A couple of months ago, in Can You Hack It? , I wrote about how I increased the testability of a service by changing it in a way that allowed me to simulate the behaviour of one of its dependencies. With that in place I could force specific code paths to be followed and so explore different scenarios easily. That was sufficient for a quick and dirty experiment but, because I was changing code, each round was slower than I'd have liked as I had to edit, compile, run, and then test. --00-- When the next opportunity to work in that area came up, on a different service, I looked for an improvement to my test approach. I realised that I could remove the compile-run step by having some configuration that would specify the response from the external service. So I taught our product to look for the URL of a downstream service in an environment variable every time it wanted to call it. This gave me very precise control of the outgoing requests which I pointed at a local mock server called ...