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Friday, February 27, 2009

unit vs. functional tests

Sometimes I find myself thinking about unit tests being useless. Not that really useless, but bothering, at least when compared to functional tests.

In the end, you should really rely on functional tests to check the health of your application, since those are the only tests that are really checking that all your stack works perfectly, top to bottom, that the database schema is ok, and that permissions and data are handled correctly.

You can even add functional tests to an existing (legacy?) system to use them as regression tests, and to be able to refactor parts of the application while knowing that everything is still right, while adding unit tests could be a pain in the ass, and not that useful, really.

Why are unit tests bothering? They are harder to maintain. You usually can change the implementation of some functionality without touching functionals, but you usually have to make changes to your unit tests. I recommend you to abuse stubs to keep the coupling to a minimum, though.

Anyway, when it al comes down to the development itself, it's when unit tests really shine. They allow you to start coding any class without worrying about other classes, thanks to mocks and stubs. They help you to think about communication between your objects or processes or whatever, because you should create your methods or functions simple enough that they can be easily tested. And finally, they are your api documentation. They allow any other developer to learn your application by example.

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