Substituting test doubles for external systems
In this section, we’ll discuss one of the biggest advantages that the hexagonal architecture brings to TDD: high testability. It also brings some workflow advantages.
Replacing the adapters with test doubles
The key advantage the hexagonal architecture brings to TDD is that it is trivially easy to replace all the adapters with test doubles, giving us the ability to test the entire domain model with FIRST unit tests. We can test the entire application core logic without test environments, test databases, or HTTP tools such as Postman or curl – just fast, repeatable unit tests. Our testing setup looks like this:
Figure 9.7 – Testing the domain model
We can see that all the adapters have been replaced by test doubles, completely freeing us from our environment of external systems. Unit tests can now cover the whole domain model, reducing the need for integration tests.
We gain several...
