How would you make a payment processor web/app technology wise?

I’m actually working on a toy payment processor for fun and to learn some new tech.

The tech I’ve chosen:

- Postgres for the DB

- Redis for caching + queues

- Go for the customer-facing backend services

- Rust for the bank-facing services

- gRPC for internal communication

- TypeScript + React for the frontend checkout app

- HashiCorp stack for deployment

Postgres for the database for the start and maybe this does suffice. Maybe think now how you could model a transaction with something like Cassandra. Only append the data. No updates to transactions. The balance is the sum over all transactions over time. Use something like a double entry bookkeeping system so you don’t make up any money out of thin air.

Keep in mind how to do reconciliation, what is a transaction, how to do idempotency.

For the backend Erlang/BEAM.