Delete tests

One of the most important things you can do is move your tests up the abstraction layers and away from unit tests. For lack of a better term, to move to integration tests. End-to-end tests are often too far from the system to easily understand what's wrong when they break, and can overwhelm a development org. Integration tests (or whatever you want to call them) are often the sweet spot: not tied to a particular implementation, able to survive fairly significant system changes, but also easy enough to debug when they break.

https://grugbrain.dev/#grug-on-testing

Integration tests and Unit tests are different tools; and each has their place and purpose. Using one "instead" of the other is a mistake.

I've never really found this to be the case in practice. When I look at well-written unit tests and well-written integration tests, they're usually doing exactly the same sort of thing and have very similar concerns in terms of code organisation and test structure.

For example, in both cases, the tests work best if I test the subject under test as a black box (i.e. interact only with its public interface) but use my knowledge of its internals to identify the weaknesses that will most require testing. In both cases, I want to structure the code so that the subject under test is as isolated as possible - i.e. no complex interactions with global state, no mocking of unrelated modules, and no complex mechanism to reset anything after the test is done. In both cases, I want the test to run fast, ideally instantaneously, so I get immediate results.

The biggest difference is that it's usually harder to write good integration tests because they're interacting with external systems that are generally slower and stateful, so I've got to put extra work into getting the tests themselves to be fast and stateless. But when that works, there's really not much difference at all between a test that tests a single function, and a test that tests a service class with a database dependency.

I'll go with a bank account, because that was one of the initial examples for automated testing.

I would write integration/system (different, but similar, imo) to test that the black box integrations with the system work as expected. Generally closer to the "user story" end of things.

I would write integration tests for smaller, targeted thing. Like making sure the sort method works in various cases, etc. Individual methods, especially ones that don't interact with data outside what is passed into them (functional methods), are good for unit testing.

What if, instead of a bank account, it's FooSystemFrobnicationPreparer? Something which is necessary today, but probably should be refactored within the next year or two?

Maybe FooSystem will be redesigned to take different inputs,maybe the upstream will change to provide different outputs, maybe responsibility will shift around due to changes in the number of dependencies and it makes sense to vertically integrate some prep to upstream to share it.

Unit tests in these circumstances - and they're the majority of unit tests, IME - can act as a drag on the quality of the system. It's better to test things like this at a component level instead of units.