The Test Pyramid

For my post this week, I found an excellent blog post by Martin Fowler on his personal website that details the “Test Pyramid.” The test pyramid is a way of representing the proportion of different tests you should be using for the purposes of keeping a balanced portfolio of tests.


As you can see by the pyramid, user-interface testing is slower and more expensive, moving to service testing which finds itself in the middle and then on towards unit testing which is speedier and less costly. This would imply that in general you want to rely more on unit testing than UI or service based testing, although all of these types of tests have a purpose.

Martin talks about the disadvantages of relying too heavily on user-interface testing. The slow testing increases build times and can require installed licenses for test automation software, which costs money and limits which machine the tests can be run on. The largest con Martin details was how brittle the tests were. That is, any small change to the system could break several tests and require them to be re-recorded. All these reasons show why the pyramid is the way it is, putting UI testing on the higher, smaller end. Fowler makes a point that often the usage of the high-level tests is as a second line of test defense. These tests are able to capture a bug you might have missed or not written tests for, and enables you to write a unit test to ensure the bug is taken care of.

I think the test pyramid is an interesting idea that highlights the importance of unit testing and helps define the priorities we should have as testers. Although it does not dismiss UI or service testing, it clearly demonstrates as testers that we should primarily be using unit testing to fix bugs in the code and make sure they stay fixed. High-level testing can be used to find things that are missed, but shouldn’t be relied on to catch bugs, and they won’t necessarily stay stable when you make changes to your program.

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