common-hexagonal-architecture

common-hexagonal-architecture

Apply Hexagonal Architecture (also known as Ports & Adapters) to keep domain logic independent of frameworks, databases, messaging systems, and UIs within a single service boundary.

When to use

Use this skill when:

• Designing or restructuring the internal architecture of a service or module
• Ensuring domain/business logic is testable without infrastructure dependencies
• Introducing a new external integration (database, API, queue, file system)
• Reviewing code to verify that infrastructure concerns do not leak into the core
• Refactoring a layered or "big ball of mud" codebase toward cleaner boundaries

Core concepts

The Hexagon (Application Core)

The hexagon contains all business logic and domain rules. It knows nothing about the outside world — no HTTP, no SQL, no file I/O. It communicates exclusively through ports.

Ports

A port is an interface (contract) that defines how the application core interacts with the outside world.

Port type	Direction	Purpose
Driving port (primary)	Inbound	Defines use cases the outside world can invoke on the core.
Driven port (secondary)	Outbound	Defines what the core needs from external systems.

• Driving ports are implemented by the application core (the use case / application service).
• Driven ports are implemented by adapters in the infrastructure layer.

Adapters

An adapter translates between the external world and a port.

Adapter type	Connects to	Examples
Driving adapter (primary)	Driving port	REST controller, CLI handler, gRPC server, message consumer
Driven adapter (secondary)	Driven port	SQL repository, HTTP client, SMTP mailer, file store

Dependency Rule

Dependencies always point inward — adapters depend on ports, ports live in the core, the core depends on nothing external.

[Driving Adapter] --> [Driving Port (interface)]
                            |
                    [Application Core]
                            |
                  [Driven Port (interface)] <-- [Driven Adapter]

Instructions

1. Define driving ports as use-case interfaces.

   • One interface per use case or cohesive group of use cases.
   • Name them after domain actions: PlaceOrder, RegisterUser, GenerateReport.
   • Accept and return only domain objects or simple DTOs — never framework-specific types.

2. Implement driving ports in application services.

   • Application services live inside the hexagon.
   • They orchestrate domain objects and call driven ports.
   • They contain no infrastructure logic (no SQL, no HTTP calls, no file I/O).

3. Define driven ports as repository/gateway interfaces.

   • One interface per external dependency concept: OrderRepository, PaymentGateway, NotificationSender.
   • Keep method signatures focused on domain needs, not technology (e.g., findByCustomerId(id) not executeQuery(sql)).
   • Driven port interfaces live in the application core alongside the domain.

4. Implement driven adapters in infrastructure.

   • Each driven adapter implements exactly one driven port interface.
   • Contains all technology-specific code (ORM calls, HTTP clients, SDK usage).
   • Can be swapped without touching the core (e.g., switch from PostgreSQL to DynamoDB).

5. Implement driving adapters at the boundary.

   • A driving adapter receives external input (HTTP request, CLI args, queue message).
   • It translates the input into a call on a driving port.
   • It translates the port's response back into an external response (JSON, exit code, ack).
   • A driving adapter must not contain business logic — only mapping and delegation.

6. Wire dependencies via injection.

   • Use constructor injection (or a composition root / DI container) to pass driven adapter instances into application services.
   • The composition root is the only place that knows about concrete adapter implementations.
   • In tests, inject fake/mock/in-memory adapters through the same ports.

7. Folder/module structure. Organize a service to clearly reflect the hexagon:

   src/
     <service-or-module>/
       core/
         domain/
           model/             # Entities, value objects, aggregates
           event/             # Domain events
           service/           # Domain services (pure logic)
         port/
           in/               # Driving port interfaces (use cases)
           out/              # Driven port interfaces (repositories, gateways)
         application/
           service/          # Application services implementing driving ports
           dto/              # Input/output DTOs for ports
       adapter/
         in/
           web/             # REST/GraphQL controllers
           cli/             # CLI command handlers
           messaging/       # Message/event consumers
         out/
           persistence/     # Database repositories
           http/            # External API clients
           messaging/       # Event publishers, queue producers
           filesystem/      # File storage adapters
       config/              # Composition root, DI wiring, framework bootstrap
   

8. Testing strategy aligned to the hexagon.

   • Unit tests: test domain model and application services with in-memory/fake driven adapters. No real I/O.
   • Integration tests: test driven adapters against real infrastructure (database, external API with testcontainers or sandbox).
   • Acceptance tests: invoke driving ports (or driving adapters) end-to-end with real or near-real adapters to verify full use cases.
   • The majority of tests should be unit tests against the core — fast, deterministic, no mocking of internals.

9. Guard the boundaries.

   • The core must never import from adapter/ packages.
   • Adapters may only import from core/port/ (and core/domain/model/ for types).
   • Enforce this with module boundaries (lint rules, package visibility, architecture tests like ArchUnit/Deptrac/dependency-cruiser).

10. Validate the architecture.

    • Can you test any use case by injecting fakes for all driven ports? If not, a dependency is leaking.
    • Can you replace a driven adapter (e.g., swap Postgres for an in-memory store) without changing the core? If not, the port is too coupled.
    • Does the driving adapter contain conditional business logic? If yes, push it into the core behind the driving port.
    • Is the composition root the only file that references concrete adapter classes? If not, wiring has leaked.