Ports and adapters, pointing inward

Fusion is layered, and the layering is the architecture. There are four layers, and there is exactly one legal direction for a dependency to point: inward, toward the domain. The domain knows nothing about anyone; everyone else knows the domain.

domain/  <-- application/
                  |
                  v
          persistence/
          presentation/

The mechanism is ports and adapters (the hexagonal architecture). A port is an interface the inner layers define for something they need (“I need to save a note”). An adapter is an outer-layer class that implements it (“here’s how you save a note to Postgres”). The inner layer depends on the interface it owns; the outer layer depends on the inner layer to implement it. Dependency points inward even though data flows out.

This is not new to you, it’s your Nest instinct with the arrows made strict. In Nest you inject an interface and register a provider for it. Same idea. The discipline Fusion adds is that the interface must live in the inner layer and the implementation in the outer one, so the domain never takes a dependency on EF Core, HTTP, or anything infrastructural.