Randomizing test execution order

Many testing framework offer optional randomization of test execution order. For example:
– Robot Framework with –randomize
– Rspec with –order random

I consider this option as very useful and use it by default for all the automated tests portfolio I run. The advantages I see are:

– we detect ordering dependencies as soon as possible.  If we execute tests A and B always in the same order, test B could work only because test A left the system in a state that is used by test B. If one day we invert the order (by renaming the tests for example, if order depend on alphabetical order) then the suite will fail and it will take us some time to understand the problem (because test A and B were maybe written months/years ago). This happens also if we insert a new test between A and B or refactor test A or B. If we run the tests in random order all the time, we will detect this issue very soon.

– we might detect bugs in the SUT that appear only in some specific sequence of actions that a random order of test could meet with luck. The problem then could be how to duplicate the bug we just bumped into. For Rspec, randomness can have predictability via seeding.  JUnit had a randomization problem when Java 7 came out, and had to think that over and came up with a deterministic but random order. There is no such thing for Robot Framework so we have to manually reproduce the test order that caused the failure.

– we won’t always run the same tests first. And usually when we read a test report, we start by the top and analyse error one by one. This could help us to not analyze the “Access”, “Audit” and “Authentication” tests first all the time…

One could argue that using a fixed test execution order is useful to run some smoke/sanity tests first, and then the rest of the porfolio. I think in that case, it is better to split that in 2 different jobs. A first “smoke test” job that runs quickly (5-10 minutes) and another “full test” that can take several hours. In Robot Framework, this can be easily achieved using tags.

Another reason to push for fixed tests execution order could be performance optimisation. Test A could be preparing the system for test B to start and when test B ends, the system is ready for test C to go. One of the reason this is a bad pattern is that you won’t be able to run only test B or only test C! If the setup of a given test is in the previous test, then you are doomed to run always the full portfolio. This is simply not bearable. When a full portfolio detect a couple of failed tests, we want to be able to run those tests once more to double-check they are failing and then start to analyse the problem.

We could also introduce randomness in the test themselves, but this is another topic… for a future post!

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