Payments. A payment gateway

A payment gateway for shops: never charge twice (idempotency key), never lose a confirmed payment (RPO = 0, transactional outbox), and survive losing a region.

Diagram of a payment gateway: an idempotency key stops a duplicate charge, the payment and its outbox are written together, and a second region stands by.

System Design — Load balancers, caches, queues and regions: build a system that holds the load