Clean Architecture + DDD + Hexagonal

Backend architecture combining DDD tactical patterns, Clean Architecture dependency rules, and Hexagonal ports/adapters for maintainable, testable systems.

This skill is an opinionated synthesis of several related architecture traditions. It is not a single canonical architecture model. Use the original source that matches the design question you are answering: DDD for domain modeling, Hexagonal Architecture for ports/adapters, Clean Architecture for dependency direction, Onion Architecture for domain-centered layering, and CQRS/Event Sourcing only for specific read/write or temporal requirements.

When to Use (and When NOT to)

Start simple. Evolve complexity only when needed. Most systems don't need full CQRS or Event Sourcing.

Pattern Boundaries

CRITICAL: The Dependency Rule

Dependencies point inward only. Outer layers depend on inner layers, never the reverse.

Infrastructure → Application → Domain
   (adapters)     (use cases)    (core)

Violations to catch:

• Domain importing database/HTTP libraries
• In this architecture style, controllers calling repositories directly instead of application use cases
• Entities depending on application services

Design validation: "Create your application to work without either a UI or a database" — Alistair Cockburn. If you can run your domain logic from tests with no infrastructure, your boundaries are correct.

Quick Decision Trees

"Where does this code go?"

Where does it go?
├─ Pure business logic, no I/O           → domain/
├─ Orchestrates domain + has side effects → application/
├─ Talks to external systems              → infrastructure/
├─ Defines HOW to interact (interface)    → port (domain or application)
└─ Implements a port                      → adapter (infrastructure)

"Is this an Entity or Value Object?"

Entity or Value Object?
├─ Has unique identity that persists → Entity
├─ Defined only by its attributes    → Value Object
├─ "Is this THE same thing?"         → Entity (identity comparison)
└─ "Does this have the same value?"  → Value Object (structural equality)

"Should this be its own Aggregate?"

Aggregate boundaries?
├─ Must be consistent together in a transaction → Same aggregate
├─ Can be eventually consistent                 → Separate aggregates
├─ Referenced by ID only                        → Separate aggregates
└─ >10 entities in aggregate                    → Split it

Rule: One aggregate per transaction. Cross-aggregate consistency via domain events (eventual consistency).

Directory Structure

src/
├── domain/                    # Core business logic (NO external dependencies)
│   ├── {aggregate}/
│   │   ├── entity              # Aggregate root + child entities
│   │   ├── value_objects       # Immutable value types
│   │   ├── events              # Domain events
│   │   ├── repository          # DDD repository interface (driven port)
│   │   └── services            # Domain services (stateless logic)
│   └── shared/
│       └── errors              # Domain errors
├── application/               # Use cases / Application services
│   ├── {use-case}/
│   │   ├── command             # Command/Query DTOs
│   │   ├── handler             # Use case implementation
│   │   └── port                # Driver port interface
│   └── shared/
│       └── unit_of_work        # Transaction abstraction
├── infrastructure/            # Adapters (external concerns)
│   ├── persistence/           # Database adapters
│   ├── messaging/             # Message broker adapters
│   ├── http/                  # REST/GraphQL adapters (DRIVER)
│   └── config/
│       └── di                  # Dependency injection / composition root
└── main                        # Bootstrap / entry point

Port placement: This skill defaults to a DDD-centered layout where aggregate repository interfaces live beside the aggregate in domain/. A stricter Hexagonal layout may instead put driven ports under application/ports/driven/. Pick one convention per codebase and keep the dependency rule intact.

DDD Building Blocks

Anti-Patterns (CRITICAL)

Implementation Order

1. Discover the Domain — Event Storming, conversations with domain experts
2. Model the Domain — Entities, value objects, aggregates (no infra)
3. Define Ports — Repository interfaces, external service interfaces
4. Implement Use Cases — Application services coordinating domain
5. Add Adapters last — HTTP, database, messaging implementations

DDD is collaborative. Modeling sessions with domain experts are as important as the code patterns.

Reference Documentation

Sources

Primary Sources

• Hexagonal Architecture — Alistair Cockburn (2005)
• Domain-Driven Design: The Blue Book — Eric Evans (2003)
• The Clean Architecture — Robert C. Martin (2012)
• Onion Architecture — Jeffrey Palermo (2008)
• Implementing Domain-Driven Design — Vaughn Vernon (2013)

Primary Pattern References

• CQRS — Martin Fowler
• Event Sourcing — Martin Fowler
• Repository Pattern — Martin Fowler (PoEAA)
• Unit of Work — Martin Fowler (PoEAA)
• Bounded Context — Martin Fowler
• Transactional Outbox — microservices.io
• Effective Aggregate Design — Vaughn Vernon

Implementation Guides

• Microsoft: DDD + CQRS Microservices
• Domain Events — Udi Dahan

Supplemental Syntheses

• Clean Architecture: Standing on the Shoulders of Giants — Herberto Graça
• Explicit Architecture — Herberto Graça (opinionated synthesis, not canonical source)
• Get Your Hands Dirty on Clean Architecture — Tom Hombergs