Art of Codeartofcode
Lesson 15 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)

Variations to practise

3 min read+10 XP

Chapters used

Approach (1), Estimation (2), Blob storage and presigned URLs (11), Replication (14), Sharding (15), Polling vs WebSockets vs push (21), Message queues (22), Pub/sub (23), Idempotency (24), CDNs (26).

Try these variations

Each is a common follow-up. Answer in two minutes, then check the sketch.

  1. "Show 'last seen' and 'typing…'." → Presence from gateway connect/disconnect and heartbeats, published to contacts with throttling; typing indicators are fire-and-forget, never stored (Chapter 21).
  2. "Delete a message for everyone." → A new "delete seq 42" event in the chat; clients that have it hide it; the server drops it if still stored.
  3. "Search my chat history." → With end-to-end encryption, search runs on the device over its local, decrypted history.
  4. "Channels with millions of followers." → One-way broadcast: store once, followers pull (fan-out on read, Chapter 23), no per-follower delivery receipts.