Art of Codeartofcode
Lesson 4 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)

38.3 What are the doors?

3 min read+10 XP

Step 3: API and data model (10 minutes)

Chat runs over a WebSocket per device (Chapter 21), with small messages both ways:

client → server { "type": "send", "chat": "c1", "client_id": "a-002", "body": "<ciphertext>" } server → client { "type": "ack", "client_id": "a-002", "seq": 2 } ← one tick server → client { "type": "msg", "chat": "c1", "seq": 2, "from": "asha", "body": "<ciphertext>" } client → server { "type": "delivered", "chat": "c1", "upto": 2 } ← device has it client → server { "type": "read", "chat": "c1", "upto": 2 } ← user saw it client → server { "type": "sync", "chat": "c1", "after": 2 } ← after reconnecting

Plus HTTPS for login, contacts, group management, and media upload (presigned URLs, Chapter 11).