Art of Codeartofcode
Lesson 5 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)

38.4 What do we store?

3 min read+10 XP
messages chat_id, seq, sender, client_id, ciphertext, sent_at (sharded by chat_id) unique (chat_id, sender, client_id) (de-duplication) cursors (user, device, chat_id) → last delivered seq, last read seq chats chat_id, members, type (direct | group), group key info sessions (user, device) → gateway server (in-memory store, TTL) devices user → devices, push tokens media encrypted blobs in object storage; the message carries the blob ID and its key

Interviewer pushback:

  • "Why a sequence number per chat instead of timestamps?" → Clocks on phones and servers disagree, and two messages can share a millisecond. A per-chat counter assigned by the server gives one clear order for everyone in the chat and makes "give me everything after 2" exact.

Memory code: "One socket per device; every message gets a client ID and a chat sequence number."