System Design: Payment Processing System A capstone system design walkthrough — designing a payment processing system end to end — covering the core domain model, the ledger as the system's source of truth, idempotency and exactly-once-effect guarantees, integrating with external payment gateways and card networks, handling asynchronous webhooks, reconciliation, fraud and risk checks, and the specific correctness and compliance demands that make payments a uniquely unforgiving system design...