Design a Chat System (WhatsApp / Slack / Messenger)

A model interview answer: real-time delivery, fan-out, ordering and presence at scale

Design a real-time chat system end to end: WebSocket architecture, message storage and ordering, 1:1 and group fan-out, delivery receipts, multi-device sync, presence, and how connection servers survive failure at millions of concurrent users.

Practice questions (5)

• Estimating the Connection Server Fleet

  Intermediate · Free
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• Choosing a Fan-out Strategy by Group Size

  Advanced
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• Idempotent Send and Delivery Acknowledgements

  Advanced
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• Designing the Message Storage Schema

  Advanced
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• Recovering from a Connection Server Crash

  Advanced
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Quizzes (1)