Art of Codeartofcode
Lesson 7 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)

38.6 How does the WebSocket gateway work?

3 min read+10 XP

Step 5: Deep dives (10 minutes)

As in Chapter 21: gateways are thin servers that only hold connections, authenticate them, and pass messages to and from the chat service.

  • About 2,000 gateways for 200 million connections at an assumed 100,000 each; event-driven servers (Erlang, Go, Netty) are the usual choice.
  • A session registry maps (user, device) → gateway, written on connect and expired by TTL if heartbeats stop.
  • The chat service sends a recipient's messages to their gateway (through the registry or a pub/sub backplane).
  • Heartbeats detect dead mobile connections; reconnects use backoff with jitter; deploys drain gateways gradually.

Memory code: "Gateways hold sockets, the registry knows who's where."