Multiplayer room state and broadcast
Design the room/session backend for a real-time multiplayer collaborative or game app where state is room-scoped (a game match, a whiteboard, a watch-party). Each room has an authoritative live state mutated by member actions and broadcast to all members. Scale: 1M concurrent rooms, up to 1,000 members in the largest rooms, sub-150ms action-to-broadcast. Rooms are created/destroyed constantly; a room's host node can crash.
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.
Clarify5:30 left
Estimate5:30 planned
Design16:30 planned
Deep dive13:30 planned
Failure9:00 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.