The Dependency Inversion Principle

In our layered architecture, the cross-layer dependencies always point downward, to the next layer. When we apply the SRP on a high level, we notice that the upper layers have more reasons to change than the lower layers.

Thus, due to the domain layer's dependency on the persistence layer, each change in the persistence layer potentially requires a change in the domain layer. But the domain code is the most important code in our application. We don't want to have to change it when something changes in the persistence code.

So, how can we get rid of this dependency?

The Dependency Inversion Principle (DIP) provides the answer.

In contrast to the SRP, the DIP means what the name suggests:

We can turn around (invert) the direction of any dependency within our codebase.

Actually, we can only invert dependencies when we have control over the code on both sides of the dependency. If we have a dependency on a third-party library, we cannot invert it, since...