Imagine an account holding one hundred dollars. Two withdrawal requests for sixty dollars each arrive at almost exactly the same moment. Both requests check the balance. Both see one hundred dollars. Both get approved. The account is now negative twenty dollars, and there is no error anywhere in the logs to explain why.
This is called the lost update anomaly, and it is one of the most dangerous bugs a financial backend can have, precisely because it never announces itself. Nothing crashes. Nothing throws. The ledger simply becomes wrong, quietly, and stays wrong until someone reconciles it against reality and finds a gap that should never have existed.
I wanted to prove I could actually solve this properly, not just explain it in theory, so I built a small NestJS project around this exact problem. The repository is on GitHub, and this article walks through what the problem actually looks like, how I solved it, and the mistakes I ran into along the way.
Repository: https://github.com/PeaceMelodi/mini-banking-ledger
Why the bug is invisible at the application level
