Most important and common question asked for developers and architect to check their design skills.
Design whatsApp or Provide architecture for whatsApp
OR
Learn how to build Realtime messaging system or Chatting application.

https://blog.whatsapp.com/196/1milli...

http://highscalability.com/blog/2013/...

http://blogbhaskaruni.blogspot.com/2...

Disclaimer: System design can vary based on the feature. this video explains the possible solution to build one. it doesn't guarantee the scale or full working. Instead this resource should be consumed to understand how it can be designed.

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