Art of Codeartofcode
Lesson 11 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)

38.10 How does group chat work?

3 min read+10 XP
  • Store once, deliver per member. A group message is one row; delivery fans out to each member's devices, like a small pub/sub topic (Chapter 23).
  • Encryption: the sender encrypts the message once with a group key (WhatsApp's security whitepaper describes "sender keys" for this), and the server fans out the same ciphertext, instead of the sender encrypting it a thousand times.
  • Cost grows with size. A 1,024-member group (WhatsApp's limit since 2022) sending 200 messages a day means about 205,000 deliveries and 409,000 delivery and read receipts a day.
  • Receipts in big groups: aggregate them ("delivered to 1,010, read by 312") and send updates in batches.
  • Membership changes need care: a new member shouldn't read old messages; a removed member must not get new ones. Re-key the group when members leave.

Memory code: "Encrypt once, store once, fan out to members, summarise the ticks."