System-Level Approval Testing Using TextTest

Approval Testing is based on the idea of managing changes to the behaviour of the system and reacting to them, rather than on trying to assert what is correct. It is a technique I have found particularly powerful when automating system or component level tests.

Assertions work well when the behaviour under test is simple, stable and well-defined, but it is often important to test behaviour that does not fit these criteria. Legacy code is one such area which is particularly suited to this kind of testing.

TextTest is a well-established and mature open source tool in this space that I first released in 2003. Over the years I have used it successfully to test a wide variety of systems.

I will present Approval Testing as an idea in a tool-neutral way, and also motivate why it can be more powerful than using assertions in many contexts. I will then illustrate this with examples from the TextTest tool to provide insight into how these ideas can be implemented in practice. I will also discuss how to apply Approval Testing to UIs, and how Capture-Replay Mocking is a powerful mocking technique that fits well with this kind of testing.