Collaborative spreadsheet engine
Design the real-time engine behind a collaborative spreadsheet. Multiple users edit cells concurrently; cells contain formulas that reference other cells (A1 = B1 + SUM(C1:C100)). On any edit you must recompute exactly the dependent cells (not the whole sheet) and push results to all collaborators in real time. Sheets can have 1M cells, deep dependency chains, and cycles must be detected. Edits must converge across users.
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.
Clarify6:00 left
Estimate6:00 planned
Design18:30 planned
Deep dive14:30 planned
Failure10:00 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.