Collaborative document offline sync
Design the sync engine for a collaborative document editor where users edit offline on phones/laptops and sync when reconnected. Two users can concurrently edit the same paragraph, and concurrent edits must merge without a server-side lock and without losing characters. Scale: docs up to a few MB, hundreds of collaborators per doc, sync over flaky mobile networks. Pick a conflict-resolution strategy and justify it against the alternative.
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.