System designHardsd-g522

Subject Event streamingLevel Senior–Staff~50 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.