Both Clean Architecture and Onion Architecture are designed to help developers build maintainable, scalable, and testable software systems. While they share similarities in their layered approach, their core principles and emphasis differ.
Clean Architecture
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Core Concept: Introduced by Robert C. Martin (Uncle Bob), Clean Architecture emphasizes the separation of concerns through concentric layers.
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Layers:
- The innermost layer contains the application’s core business logic (use cases and entities).
- Surrounding layers handle interfaces, adapters, and frameworks.
- The outermost layer deals with technical concerns like UI, database access, and frameworks.
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Dependency Management: Clean Architecture relies on dependency inversion, ensuring that high-level modules are not dependent on low-level modules. This decouples components, making it easier to replace or modify parts of the system.
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Focus: The primary focus is on business rule independence, ensuring the core logic remains unaffected by changes in external systems like databases or UI frameworks.
Onion Architecture
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Core Concept: Introduced by Jeffrey Palermo, Onion Architecture organizes the system as concentric circles resembling an onion.
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Layers:
- The innermost layer houses the core domain model—entities and domain logic.
- Middle layers include services and repositories.
- The outermost layer addresses technical concerns like infrastructure and UI.
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Dependency Management: Dependencies flow from the outer layers to the innermost core, isolating the core domain model from external dependencies. This ensures the domain remains unaffected by technical details.
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Focus: Onion Architecture prioritizes a clear separation of concerns, with the domain model at the center and technical concerns on the periphery.
Key Differences
| Feature | Clean Architecture | Onion Architecture |
|---|---|---|
| Origin | Robert C. Martin (Uncle Bob) | Jeffrey Palermo |
| Core Emphasis | Business rule independence | Clear separation of concerns |
| Core Layer | Use cases and entities | Domain model |
| Dependency Management | Uses dependency inversion | Dependencies flow inward toward the core |
| Visualization | Layers of concentric circles | Concentric circles resembling an onion |
Similarities
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Layered Design: Both architectures rely on concentric layers, emphasizing modularity and separation of concerns.
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Isolation of Core Logic: In both, the core logic is isolated from external concerns, ensuring maintainability and scalability.
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Focus on Independence: Both architectures seek to create systems where core components remain independent of external frameworks or technologies.
Conclusion
While both Clean Architecture and Onion Architecture aim to create modular, testable, and maintainable systems, their focus and visualization differ slightly:
- Clean Architecture prioritizes business rule independence and heavily emphasizes dependency inversion.
- Onion Architecture highlights separation of concerns, with the domain model as the central focus.
Understanding your project’s requirements and priorities will help you decide which architecture best suits your needs.