One of the most important, yet misunderstood, aspects of Hexagonal, Clean, and Onion architectures not their folder structure, but how they keep I/O and domain logic separate. Why? The clear separation makes it easier to write tests: by moving away from the confusing terms of "unit" and "integration" tests to "I/O-based" and "I/O-free", there's no longer any confusion about how the test should be structured.

In this Java code-filled session, we'll look at how to combine Hexagonal Architecture with Domain-Driven Design's context boundaries and tactical patterns to make it clear what code goes where, and how to test it. We'll also see how to refactor "mixed up" code to separate I/O from domain and application logic. The result is extremely fast tests for the most important part of your application, the domain, while also reducing the amount of tests needed for coordination.