clean-ddd-hexagonal — Skillopedia

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 spe…

);\n\n await expect(rule).toPassAsync();\n });\n\n it('domain events should be named in past tense', async () => {\n const rule = filesOfProject()\n .inFolder('domain/**/events')\n .should()\n .matchPattern('.*(Created|Updated|Deleted|Confirmed|Shipped|Cancelled)\\\\.ts

Important: agents should read /llm.txt, /llms.txt, or /.well-known/skills.json to discover the public Skillopedia API.

);\n\n await expect(rule).toPassAsync();\n });\n });\n});\n```\n\n---\n\n## Test Organization\n\n```\ntests/\n├── unit/\n│ ├── domain/\n│ │ ├── order/\n│ │ │ ├── order.test.ts\n│ │ │ ├── order_item.test.ts\n│ │ │ └── value_objects.test.ts\n│ │ └── shared/\n│ │ ├── money.test.ts\n│ │ └── email.test.ts\n│ └── application/\n│ ├── place_order/\n│ │ └── handler.test.ts\n│ └── confirm_order/\n│ └── handler.test.ts\n├── integration/\n│ ├── persistence/\n│ │ └── postgres_order_repository.test.ts\n│ ├── messaging/\n│ │ └── rabbitmq_event_publisher.test.ts\n│ └── http/\n│ └── orders_api.test.ts\n├── e2e/\n│ └── order_workflow.test.ts\n├── architecture/\n│ └── dependency_rules.test.ts\n├── fixtures/\n│ ├── order_fixtures.ts\n│ └── product_fixtures.ts\n└── helpers/\n ├── test_database.ts\n └── mock_factories.ts\n```\n\n---\n\n## Test Fixtures & Builders\n\n```typescript\n// tests/fixtures/order_fixtures.ts\nexport class OrderBuilder {\n private customerId: CustomerId = CustomerId.from('default-customer');\n private items: Array\u003c{ productId: ProductId; quantity: Quantity; price: Money }> = [];\n private status: 'draft' | 'confirmed' | 'shipped' | 'cancelled' = 'draft';\n\n withCustomer(id: string): this {\n this.customerId = CustomerId.from(id);\n return this;\n }\n\n withItem(productId: string, quantity: number, price: number): this {\n this.items.push({\n productId: ProductId.from(productId),\n quantity: Quantity.create(quantity),\n price: Money.create(price, 'USD'),\n });\n return this;\n }\n\n confirmed(): this {\n this.status = 'confirmed';\n return this;\n }\n\n build(): Order {\n const order = Order.create(this.customerId);\n\n for (const item of this.items) {\n order.addItem(item.productId, item.quantity, item.price);\n }\n\n if (this.status === 'confirmed') {\n order.setShippingAddress(new AddressBuilder().build());\n order.confirm();\n }\n\n order.clearEvents(); // Clear events from building\n return order;\n }\n}\n\n// Usage\nconst order = new OrderBuilder()\n .withCustomer('cust-123')\n .withItem('prod-1', 2, 10.00)\n .withItem('prod-2', 1, 25.00)\n .confirmed()\n .build();\n```\n\n---\n\n## Key Testing Principles\n\n1. **Test behavior, not implementation** - Focus on what, not how\n2. **Domain tests need no mocks** - Domain layer is pure\n3. **Mock at port boundaries** - Application tests mock driven ports\n4. **Integration tests use real infra** - Test actual database, message broker\n5. **Fast unit tests, slower integration** - Run unit tests frequently\n6. **Test business rules in domain** - Not in application or infrastructure\n","content_type":"text/markdown; charset=utf-8","language":"markdown","size":19104,"content_sha256":"94f27ba8cc178654f00468cf5f2e7a0957a12e268f43161d8f6dd1afbf43e118"}],"content_json":{"type":"doc","content":[{"type":"heading","attrs":{"level":1},"content":[{"text":"Clean Architecture + DDD + Hexagonal","type":"text"}]},{"type":"paragraph","content":[{"text":"Backend architecture combining DDD tactical patterns, Clean Architecture dependency rules, and Hexagonal ports/adapters for maintainable, testable systems.","type":"text"}]},{"type":"paragraph","content":[{"text":"This skill is an ","type":"text"},{"text":"opinionated synthesis","type":"text","marks":[{"type":"strong"}]},{"text":" 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.","type":"text"}]},{"type":"heading","attrs":{"level":2},"content":[{"text":"When to Use (and When NOT to)","type":"text"}]},{"type":"table","attrs":{"layout":null},"content":[{"type":"tr","content":[{"type":"th","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Use When","type":"text"}]}]},{"type":"th","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Skip When","type":"text"}]}]}]},{"type":"tr","content":[{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Complex business domain with many rules","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Simple CRUD, few business rules","type":"text"}]}]}]},{"type":"tr","content":[{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Long-lived system (years of maintenance)","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Prototype, MVP, throwaway code","type":"text"}]}]}]},{"type":"tr","content":[{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Team of 5+ developers","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Solo developer or small team (1-2)","type":"text"}]}]}]},{"type":"tr","content":[{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Multiple entry points (API, CLI, events)","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Single entry point, simple API","type":"text"}]}]}]},{"type":"tr","content":[{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Need to swap infrastructure (DB, broker)","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Fixed infrastructure, unlikely to change","type":"text"}]}]}]},{"type":"tr","content":[{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"High test coverage required","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Quick scripts, internal tools","type":"text"}]}]}]}]},{"type":"paragraph","content":[{"text":"Start simple. Evolve complexity only when needed.","type":"text","marks":[{"type":"strong"}]},{"text":" Most systems don't need full CQRS or Event Sourcing.","type":"text"}]},{"type":"heading","attrs":{"level":2},"content":[{"text":"Pattern Boundaries","type":"text"}]},{"type":"table","attrs":{"layout":null},"content":[{"type":"tr","content":[{"type":"th","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Pattern","type":"text"}]}]},{"type":"th","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Primary Question","type":"text"}]}]},{"type":"th","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Use It For","type":"text"}]}]},{"type":"th","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Do Not Treat 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Architecture","type":"text","marks":[{"type":"strong"}]}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"How does the application interact with the outside world?","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Ports, driver adapters, driven adapters, testable application core","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"A mandate for six sides or one exact package layout","type":"text"}]}]}]},{"type":"tr","content":[{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Clean 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Architecture","type":"text","marks":[{"type":"strong"}]}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"How do we keep the domain model central?","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Domain-centered layers and dependency inversion","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"A separate requirement when Clean/Hexagonal already solve the local problem","type":"text"}]}]}]},{"type":"tr","content":[{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"CQRS","type":"text","marks":[{"type":"strong"}]}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Do reads and writes need different models?","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Bounded contexts with divergent read/write workloads","type":"text"}]}]},{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"A default application architecture","type":"text"}]}]}]},{"type":"tr","content":[{"type":"td","attrs":{"colspan":1,"rowspan":1,"colwidth":null,"alignment":""},"content":[{"type":"paragraph","content":[{"text":"Event 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Outer layers depend on inner layers, never the reverse.","type":"text"}]},{"type":"code_block","attrs":{"wrap":false,"language":""},"content":[{"text":"Infrastructure → Application → Domain\n (adapters) (use cases) (core)","type":"text"}]},{"type":"paragraph","content":[{"text":"Violations to catch:","type":"text","marks":[{"type":"strong"}]}]},{"type":"bullet_list","content":[{"type":"list_item","content":[{"type":"paragraph","content":[{"text":"Domain importing database/HTTP libraries","type":"text"}]}]},{"type":"list_item","content":[{"type":"paragraph","content":[{"text":"In this architecture style, controllers calling repositories directly instead of application use cases","type":"text"}]}]},{"type":"list_item","content":[{"type":"paragraph","content":[{"text":"Entities depending on application services","type":"text"}]}]}]},{"type":"paragraph","content":[{"text":"Design validation:","type":"text","marks":[{"type":"strong"}]},{"text":" \"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.","type":"text"}]},{"type":"heading","attrs":{"level":2},"content":[{"text":"Quick Decision Trees","type":"text"}]},{"type":"heading","attrs":{"level":3},"content":[{"text":"\"Where does this code go?\"","type":"text"}]},{"type":"code_block","attrs":{"wrap":false,"language":""},"content":[{"text":"Where does it go?\n├─ Pure business logic, no I/O → domain/\n├─ Orchestrates domain + has side effects → application/\n├─ Talks to external systems → infrastructure/\n├─ Defines HOW to interact (interface) → port (domain or application)\n└─ Implements a port → adapter (infrastructure)","type":"text"}]},{"type":"heading","attrs":{"level":3},"content":[{"text":"\"Is this an Entity or Value Object?\"","type":"text"}]},{"type":"code_block","attrs":{"wrap":false,"language":""},"content":[{"text":"Entity or Value Object?\n├─ Has unique identity that persists → Entity\n├─ Defined only by its attributes → Value Object\n├─ \"Is this THE same thing?\" → Entity (identity comparison)\n└─ \"Does this have the same value?\" → Value Object (structural equality)","type":"text"}]},{"type":"heading","attrs":{"level":3},"content":[{"text":"\"Should this be its own Aggregate?\"","type":"text"}]},{"type":"code_block","attrs":{"wrap":false,"language":""},"content":[{"text":"Aggregate boundaries?\n├─ Must be consistent together in a transaction → Same aggregate\n├─ Can be eventually consistent → Separate aggregates\n├─ Referenced by ID only → Separate aggregates\n└─ >10 entities in aggregate → Split it","type":"text"}]},{"type":"paragraph","content":[{"text":"Rule:","type":"text","marks":[{"type":"strong"}]},{"text":" One aggregate per transaction. Cross-aggregate consistency via domain events (eventual consistency).","type":"text"}]},{"type":"heading","attrs":{"level":2},"content":[{"text":"Directory Structure","type":"text"}]},{"type":"code_block","attrs":{"wrap":false,"language":""},"content":[{"text":"src/\n├── domain/ # Core business logic (NO external dependencies)\n│ ├── {aggregate}/\n│ │ ├── entity # Aggregate root + child entities\n│ │ ├── value_objects # Immutable value types\n│ │ ├── events # Domain events\n│ │ ├── repository # DDD repository interface (driven port)\n│ │ └── services # Domain services (stateless logic)\n│ └── shared/\n│ └── errors # Domain errors\n├── application/ # Use cases / Application services\n│ ├── {use-case}/\n│ │ ├── command # Command/Query DTOs\n│ │ ├── handler # Use case implementation\n│ │ └── port # Driver port interface\n│ └── shared/\n│ └── unit_of_work # Transaction abstraction\n├── infrastructure/ # Adapters (external concerns)\n│ ├── persistence/ # Database adapters\n│ ├── messaging/ # Message broker adapters\n│ ├── http/ # REST/GraphQL adapters (DRIVER)\n│ └── config/\n│ └── di # Dependency injection / composition root\n└── main # Bootstrap / entry point","type":"text"}]},{"type":"paragraph","content":[{"text":"Port placement:","type":"text","marks":[{"type":"strong"}]},{"text":" This skill defaults to a DDD-centered layout where aggregate repository interfaces live beside the aggregate in ","type":"text"},{"text":"domain/","type":"text","marks":[{"type":"code_inline"}]},{"text":". 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Important: agents should read /llm.txt, /llms.txt, or /.well-known/skills.json to discover the public Skillopedia API.