Programming Tutorials

  A dropped network connection during a checkout flow is a critical failure point in distributed systems. A client sends a payment request, the connection hangs, and the client automatically retries the operation. Without server-side safeguards, this sequence of events directly leads to a double charge. FinTech API design requires defensively engineering against these unobservable failure states. To achieve strict payment processing API safety, backend architectures must guarantee that no matter how many times a client retries a specific operation, the state mutation only occurs once. The Root Cause: Network Boundary Uncertainty Understanding why duplicates occur requires examining HTTP semantics and network topology. HTTP methods like  GET ,  PUT , and  DELETE  are idempotent by definition—executing them multiple times yields the same server state. However,  POST , the standard method for resource creation and payment execution, is not. When a client dispa...