Today we discuss the design of Whatsapp messenger. This is a messaging architecture. It offers free texting in a SMS world. There are no ads, no gimmicks, no games and no feeds. They have hundreds of nodes, thousands of cores, hundreds of terabytes of RAM, serve billions of smartphones. Their server infrastructure is based on Erlang/FreeBSD.  The Erlang is a programming language that is used to build massively scalable soft real-time systems with requirements on high availability.  With this programming language they send out about 70 million messages per second.
This architecture is famous for “it just works” because only a phone number is used as the address to send messages to and messages can be voice, video, images or text.   It also features voice message, group chats and read-receipts.
The backend consists of Erlang, Jabber, FreeBSD, Yaws web server, PHP, BEAM, and custom XMPP. The Frontend consists of seven client platforms for the different mobile devices and SQLLite. Every user is an actor. Messages flow between actors when they are online or they are stored offline. When the actor comes online, he pulls the messages sent to him.  User and actor differ in that the actor notion or proxy exists only in the Whatsapp backend framework. Message sent from user goes to actor inbox which then sends the message to the other actor who then repeatedly sends to the user behind the actor. When that user comes online, the messages appear.  The Mnesia backend DB stores the actors and their inboxes. The message is encrypted with a key as a tag. They use tools for system activity monitoring on all their server operating systems as well as BEAM Processor hardware perf counters and dtrace. BEAM is the erlang VM that is used to process the messages.
The highlights of this architecture is its performance oriented design. Even similar feature from Facebook also written in erlang could not match the widespread adoption of this messenger.  They have proven that Erlang and FreeBSD have unmatched SMP scalability.
#codingexercise
Let C be the cost of a string S defined as the length of the longest substring of S which has all characters same.
int GetCost(string S)
{
int max = 0;
for (int i =0; i < S.Length; i++)
{
  int cost = 0;
  for (int j =i; j < S.Length; j++)
  {
     if (S[j] != S[i]) break;
     cost += 1;
   }
if (cost > max)
     max = cost;
}
return max;
}

Given a language L with c distinct characters and strings all of length n, find the expected value of cost as Sum(cost)/number_of_strings
void FormStringInL(List<char> c, int n, ref List<char> b, ref List<string> combinations)
{
if (b.Length == n) {combinations.Add(ConvertToString(b)); return;}
foreach(var a in c)
{
b.Add(a);
FormStringInL(c, n, ref b, ref combinations);
b.RemoveLast();
}
}
value = combinations.Sum(x => GetCost(x))/combinations.Count;
Console.WriteLine(value);
# other System Design : https://1drv.ms/w/s!Ashlm-Nw-wnWsBPfKqVUIHGkJ4Rf

#tree delete operation for node z
if either left or right child of z exists to delete for node
   find the tree successor as y or set it to z
if y has a child, let it take its place and fix the parent of that child
if parent of y was null then it was root, fix root otherwise fix parents left or right
if y is not the same as z
    then copy z to y
return y
Node TreeSuccessor(Node z)
(
if (z == null) return null;
if (z.right)
    return TREE_MINIMUM(z.right);
Node parent = z.parent;
while(parent && parent.right == z)
{
 z = parent;
 parent = parent.parent;
 }
return parent;
}