1. Hexagonal Architecture (Ports and Adapters)
Concept
- Proposed by Alistair Cockburn.
- Focuses on
decoupling the core business logic(domain) from external systems (UI, databases, message queues) using Ports and Adapters.
Structure
- Core (Domain Model): Contains business logic.
- Ports: Interfaces that define input/output operations.
- Adapters: Implement ports to interact with external systems.
Key Characteristics
- Inbound Adapters: Handle user inputs (HTTP controllers, CLI, etc.).
- Outbound Adapters: Handle outputs (DB, MQ, APIs).
- Easily testable: Adapters can be replaced without affecting core logic.
Use Case
- Applications that need flexibility with multiple I/O interfaces (web, CLI, MQ).
2. Onion Architecture
Concept
- Introduced by Jeffrey Palermo.
- Emphasizes
dependency inversion, where the core business logic is independent of external concerns.
Structure
- Core (Domain Layer): Entities and business rules.
- Domain Services: Business logic that doesn’t fit in entities.
- Application Layer: Use cases and service interfaces.
- Infrastructure Layer: Implements interfaces (DB, external APIs).
- Presentation Layer: UI or API controllers.
Key Characteristics
- Dependencies point inward (from outer layers to inner layers).
- Strong focus on Domain logic.
- Highly testable due to dependency inversion.
Use Case
- Domain-Driven Design (DDD) projects with complex business rules.
3. Clean Architecture
Concept
- Created by Robert C. Martin (Uncle Bob).
- Combines principles from Hexagonal and Onion architectures.
- Prioritizes business rules and enforces clear separation between layers.
Structure
- Entities (Core): Enterprise-wide business rules.
- Use Cases (Application): Specific business rules.
- Interface Adapters: Converts data between use cases and external interfaces.
- Frameworks & Drivers: External systems (DB, UI, MQ).
Key Characteristics
- Dependency Rule: Inner layers must not depend on outer layers.
- Testable and maintainable.
- Frameworks are easily replaceable.
Use Case
- Enterprise-level applications requiring scalability and maintainability.
Comparison Table
| Feature | Hexagonal Architecture | Onion Architecture | Clean Architecture |
|---|---|---|---|
| Focus | Decoupling I/O via Ports & Adapters | Dependency Inversion & Domain Core | Clear Separation & Business Rules |
| Layers | Core, Ports, Adapters | Domain, Application, Infrastructure | Entities, Use Cases, Adapters, Frameworks |
| Dependency Direction | Toward Core | Toward Domain Core | Toward Business Rules |
| Testability | High | High | High |
| External Systems | Managed via Adapters | Managed in Infrastructure Layer | Managed in Frameworks Layer |
| Best for | Systems with multiple I/O interfaces | DDD-based complex business logic | Large, scalable applications |
Which Architecture to Choose?
- Hexagonal Architecture: Best when the application interacts with various I/O systems (REST API, CLI, MQ).
- Onion Architecture: Suitable for applications with complex domains requiring pure business logic.
- Clean Architecture: Ideal when scalability, testability, and maintainability are top priorities.
In practice, these architectures are not mutually exclusive. Many systems blend concepts from all three to fit specific requirements.