[crdoconnor]

If your end to end test uncovers a bug in a lower level module that has a clean and stable abstraction, yes it makes sense to write a test that surrounds that instead.

That needn't be a unit test. It could simply be a lower level integration test.

[scott_s]

In some cases maybe, but it's not effective in my case. In order to catch failures in this particular component, the most effective thing is to check that a large set of invariants is still true. An integration test of some kind only checks for behavior; does this component work when interacting with the rest of the system? That can show a failure exists, but is not useful for showing why. It's the unit tests which continually check the large set of invariants that can really get to the why: if a particular invariant is no longer true, the path to the problem is usually clear.

Maybe that should be a number 6): easily tested invariants.

[JanisL]

Reading this makes me realize I had a bias against making lower level integration tests in this situation, definitely something I'll have to look for in the future.