We present our experience applying Test-Driven Development (TDD) at all levels of the development process. TDD at the class level is now well understood (if not always well practiced). We find that the benefits we get from writing tests first and using them to drive design apply at the system level too. That's why we try to address integration and system testing as early as possible. The sooner the system is in a deployable state, the better equipped we are to react to changing business needs by delivering new features.
Our experience is that putting testing at the front of our development process, and paying attention to what we learn from them, flushes out architectural issues such as concurrency and distribution. The result is systems that are easier both to maintain and to support. We can also avoid some of common testing pitfalls, such as unreliability, slow execution, and brittleness.
