Art of Codeartofcode
Lesson 13 of 17

38. Design a Chat App (WhatsApp)

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The interview prompt
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38.1 What do you ask first?
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38.2 How big is this, really?
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38.3 What are the doors?
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38.4 What do we store?
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38.5 What does the whole system look like?
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38.6 How does the WebSocket gateway work?
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38.7 How do message IDs and ordering work?
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38.8 How do delivery and read ticks work?
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38.9 How does offline delivery work?
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38.10 How does group chat work?
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38.11 How does media work?
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38.12 What would you say in the last five minutes?
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Score yourself
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Variations to practise
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Sources
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Code lab
38. Design a Chat App (WhatsApp)

38.12 What would you say in the last five minutes?

3 min read+10 XP

Step 6: Wrap-up (5 minutes)

  • Bottlenecks: connection count (gateway tier), message writes (store sharded by chat), receipts in large groups (aggregation).
  • Single points of failure: none central; gateways are interchangeable; chat shards are replicated with failover (Chapter 14); session registry entries are rebuilt on reconnect.
  • Correctness: client IDs for de-duplication, per-chat sequence numbers for order, store-then-deliver for no loss, per-device cursors for sync.
  • Privacy: end-to-end encryption means servers route ciphertext; metadata (who talks to whom, when) still needs protection and minimal retention.
  • Next with more time: voice and video calls (separate media servers, WebRTC), message editing and deleting (as new events in the chat), backups, and spam/abuse controls that work without reading message content.

Memory code: "Ordered, de-duplicated, stored before delivered, synced by cursor."