Offline-first collaborative sync
Design the real-time sync backend for a collaborative offline-first app (think a shared notes/whiteboard tool) where 10M users edit documents that must converge across devices, work offline, and merge without a central lock. Edits stream over WebSocket when online and queue locally when offline. You need <500 ms propagation between online collaborators on a doc and automatic conflict resolution. How do edits flow, and how do you guarantee convergence?
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.