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Problem

Build a payment processing system like Stripe. The system must accept card payments, enforce idempotency (same charge retried N times = charged exactly once), perform fraud screening before authorization, and maintain a double-entry ledger of every financial movement. Financial correctness is non-negotiable.

Functional Requirements

• Accept and process card payments via REST API
• Idempotency keys — same request retried N times = charged once
• Fraud scoring before authorization
• Double-entry ledger for every money movement
• Refunds, disputes, and bank reconciliation

Non-Functional

• 1M transactions per day at peak
• Payment processing < 3 seconds end-to-end
• Zero tolerance for double-charges or lost transactions
• PCI-DSS compliance — card data never touches application servers