Design stateful windowed stream aggregation
Design a stateful stream-processing pipeline that computes per-key windowed aggregations (e.g. 5-minute rolling revenue per merchant) over an event stream where events arrive out of order and late (mobile clients buffer offline for minutes/hours). 300K events/sec, results must be correct despite late data, and the operator state (per-key windows) can be large and must survive crashes. Emit results with bounded latency.
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.