System designHardsd-g181

Subject Batch vs streamLevel Senior–Staff~50 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

A payments company shows merchants a 'today's volume' counter that must update within seconds, AND a finalized end-of-day settlement report that must be exactly correct to the cent (it triggers payouts). Transactions can be retried, reversed, or disputed hours later. ~30k txns/sec peak. Design how you serve the live counter and the authoritative report. Decide whether you run one pipeline or two, and how the two numbers reconcile so merchants don't see the live counter contradict the official report.

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.