Payments

Payment systems are the hardest type of event-driven system to get right. A misplaced message in a recommendation engine means a user sees the wrong product. A misplaced message in a payment system means money moves to the wrong account — and that is a regulatory incident with real financial consequences. We have designed and built event-driven payment platforms for fintechs processing hundreds of thousands of transactions daily. Here is the pattern that works. ...

Processing payments at scale is one of the hardest problems in distributed systems. Every transaction must be exactly-once, the system must be available 99.999% of the time, and reconciliation must be perfect. A payment gateway that processes 100 million transactions per month cannot afford to lose a single transaction or double-charge a single customer. We have built payment gateway infrastructure for fintechs processing over $10 billion in annual transaction volume. The architecture decisions that matter are not about speed — they are about correctness, idempotency, and the ability to recover from partial failures without losing money. ...