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Payment Gateway System Design
Visual Problem Diagram
Scenario
A shopper double-clicks Pay and two $2,400 charges settle because idempotency keys never reached the processor—support tickets spike while your ledger still shows one order. The gateway interview is idempotency, PCI scope, reconciliation, and exactly-once money movement semantics, not drawing Visa logos.
Design a payment gateway that merchants use to charge customers via cards and wallets with authorize/capture/refund flows. Money bugs are irreversible—idempotency, clear state machines, and reconciliation matter more than raw QPS.
You should support payment intents, idempotent APIs, webhooks, fraud hooks, and internal ledger. Be ready to explain PCI boundaries, duplicate retry safety, and settlement reconciliation.
Constraints
Create payment, authorize, capture, refund, idempotent APIs, merchant webhooks, fraud check hook
99.99% availability, < 500 ms p95 auth, ACID ledger, PCI scoped card handling
10K TPS peak, millions merchants, billions $/month, global multi-currency
Stages ahead
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