__link">
-      Introduction
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/overview/" class="md-nav__link">
-      Overview of Big Data
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/usage/" class="md-nav__link">
-      Usage of Big Data techniques
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/evolution/" class="md-nav__link">
-      Evolution of Hadoop
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/architecture/" class="md-nav__link">
-      Architecture of Hadoop
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/tasks/" class="md-nav__link">
-      Tasks and conclusion
-    </a>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-        
-      </ul>
-    </nav>
-  </li>
-
-    
-      
-      
-      
-
-
   <li class="md-nav__item md-nav__item--nested">
     
     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
diff --git a/systems_design/intro/index.html b/systems_design/intro/index.html
index 22b54c0..359740a 100644
--- a/systems_design/intro/index.html
+++ b/systems_design/intro/index.html
@@ -407,18 +407,140 @@
       
       
 
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
+    <label class="md-nav__link" for="nav-4">
+      Data
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Data" data-md-level="1">
+      <label class="md-nav__title" for="nav-4">
+        <span class="md-nav__icon md-icon"></span>
+        Data
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
+      Big Data
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Big Data" data-md-level="2">
+      <label class="md-nav__title" for="nav-4-1">
+        <span class="md-nav__icon md-icon"></span>
+        Big Data
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/overview/" class="md-nav__link">
+      Overview of Big Data
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/usage/" class="md-nav__link">
+      Usage of Big Data techniques
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/evolution/" class="md-nav__link">
+      Evolution of Hadoop
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/architecture/" class="md-nav__link">
+      Architecture of Hadoop
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/tasks/" class="md-nav__link">
+      Tasks and conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
   
 
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" checked>
-    <label class="md-nav__link" for="nav-4">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" checked>
+    <label class="md-nav__link" for="nav-5">
       Systems Design
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
-      <label class="md-nav__title" for="nav-4">
+      <label class="md-nav__title" for="nav-5">
         <span class="md-nav__icon md-icon"></span>
         Systems Design
       </label>
@@ -581,128 +703,6 @@
       
 
 
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" >
-    <label class="md-nav__link" for="nav-5">
-      Data
-      <span class="md-nav__icon md-icon"></span>
-    </label>
-    <nav class="md-nav" aria-label="Data" data-md-level="1">
-      <label class="md-nav__title" for="nav-5">
-        <span class="md-nav__icon md-icon"></span>
-        Data
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item md-nav__item--nested">
-    
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5-1" type="checkbox" id="nav-5-1" >
-    <label class="md-nav__link" for="nav-5-1">
-      Big Data
-      <span class="md-nav__icon md-icon"></span>
-    </label>
-    <nav class="md-nav" aria-label="Big Data" data-md-level="2">
-      <label class="md-nav__title" for="nav-5-1">
-        <span class="md-nav__icon md-icon"></span>
-        Big Data
-      </label>
-      <ul class="md-nav__list" data-md-scrollfix>
-        
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/intro/" class="md-nav__link">
-      Introduction
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/overview/" class="md-nav__link">
-      Overview of Big Data
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/usage/" class="md-nav__link">
-      Usage of Big Data techniques
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/evolution/" class="md-nav__link">
-      Evolution of Hadoop
-    </a>
-  </li>
-
-        
-          
-          
-          
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-
-  <li class="md-nav__item">
-    <a href="../../big_data/architecture/" class="md-nav__link">
-      Architecture of Hadoop
-    </a>
-  </li>
-
-        
-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/tasks/" class="md-nav__link">
-      Tasks and conclusion
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-      </ul>
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-  </li>
-
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-  </li>
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-    
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     <input class="md-nav__toggle md-toggle" data-md-toggle="nav-6" type="checkbox" id="nav-6" >
@@ -921,7 +921,7 @@ Jim Waldo, Sun Microsystems, On System Design</em></p>
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       <nav class="md-footer-nav__inner md-grid" aria-label="Footer">
         
-          <a href="../../python_web/sre-conclusion/" class="md-footer-nav__link md-footer-nav__link--prev" rel="prev">
+          <a href="../../big_data/tasks/" class="md-footer-nav__link md-footer-nav__link--prev" rel="prev">
             <div class="md-footer-nav__button md-icon">
               <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12z"/></svg>
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@@ -930,7 +930,7 @@ Jim Waldo, Sun Microsystems, On System Design</em></p>
                 <span class="md-footer-nav__direction">
                   Previous
                 </span>
-                SRE Aspects of The App and Conclusion
+                Tasks and conclusion
               </div>
             </div>
           </a>
diff --git a/systems_design/scalability/index.html b/systems_design/scalability/index.html
index da0a1e2..49c2bdd 100644
--- a/systems_design/scalability/index.html
+++ b/systems_design/scalability/index.html
@@ -407,18 +407,140 @@
       
       
 
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" >
+    <label class="md-nav__link" for="nav-4">
+      Data
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Data" data-md-level="1">
+      <label class="md-nav__title" for="nav-4">
+        <span class="md-nav__icon md-icon"></span>
+        Data
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item md-nav__item--nested">
+    
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4-1" type="checkbox" id="nav-4-1" >
+    <label class="md-nav__link" for="nav-4-1">
+      Big Data
+      <span class="md-nav__icon md-icon"></span>
+    </label>
+    <nav class="md-nav" aria-label="Big Data" data-md-level="2">
+      <label class="md-nav__title" for="nav-4-1">
+        <span class="md-nav__icon md-icon"></span>
+        Big Data
+      </label>
+      <ul class="md-nav__list" data-md-scrollfix>
+        
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/intro/" class="md-nav__link">
+      Introduction
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/overview/" class="md-nav__link">
+      Overview of Big Data
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/usage/" class="md-nav__link">
+      Usage of Big Data techniques
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/evolution/" class="md-nav__link">
+      Evolution of Hadoop
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/architecture/" class="md-nav__link">
+      Architecture of Hadoop
+    </a>
+  </li>
+
+        
+          
+          
+          
+
+
+  <li class="md-nav__item">
+    <a href="../../big_data/tasks/" class="md-nav__link">
+      Tasks and conclusion
+    </a>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+        
+      </ul>
+    </nav>
+  </li>
+
+    
+      
+      
+      
+
   
 
 
   <li class="md-nav__item md-nav__item--active md-nav__item--nested">
     
-    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-4" type="checkbox" id="nav-4" checked>
-    <label class="md-nav__link" for="nav-4">
+    <input class="md-nav__toggle md-toggle" data-md-toggle="nav-5" type="checkbox" id="nav-5" checked>
+    <label class="md-nav__link" for="nav-5">
       Systems Design
       <span class="md-nav__icon md-icon"></span>
     </label>
     <nav class="md-nav" aria-label="Systems Design" data-md-level="1">
-      <label class="md-nav__title" for="nav-4">
+      <label class="md-nav__title" for="nav-5">
         <span class="md-nav__icon md-icon"></span>
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@@ -709,128 +831,6 @@
       
 
 
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-          
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-      Introduction
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-
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-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/overview/" class="md-nav__link">
-      Overview of Big Data
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-
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-          
-          
-          
-
-
-  <li class="md-nav__item">
-    <a href="../../big_data/usage/" class="md-nav__link">
-      Usage of Big Data techniques
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-
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-          
-          
-          
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-
-  <li class="md-nav__item">
-    <a href="../../big_data/evolution/" class="md-nav__link">
-      Evolution of Hadoop
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-          
-          
-
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-    <a href="../../big_data/architecture/" class="md-nav__link">
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-
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-    <a href="../../big_data/tasks/" class="md-nav__link">
-      Tasks and conclusion
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