Keep a shared payment account correct under concurrent payments

Two people share one online payment account and pay with it at the same moment. How do you make sure the balance never goes wrong? Answer from the API side (idempotency, request design) and the data side (transactions, locking or optimistic concurrency), then explain how you would handle a very large number of requests.

The same screen asks quick follow-ups: the HTTP methods and which you use most and why, how relational and NoSQL databases differ, and whether and how you use Kubernetes.