Onion architecture is a software architectural style that prioritizes a strong and cohesive system core while keeping libraries and dependencies on the outer layers. It emphasizes maintaining a stable and efficient core by strategically managing dependencies.
Understanding the Layers of Onion Architecture
The onion architecture is structured in concentric layers, much like an onion. Each layer has a specific role, and the key principle is that inner layers don't depend on outer layers, but outer layers can depend on inner layers. This dependency rule ensures the core remains independent from external changes.
Core Layer (Entities/Domain)
- The innermost layer.
- Contains the core business logic and entities (data structures).
- Has no dependencies on any other layer, making it highly stable and reusable.
Use Case Layer (Application Business Logic)
- Contains use cases of the application that implements the business logic.
- Depends on the inner Core Layer.
- Defines how the application interacts with entities.
Interface Adapters Layer (Controllers/Repositories)
- This layer contains interfaces for databases, UI frameworks, and external services.
- Translates data to a format suitable for the outer layers, making the system more independent of the framework.
Frameworks and Drivers Layer (UI/Database/External Services)
- The outermost layer.
- Contains the actual implementations of interfaces, like UI, databases, and external APIs.
- Depends on all other layers.
Key Principles and Advantages
- Dependency Inversion: Inner layers are independent of outer layer implementation details.
- Testability: Easier to test due to clear separation of concerns and less coupled components.
- Maintainability: Changes in the UI or framework are less likely to affect the core logic.
- Flexibility: Easier to adapt to new technologies and frameworks.
- According to the provided reference, onion architecture helps maintain libraries and dependencies on the extremities of a software system while sustaining a strong and cohesive system core.
- A stable and efficient system core is essential when basing a system's architecture on that of an onion.
Practical Insights
- When developing an application, start from the core (entities) and work outward.
- Use interfaces to define communication between layers.
- Keep the core as stable as possible.
- Refactor code regularly to maintain the architecture's integrity.
Example of Layer Dependency
| Layer | Dependencies |
|---|---|
| Core (Domain/Entities) | None |
| Use Cases | Core |
| Interface Adapters | Use Cases, Core |
| Frameworks/Drivers | All other layers |
