System Design Article

Design a Payment System (Stripe)

Difficulty: Hard

Design a Stripe-style payment platform that processes 100M payments per day across 50 currencies and dozens of payment methods, where the central requirement is financial correctness: never charge a customer twice, never lose a payment, always reconcile to the cent. The interview centerpiece is the trio of idempotency keys, the payment intent state machine, and the immutable double-entry ledger - together they make the system safe in the face of network failures, partial outages, and adversarial retries. We also cover webhook delivery with signing and exponential backoff, PCI scope minimization through tokenization, multi-region availability, and the reconciliation jobs that compare our ledger to the bank's settlement files every night.