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title = "On Unit Tests and Layers, Part II"
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date = 2017-09-15
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[taxonomies]
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tags = ["unit tests"]
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After coming with a discussion about unit tests and layers, I got a
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bunch of other insights, specially from a video of Gary Bernhardt
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about "Fast Test, Slow Test".
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<!-- more -->
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Just after posting about
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[who one could see the layers through unit testing](@/code/on-unittests-and-layers.md),
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I finally watched a video of Gary
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Bernhardt (of the "DestroyAllSoftware" fame) about "Fast Test, Slow Test".
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Basically, what Gary is going after is saying "write tests for the layer ONLY,
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so all your tests are fast". Ok, I can get behind it, in a way. Because layers
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have their behaviour -- in a "mechanical" sense, since they don't require
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human interaction, but behaviour nonetheless -- so you're testing behaviour.
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But that also rises one question: *What are you testing?* Are you testing the
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*component* or the *application*? What do you deliver, anyway? *Components* or
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an *application*? How do you make sure you're delivering an application in the
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proper way?
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Testing layer behaviour also has a bad side effect: If you're application
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doesn't need a certain part of your layer -- say, you wrote a validator in the
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model layer, but the powers to be decided it wasn't required anymore --, how
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do you make sure it will go away? Your tests will still test those validators
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-- after all, you're testing your model layer -- and your coverage will still
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point that that piece of code is needed and you'll end up with a bunch of dead
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code that is kept alive only because the tests require them.
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Also, because Gary points out that the "integration tests" are still required,
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you'll end up with a lot more tests than necessary. Why not focus on the
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behaviour your *application* should have instead of the behaviour your
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*layers* have?
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I'm not against layer testing per-se, I'm just against writing tests that do
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not reflect the general expected behaviour of the application and duplicating
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tests because you're testing layer after layer and then testing them all
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together. You should test the *value* of your application, not its components.
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On a side note, Gary still does the same mistake everyone does, calling the
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"all layers" tests "system tests". That's wrong. Just because you're going
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through different layers it doesn't mean it can't be a unit test. It depends
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only on itself? Does it test behaviour? Congratulations, you have a unit test.
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