The Problem If you've ever built a fintech product, you know this truth: the hardest part isn't moving money — it's making sure you never lose it. A wallet engine sounds simple on the surface. Credit. Debit. Balance. But the moment you introduce concurrency, network failures, and the need for auditability, it becomes one of the most demanding distributed systems problems you can tackle. I built a wallet engine that needed to: Handle credits, debits, and wallet-to-wallet transfer...