Challenge
The product began as a payment aggregator on an off-the-shelf platform. A few years in, the box ran out: every new currency, gateway and limit rule ran into decisions somebody else had made before us. It needed a full move to our own platform, without taking the service down and without losing years of accumulated search addresses.
Solution
The wallet was rewritten from scratch, and the old system was kept as a reference for partner protocols. That saved months, because payment API documentation rarely describes what actually happens.
Underneath sits a rule we never bent: any money operation runs as a function inside the database rather than in application code. The database provides atomicity, and double spending stops being a question of how careful a developer was that day. Money is never held in floating point either.
Architecture and implementation
What the system covers:
· top-ups and withdrawals through processing gateways
· payments for services through aggregators, kept in a separate register
· multi-currency balances with identity checks and tiered limits
· background queues for the slow operations that depend on partners
Stack: NestJS and TypeScript on the server, Next.js in the user's office, PostgreSQL with calculation functions, Redis for queues.
Result
The product is almost ten years old and still serves people who live in one country and pay in another. It is our longest-running payment service, and the most instructive one: in ten years, everything that can happen to money in transit has happened here at least once.
