Code Room
System designHard
Question
Design the realtime sync backend for a collaborative whiteboard where each open board is a 'room': users join a board and broadcast cursor moves and shape edits to others in that room. Scale: 2M concurrent users across 500k active boards, edits should propagate within ~100ms, and a user's connection must route to wherever that board's other participants are. Cover room routing across many servers, what you broadcast vs persist, and reconnect.
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.
Learn the concepts
Loading whiteboard…
Run or narrate your approach, then ask the coach.