When to use each test level

With a clear understanding of the different test levels and their benefits, we have to decide whether to invest more in unit testing or system testing and determine which components should be tested via unit testing and which components should be tested via system testing. A wrong decision may have a considerable impact on the system’s quality: a wrong level may cost too many resources and may not find sufficient bugs. As you may have guessed, the best answer here is, “It depends.”

Some developers—including me—favor unit testing over other test levels. This does not mean such developers do not do integration or system testing; but whenever possible, they push testing toward the unit test level. A pyramid is often used to illustrate this idea, as shown in figure 1.8. The size of the slice in the pyramid represents the relative number of tests to carry out at each test level.

Figure 1.8 My version of the testing pyramid. The closer a test is to the top, the more real and complex the test becomes. At the right part you see what I test at each test level.

Unit testing is at the bottom of the pyramid and has the largest area. This means developers who follow this scheme favor unit testing (that is, write more unit tests). Climbing up in the diagram, the next level is integration testing. The area is smaller, indicating that, in practice, these developers write fewer integration tests than unit tests. Given the extra effort that integration tests require, the developers write tests only for the integrations they need. The diagram shows that these developers favor system tests less than integration tests and have even fewer manual tests.