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I'm confused as to where any mention of integration tests are? Integration tests can't use Mocks, as then they wouldn't be testing the integration. Unit tests c

I'm confused as to where any mention of integration tests are? Integration tests can't use Mocks, as then they wouldn't be testing the integration. Unit tests can use Mocks, but as the article and GP and Mark Seemann[0] points out, that is always worse than writing your logic in a pure way, if you can. [0]: https://blog.ploeh.dk/2017/02/02/dependency-rejection/ https://blog.ploeh.dk/2017/02/02/dependency-rejection/

Spivak 5y ago
Because you don’t gain as much as you think when you do this. Before your external external service touches your integration point and then runs some logic. You mock the external service to unit test the internal logic. Then you run integration tests to exercise the integration point. In the new world you have an external service talking to an integration point which then passes the data to your internal logic. You don’t need the mocks anymore because you can just call your logic function with your data that would have come from the mock. Great! You run integration tests again to test the integration point. But now you have another linkage at the call site. Your internal logic function now has a dependency on the output of your integration point and that’s an invariant that has to be tested to catch someone modifying one but not the other. With enough discipline you might be able to make the type system do these kinds of checks for you but IRL very few people do enough to say catch an external string field changing format slightly.
- ItsMonkk 5y ago
  I think I was confused because I naturally do what you say few people do. My external calls all parse into an object that you can then pass into the unit. It's the integration tests job to ensure that the parse works. Then its the unit tests job to ensure that you run enough that you can be reasonably sure that you cover all cases.
- u801e 5y ago
  > You mock the external service to unit test the internal logic You only need the output produced by that external service or the the input expected by that service. For example, I can pass in a data structure or object representing the HTTP request to the method that I'm unit testing, but I don't need to mock a client to generate those requests to test that logic.

Spivak 5y ago

Because you don’t gain as much as you think when you do this. Before your external external service touches your integration point and then runs some logic. You mock the external service to unit test the internal logic. Then you run integration tests to exercise the integration point. In the new world you have an external service talking to an integration point which then passes the data to your internal logic. You don’t need the mocks anymore because you can just call your logic function with your data that would have come from the mock. Great! You run integration tests again to test the integration point. But now you have another linkage at the call site. Your internal logic function now has a dependency on the output of your integration point and that’s an invariant that has to be tested to catch someone modifying one but not the other. With enough discipline you might be able to make the type system do these kinds of checks for you but IRL very few people do enough to say catch an external string field changing format slightly.

ItsMonkk 5y ago
I think I was confused because I naturally do what you say few people do. My external calls all parse into an object that you can then pass into the unit. It's the integration tests job to ensure that the parse works. Then its the unit tests job to ensure that you run enough that you can be reasonably sure that you cover all cases.
u801e 5y ago
> You mock the external service to unit test the internal logic You only need the output produced by that external service or the the input expected by that service. For example, I can pass in a data structure or object representing the HTTP request to the method that I'm unit testing, but I don't need to mock a client to generate those requests to test that logic.