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Hexagonal Architecture + Domain Driven Design. How to perform a correct implementation?

Hello!

Currently, I am trying to implement these two architectures together with Java and Spring (although technology shouldn't matter I think).

But I'm encountering problems getting them to work together.

I understand that my application should have a structure with 3 layers and 2 zones (inside and outside the hexagon):

**Domain:** Entities that should be self-contained with their own functionality, Repositories, ValueObjects, Factories, Events, etc. This layer is the center of the hexagon.

**Application:** Where the ports should be found. I've also seen that this is where REST or "input" adapters are typically implemented. This layer is at the edge of the hexagon.

**Infrastructure:** the implementation of the output ports, where external libraries and frameworks should already be used. This layer is outside the hexagon.

But this raises several questions for me. Why are adapters typically implemented in the application layer or in the infrastructure layer, depending on whether they are input or output?

What do repositories and domain services do? Shouldn't these be in the application layer as interfaces, and infrastructure should implement them? After all, all the logic should go in the entity.


Right now I have this structure (after following several guides and reviewing some repositories on GitHub):

   Shop (Base Module)

    Shop-Domain (Submodule)
        src/main/java
        ├─── net.test.sell.domain
        │    └─── error (exceptions related)
        │    └─── model (Entities, ValueObjects, Events, etc...)
        ├─── net.test.workers.domain
        ....
     
    Shop-Application (Submodule)
        src/main/java
        ├─── net.test.sell.aplication
        │    └─── ports
        |          └─── input (implementation of use cases)
                   └─── output (interfaces for infrastructure submodule)
        │    └─── use_cases (interfaces)
        ├─── net.test.workers.application
    
    Shop-Infrastructure (Submodule)
        src/main/java
        ├─── net.test.sell.infrastructure
        │    └─── database
        |          └─── adapters (implementation output interfaces)
        │    └─── rest_api(interfaces)
        |          └─── controllers (just REST Controllers)
        ├─── net.test.workers.infrastructure
        ...

 

I also have some questions regarding the most appropriate folder structure for this architecture.


Why do InputPorts use UseCases and OutputPorts are interfaces that are later implemented in infrastructure?

Is it a good practice to implement each small UseCase in a separate interface? Or can everything be done in one?

Finally. What is the correct naming convention for the different projects and packages?

Thanks in advance!