Realtime

Cards

1. Chat apps show "sent" when the _____ has stored a message, "delivered" when it reached the recipient's _____, and "read" when the recipient opened it. easy Fill in the blank
2. How does long polling work, and what does it cost compared with a WebSocket? easy Flashcard
3. The recipient's phone app is closed. How does a new chat message reach them? easy Multiple choice
4. A page shows live sports scores. Only the server sends updates. Which is the simplest fit over plain HTTP, with reconnects built into the browser? easy Multiple choice
5. How should "is typing…" indicators be handled differently from messages? easy Flashcard
6. At peak, 10 million clients hold WebSocket connections. One server comfortably holds 50,000. How many servers do you need? easy Estimate
7. When should a chat server acknowledge a message to its sender? medium Flashcard
8. How does a chat service know a user has gone offline when their phone just loses signal (no "goodbye" is ever sent)? medium Flashcard
9. Why should a chat app never rely on the push notification itself to carry the message? medium Flashcard
10. A gateway holding a million WebSocket connections restarts. How do you keep the reconnects from overwhelming the other servers? medium Multiple choice
11. Why are WebSocket servers harder to run than stateless HTTP servers? medium Flashcard
12. After reconnecting, a chat client asks for every message after the last _____ it has seen, so nothing that arrived while it was offline is lost. medium Fill in the blank
13. How should a chat service order the messages in a conversation? hard Flashcard
14. Why don't large chat apps push every online/offline change to every member of big groups? hard Multiple choice
15. A chat has 50 WebSocket servers. A message for Bob arrives at server 1, but Bob is connected to server 7. How does it reach him? hard Multiple choice