The requirement of predictability also relates to isolation. Test cases have to run self-sufficiently, without affecting other tests. Changing and maintaining test cases should also have no impact on other test scenarios.

Besides leveraging predictability and maintainability, isolating tests also has an impact on the reproducibility of errors. Complex test scenarios may contain a lot of concerns and responsibilities that can make it hard to find the root causes of failing tests. Isolated tests with smaller scope, however, limit the possibilities of causes and enable developers to find bugs faster.

The several test scopes an enterprise project usually has, which we will see later in this chapter, also come with several test isolation layers. Tests with a small scope, such as unit tests, run more isolated than, for example, end-to-end tests. It certainly makes sense to write test cases in different scopes, which implies different test isolation layers.