Real-Time Backends

What is inside

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