Real-Time Backends

What is inside

Every section, and every one of them opens in the browser. Nothing here is hidden: if a section you need is not in this list, the notebook is not the right one and you should not spend a week on it.

1. How to use this book
2. The whole picture on one page
3. 1The requirement: two people, one board
4. 2The shape of every request you have written so far
5. 3Polling, and why it is not stupid
6. 4The arithmetic of polling
7. 5Long polling, and the gap
8. 6The two inversions
9. 7A 200 that does not close
10. 8The wire format: four fields and a blank line
11. 9Reconnection you did not write
12. 10Why nobody streams an LLM with EventSource
13. 11The proxy that eats your stream
14. 12Six connections per origin
15. 13What SSE cannot do
16. 14The handshake, byte by byte
17. 15What 101 actually changed
18. 16The frame
19. 17Masking, and the proxy it was designed to protect
20. 18Ping, pong, and the connection that is already dead
21. 19Close codes, and the one you cannot send
22. 20Choosing: polling, long polling, SSE, WebSocket
23. 21Ceiling 1: file descriptors
24. 22The thousand-descriptor limit is language-dependent
25. 23Ceiling 2: the client's ports, not the server's
26. 24Ceiling 3: memory, and where 10 KB goes
27. 25The ceiling the benchmark did not measure
28. 26Compression, and the budget it destroys
29. 27What you just broke
30. 28Sticky sessions are not the answer, and not for the reason you think
31. 29Pub/sub: the instances talk to each other
32. 30Redis pub/sub is at-most-once, and deploys are when it bites
33. 31Ordering, and why sequence numbers come first
34. 32The catch-up protocol
35. 33Fan-out: events times subscribers
36. 34Coalescing: the other lever
37. 35The reconnect storm
38. 36Deploys: draining a connection that never ends
39. 37Authenticating a socket
40. 38The token that never expires
41. 39Cross-site WebSocket hijacking
42. 40Presence: who is online
43. 41Collaborative editing, in one page
44. 42Where this goes next: WebTransport
45. 43The delivery script
46. 44Whiteboard drawing order
47. 45Follow-up question bank
48. Appendix A. Numbers to memorise
49. Appendix B. Glossary
50. Appendix C. Self-test

How to read it

Read with a pen. Every notebook opens with a question to answer before you start and asks you to redo the answer at the end, and the margin in the Print edition exists so you have somewhere to be wrong first. The Tablet edition is 16:9 with vector text, so note apps draw on it rather than treating it as a photograph.