Overlapping magisteria

I've written that unit tests and acceptance tests represent two non-overlapping magisteria. I've realised I need to revise this.

Unit tests and acceptance tests specify internal and external behaviour respectively. There is no inherent reason why external behaviour should be trivially translatable to internal behaviour, because the structure of the internals need not bear any resemblance to the model the system presents to the world.

However, that is only true of systems in general. Systems produced using a domain-driven design methodology deliberately enforce structural similarity between the system's implementation and the domain it represents.

Because of this, it's possible to make inferences between external and internal properties of the system. For example, if a

LoginFailure

object returns

true

to the method

shouldLockAccount()

when its

int

field called

previousAttempts

is

2

or greater, there's a good chance that if a user thrice enters the wrong password that their account will be disabled.

If we know something about a well-designed system's external behaviour, we can generally extrapolate about its internal behaviour (and vice-versa).

Acceptance tests make big claims

If we have good acceptance tests, why do we need unit tests?

Acceptance tests tell us a lot. When they pass, they tell us things like:

• A feature works from an end-user perspective.
• All the components required to serve user requests have been wired up correctly. 
• The supporting infrastructure has been installed and configured in the test environment.
• The user interface has not significantly changed since the acceptance tests were first written.
• An intermittant network outage did not occur.

But when they fail, they tell us very little. They make such ambitious claims about the state of the software that failures could indicate a large number of disparate underlying issues.

Unit tests, on the other hand, claim very little. If a unit test fails, it is immediately obvious that there is a problem with a very specific section of code.

In general, the easier it is for a test to fail, the less it tells us when it does. Failures from tests that intermittently break for no reason at all aren't very informative. But if there's a little red dot next to that test that "couldn't possibly fail" then you've just discovered something startlingly new about your system.

Relying on acceptance tests makes sense if the code-base is relatively static, because failures will be relatively rare. But software that must change to remain valuable (i.e. almost all software) also needs fine-grained unit tests to give specific and actionable feedback on failures.