Presence in massive rooms
Design a presence and typing-indicator system for very large rooms. Some rooms have 500K concurrent members; presence (online/away/offline) and 'X is typing…' must feel real-time (<1s) but you cannot broadcast every keystroke or every member's status change to all 500K clients — that's a fan-out explosion. Members join/leave constantly and connections are flaky mobile sockets.
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify5:00 left
Estimate5:00 planned
Design15:00 planned
Deep dive12:00 planned
Failure8:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.