System designHardsd-g111

Subject PaymentsLevel Senior–Staff~50 minCommon in Security · Networking & APIs interviewsIndustries Technology, Software development

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.