It Grows Hard to Test

A common evolution within a layered architecture is that layers are being skipped. We access the Persistence layer directly from the Web layer since we're only manipulating a single field of an Entity, and for that we need not bother the Domain layer, right?

Figure 1.4: Skipping the domain layer tends to scatter domain logic across the code base

Again, this feels OK the first couple of times, but it has two drawbacks if it happens often (and it will, once someone has done the first step).

First, we're implementing domain logic in the Web layer, even if it's only manipulating a single field. What if the use case expands in the future? We're most likely going to add more domain logic to the Web layer, mixing responsibilities and spreading essential domain logic all over the application.

Second, in the tests of our Web layer, we not only have to mock away the domain layer, but also the persistence layer. This adds complexity to the unit test. And a complex test setup is the first step toward no tests at all because we don't have time for them.

As the web component grows over time, it may accumulate a lot of dependencies to different persistence components, adding to the test's complexity. At some point, it takes more time for us to understand and mock away the dependencies than to actually write test code.