System designHardsd-g008

Subject Exactly once deliveryLevel Senior–Staff~45 minCommon in Networking & APIs interviewsIndustries Technology, Software development

Question

Design the charge-execution service for a payments platform that must move money exactly once per user-initiated charge, even though the upstream payment-processor API can time out (you don't know if the charge went through), the client retries on network errors, and your own workers can crash mid-flight. Throughput is ~5k charges/sec. Explain how you achieve effective exactly-once semantics end to end and what 'exactly once' really means here.

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.