Payment Processing Engine

Idempotent charge API backed by a double-entry ledger, with HMAC-signed async webhook delivery.

status in-progress

stack TypeScript, Node.js, PostgreSQL, Redis, Docker

A payment processing engine designed around an idempotent charge API backed by a double-entry ledger rather than a mutable balance field, so financial correctness holds under retries and concurrent requests.

problem

A balance column plus a retryable endpoint is how you end up double-charging customers. Correctness has to be structural, not defensive.

solution

A double-entry ledger as the source of truth and an idempotency key on every charge, so retries and concurrent requests converge on the same state.

architecture

Idempotent charge API keyed on client-supplied idempotency keys
Double-entry ledger in PostgreSQL as source of truth
Async webhook delivery via a Redis-backed job queue
HMAC-signed webhook payloads with exponential-backoff retries

results

In progress.