Summary

In this chapter, we learned how to apply the SOLID principles to decouple external systems completely, leading to an application architecture known as the hexagonal architecture. We saw how this allows us to use test doubles in place of external systems, making our tests simpler to write, with repeatable results. This, in turn, allows us to test entire user stories with a FIRST unit test. As a bonus, we isolate ourselves from future changes in those external systems, limiting the amount of rework that would be required to support new technologies. We’ve seen how the hexagonal architecture combined with dependency injection allows us to support several different external systems choices and select the one we want at runtime via configuration.

The next chapter will look at the different styles of automated testing that apply to the different sections of a hexagonal architecture application. This approach is summarized as the Test Pyramid, and we shall learn more about...