The Domain is King

It should have become obvious in the previous chapters that the main feature of a hexagonal architecture style is that we can develop the domain code free from diversions such as persistence concerns and dependencies upon external systems.

Evolving domain code free from external influence is the single most important argument for the hexagonal architecture style.

This is why this architecture style is such a good match for Domain-Driven Design (DDD) practices. To state the obvious, in DDD the domain drives the development. And we can best reason about the domain if we don't have to think about persistence concerns and other technical aspects at the same time.

I would even go so far as to say that domain-centric architecture styles such as the hexagonal style are enablers of DDD. Without an architecture that puts the domain into the center of things, without inverting the dependencies toward the domain code, we have no chance of really doing DDD; the design will always...