(9) How does messaging work in WhatsApp? - Quora

(9) How does messaging work in WhatsApp? - Quora

Rick Reed here talks about some of the things his team did to do to scale their application -  Rick Reed, Software engineer at WhatsApp.
The rest of the answer is my guess on what a naive version of WhatsApp could do to implement it's features. WhatsApp or most of the other messaging apps rarely work on a peer to peer basis. So it wouldn't open a connection (from your device) to each of your friends' devices. Instead your device connects to their server. It could then use a custom TCP protocol or maybe HTTP to communicate your messages to the server. The server in return would dispatch them to your friends' devices. If your friend had their app open or at least the app process running there might be a live connection to the server. WhatsApp will use that connection to send them your messages. If their app is "offline" then they might choose to send them a push notification instead.
WhatsApp has chosen Erlang a language built for writing scalable applications that are designed to withstand errors. Erlang uses an abstraction called the Actor model for it's concurrency - http://en.wikipedia.org/wiki/Act.... Instead of the more traditional shared memory approach, actors communicate by sending each other messages. Actors unlike threads are designed to be lightweight. Actors could be on the same machine or on different machines and the message passing abstractions works for both. A simple implementation of WhatsApp could be:
Each user/device is represented as an actor. This actor is responsible for handling the inbox of the user, how it gets serialized to disk, the messages that the user sends and the messages that the user receives. Let's assume that Alice and Bob are friends on WhatsApp. So there is an an Alice actor and a Bob actor.

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