Design a real-time chat system that supports one-on-one and group messaging. Users should see messages in real time, and messages should be persisted. The system should handle 50 million daily active users.
Cover: connection management, message delivery, storage, online/offline handling, and group chat fan-out.
How to approach it
- Hint 1
WebSockets provide the persistent, bidirectional connection needed for real-time delivery.
- Hint 2
Think about how to route a message when the recipient is connected to a different server.
- Hint 3
For group chats, consider fan-out on write vs. fan-out on read.
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