Art of Codeartofcode
Lesson 12 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)

38.11 How does media work?

3 min read+10 XP

Photos and videos are too big for the chat path:

  1. The sender's app encrypts the file with a fresh random key.
  2. It uploads the encrypted blob to object storage with a presigned URL (Chapter 11).
  3. It sends a normal chat message containing the blob ID, the key and a thumbnail, itself end-to-end encrypted.
  4. Recipients download the blob (through a CDN, Chapter 26) and decrypt it with the key from the message.

The server never sees the image in the clear, and forwarding a photo to ten chats can reuse the same uploaded blob. Blobs expire after everyone has downloaded them (or after a set time).

Memory code: "Upload the locked box, send the key in the message."