Hexagonal architecture - Coding Assistant Guidelines

1. Domain Layer Purity

• The domain layer contains entities, value objects, and aggregates.
• It must not depend on:
  • Application layer
  • Infrastructure layer
  • External libraries (except standard language libs)

2. Application Layer Depends Only on Domain

• Application services (use cases) can depend on the domain layer.
• They must not depend on the infrastructure layer.

3. Infrastructure Implements Ports Only

• The infrastructure layer (DB, APIs, messaging, file systems, etc.) should implement ports defined in the domain or application.
• It must not introduce its own business logic.

4. Adapters Contain No Business Logic

• Infrastructure adapters (e.g., database repositories, API clients) should contain only integration logic.
• They should not implement domain rules such as validation, calculations, or business decisions.

5. Ports Are Stable Contracts

• Ports define the interfaces for repositories, services, or external dependencies.
• They must not import or depend on the infrastructure layer.

6. Entry Points Live in Interfaces Layer

• All external entry points (REST controllers, GraphQL resolvers, CLI commands, etc.) belong to the interfaces layer.
• They should call application services only, not the domain or infrastructure directly.

7. Tests Respect Boundaries

• Unit tests should mock dependencies across layers.
• Integration tests should wire adapters together but always go through ports.