Payments / Card Processing System

Problem statement

Legacy processing pipelines were tightly coupled and difficult to audit. Failures during settlement required manual intervention and increased reconciliation time.

Architecture overview

The system is a set of stateless services behind a single payment API. State transitions are recorded in an append-only ledger, with a reconciliation worker that validates settlement outcomes against external processors.

Technical decisions & tradeoffs

• Adopted an append-only ledger to support auditability and replay, trading write amplification for traceability.
• Introduced explicit state machines to validate transitions and prevent partial updates.
• Built adaptive retry queues to reduce backpressure during external processor downtime.

Lessons learned

State machines and ledgering simplified incident response. Clear ownership boundaries helped teams ship faster without breaking settlement guarantees.