How would you design a payment processing system?

Requirements: exactly-once processing, high availability, security, compliance. Flow: payment request -> validation -> authorization (payment gateway) -> capture -> settlement. Idempotency: unique transaction IDs, store processed transactions. Double-entry: every credit has debit, reconciliation. State machine: pending -> authorized -> captured -> settled. Failure handling: retry with backoff, compensation, manual review queue. Security: tokenization, PCI compliance, encryption. Features: refunds, disputes, multiple payment methods, fraud detection. Reconciliation: match with bank statements. Audit trail: immutable event log. Consistency critical.