Hexagonal Architecture (Ports & Adapters)

Overview

Design systems with clear separation between business logic and infrastructure. The domain core has zero dependencies on outer layers; all external interactions flow through explicitly defined ports and adapters.

Core Concepts

Ports

• Interfaces that express application intent independent of technology
• Inbound ports (driving): define use cases the application exposes (e.g., CreateOrderUseCase)
• Outbound ports (driven): define what the application needs from the outside world (e.g., OrderRepository, PaymentGateway)

Adapters

• Concrete implementations that connect ports to external systems
• Inbound adapters: REST controllers, CLI handlers, message consumers, GraphQL resolvers
• Outbound adapters: database repositories, HTTP clients, message publishers, file storage

Dependency Rule

• All dependencies point inward toward the domain core
• Domain knows nothing about adapters, frameworks, or infrastructure
• Adapters depend on ports, never the reverse

Project Structure

src/
├── domain/              # Pure business logic, no framework dependencies
│   ├── model/           # Entities, value objects, aggregates
│   ├── service/         # Domain services
│   └── event/           # Domain events
├── application/         # Use cases / application services
│   ├── port/
│   │   ├── inbound/     # Use case interfaces (driving ports)
│   │   └── outbound/    # Repository/gateway interfaces (driven ports)
│   └── service/         # Use case implementations
├── infrastructure/      # Framework and technology concerns
│   ├── config/          # Dependency injection, app configuration
│   └── persistence/     # Database migrations, ORM config
└── adapter/
    ├── inbound/         # Controllers, CLI, event consumers
    └── outbound/        # Repository impls, API clients, message publishers

Steps

1. Identify Boundaries

• Enumerate all external dependencies (databases, APIs, message brokers, file systems)
• Identify each as an inbound or outbound interaction

2. Define Ports

• Create an inbound port interface for each use case the application exposes
• Create an outbound port interface for each external dependency the application needs

3. Implement Adapters

• Build an adapter for each port connecting to the concrete technology
• Verify all dependencies point inward (adapters depend on ports, never the reverse)

4. Validate Structure

• Confirm domain layer has zero imports from infrastructure or adapter layers
• Confirm tests can substitute adapters without touching domain logic

Output

Report structural compliance as a list of violations (port without adapter, domain importing infrastructure, etc.) or confirm the structure passes all checks. Be concise — violations only; no narration for passing checks.

Constraints

• Do not embed framework dependencies in the domain layer
• Do not bypass ports with direct infrastructure access from application services

Guidelines

• Every external dependency gets its own adapter behind a port
• Test domain logic by substituting adapters (e.g., in-memory repository for unit tests)
• Introduce a new port when a new category of external interaction appears
• Reuse existing ports when the interaction pattern is the same
• Keep the application layer thin: orchestrate domain objects, don't duplicate domain logic