I appreciate your comment and your experience. And I agree: integration tests tend to be time consuming to run. At work we actually had similar issues (and many

I appreciate your comment and your experience. And I agree: integration tests tend to be time consuming to run. At work we actually had similar issues (and many of our integration tests need to exercise cloud APIs) before a rewrite that made the time manageable. I just wanted to pick up one of your points: > I've gone into the details I end up with interface classes My reply to this is that functional programming languages tend to manage entirely without, just by doing “dependency injection” via higher-order functions (in fact, outside Java I’ve never [needed to] use a DI framework). This (often? always? I honestly don’t know!) makes mocking possible without impacting the overall architecture. And like you, I practice strict TDD for all my current own hobby projects, and I manage without interfaces. But I don’t want to make a claim of generality here: it’s entirely possible that this approach doesn’t work everywhere, or even in the majority of cases.