Design Whatsapp

Overview–

WhatsApp is a messaging service that allows users to send and receive encrypted messages and calls from their phones and computers. WhatsApp was famously originally built on Erlang (no longer!) and renowned for handling high scale with limited engineering and infrastructure outlay.

Requirements–
Start group chat
send/receive messages
send/receive media
Access messages after I've been offline.

Non-Functional Requirements—
Delivered with low latency < 500ms
Guarantee delivery messages
Billions of users, high throughput
Messages not stored unnecessarily.
Fault-tolerant

Core Entities—
Messages
Users
Chat
Client

API-
Commands Sent Commands Received
createChat - newMessage
sendMessage - chatUpdate
createAttachment
modifyParticipants

High-Level Design–