Card payment authorization and routing
Design a card payment gateway that authorizes ~8,000 transactions/sec at peak across 4 acquiring banks, routing each charge to the cheapest/most-likely-to-approve PSP. Each authorization must be exactly-once toward the bank (no double-charge) even when the merchant client retries, and the merchant needs a synchronous approve/decline within 2s p99. Walk the components, the data model, and how you guarantee no double-authorization under client retries and partial network failures.
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.