Hexagonal Architecture Guide

This skill guides the implementation of Hexagonal Architecture to ensure clean separation of concerns and testability.

Architectural Layers

1. Domain Layer (Core)

The heart of the application. It contains business logic and rules.

• •Dependencies: NONE. It must not depend on any outer layer.
• •Contents: Entities, Value Objects, Aggregates, Domain Services, Repository Interfaces (Ports).

2. Application Layer

Orchestrates the domain objects to perform specific user tasks (Use Cases).

• •Dependencies: Domain Layer.
• •Contents: Use Cases (Application Services), DTOs (Data Transfer Objects).
• •Responsibility:
  • •Receives input from the Driving Adapters (controllers).
  • •Validates input.
  • •Calls Domain objects or services.
  • •persist state using Repository Ports.

3. Infrastructure Layer (Adapters)

Contains the implementation of the interfaces defined in the Domain/Application layers and the entry points to the application.

• •Dependencies: Application Layer, Domain Layer.
• •Contents:
  • •Driving Adapters: HTTP Controllers (Express/Fastify), Event Listeners, CLI commands.
  • •Driven Adapters: Database Repositories (Postgres, Mongo), External APIs (Stripe, Email), File System.

Dependency Rule

Source Code Dependencies must point only inward, toward the Domain. Infrastructure -> Application -> Domain

4. Use Cases (Application Services)

Use Cases orchestrate the flow of data between the User (via DTOs) and the Domain.

Template:

interface CreateOrderDTO {
  customerId: string;
  items: { productId: string; quantity: number }[];
}

export class CreateOrderUseCase {
  constructor(
    private readonly orderRepository: OrderRepository,
    private readonly customerRepository: CustomerRepository
  ) {}

  public async execute(dto: CreateOrderDTO): Promise<void> {
    const customer = await this.customerRepository.findById(dto.customerId);
    if (!customer) throw new Error("Customer not found");

    const order = new Order(generateId(), customer.id);

    // ... logic to add items

    await this.orderRepository.save(order);
  }
}

Rules:

• •Accept DTOs, not raw HTTP requests.
• •Return DTOs or void, not Domain Entities (to avoid leaking domain logic to the controller).
• •Orchestrate, don't implement core business rules (delegating them to Entities or Domain Services).

Folder Structure

src/
  modules/
    [module-name]/
      domain/        # Inner Hexagon (Core)
        models/
        repositories/ (Interfaces)
      application/   # Use Cases
        use-cases/
        dtos/
      infrastructure/ # Outer Hexagon (Adapters)
        http/
          controllers/
          routes/
        persistence/
          repositories/ (Implementations)
          mappers/

Implementation Rules

1. •Repositories: Define the interface in domain/repositories. Implement it in infrastructure/persistence.
2. •DTOs: Use DTOs to transfer data between the Controller and the Use Case. Do not pass Domain Entities to the Controller.
3. •Mappers: Use Mappers in the Infrastructure layer to convert Database Models to Domain Entities and vice versa.