Saturday, 4 February 2017

All our tests should always pass

Continuous Integration and automation enforce that our tests should be of a very high quality and always pass.  I don't think I always agree.  Tests that regularly pass could be concealing quality problems that could damage your product.

In any CI pipeline new builds are tested with a set of automated testing.  Tools such as chef or jenkins are very good at orchestrating this process.  However they do rely on a set of tests that are almost 'guaranteed' to run.  However if the same tests are always used and they always pass you must ask yourself the following questions.


  1. Are the tests doing what you think that they are.  Are they testing the code you think they are, are they just exercising the code and not testing it?  Beware tests that don't test what you think they do
  2. Are the tests and the newly delivered code in different areas of the product.  If your tests are regression testing one part of the product which is logically separated from the code you are delivering then you might not be testing as fully as possible.
  3. Beware the pesticide paradox.  If the same tests have been used for a long time then it is likely that the code has become 'immune' to the tests.  Areas of the code that are actively tested by the tests are likely to have become so well written over time that a developer is unlikely to introduce an error into this area.
  4. The tests have become stale.  The tests although they exercise the code that is being developed they may not do it in ways that you would expect your customers to use the product.  Maybe they use a legacy way of invoking the product or they use methods that have long fallen out of favour.
Tests that fall into any of the above categories might be lulling you into a false sense of security.  Sure they pass but do they do what you think that they do.   To help keep your pipeline of high quality I have the following recommendations:
  1. Rotate your CI tests.  At different stages of your testing you will use different tests.  Rotating the tests to different stages in the pipeline helps to drive the code differently at different stages of integration and could find some easy to spot defects
  2. Calculate a test efficiency rating by dividing the number of regressions the test has found by the number of times it has been executed.  Tests that are inefficient might be incorrectly testing the code or be testing an area so stabilised that it is unlikely to be regressed.  It might be worth only running this test occasionally
  3. Constantly add new tests into the pipeline.  Writing a new test for an existing part of the product can be a great learning exercise and pull out new defects.
In summary - although you need code to progress through your CI pipeline it is good and almost healthy to find regressions as a regular activity.  If your tests always pass don't become smug, they could be just not telling you that there is a problem
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