Exactly-once charge execution
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.
Clarify5:00 left
Estimate5:00 planned
Design15:00 planned
Deep dive12:00 planned
Failure8:00 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.