>> Onion Architecture for dummies

Ever wondered how to structure your software projects for better maintainability and scalability? Look no further! In this blog post, we’ll explore the concept of Onion Architecture in a way that’s easy to understand, even if you’re new to software design patterns.

What is Onion Architecture?

In simple terms, Onion Architecture is a way of organizing your code into layers, like the layers of an onion. Each layer has a specific role and communicates only with the layers directly adjacent to it. This design helps keep your core business logic independent from external concerns like databases or user interfaces.

The Layers of Onion Architecture

1. Core Layer: This is the innermost layer and contains your domain entities and business logic. It should not depend on any other layers.
2. Application Layer: This layer contains application services that orchestrate the business logic. It depends on the Core Layer but not on any external systems.
3. Infrastructure Layer: This layer deals with external systems like databases, file systems, or web services. It depends on the Application Layer but not on the Core Layer.
4. Presentation Layer: This is the outermost layer that handles user interactions, such as web pages or APIs. It depends on the Application Layer but not on the Core Layer.

Think of it like this:

[ Presentation Layer ]
          ↑
[ Infrastructure Layer ]
          ↑
[ Application Layer ]
          ↑
[     Core Layer     ]

Each layer can only interact with the layer directly beneath it, ensuring a clear separation of concerns.

Benefits of Onion Architecture

• Separation of Concerns: Each layer has a clear responsibility, making it easier to manage and understand the code.
• Testability: Since the core logic is independent of external systems, you can easily write unit tests for your business logic.
• Maintainability: Changes in one layer have minimal impact on other layers, making it easier to update and maintain the codebase.
• Flexibility: You can swap out external systems (like databases) without affecting the core business logic.

How to Implement Onion Architecture

1. Define your domain entities in the Core Layer as plain Java classes.
2. Create service interfaces in the Application Layer that define the operations your application will perform
3. Implement the service interfaces in the Infrastructure Layer, where you can interact with databases or other external systems.
4. Build your controllers or UI components in the Presentation Layer that interact with the Application Layer services.

Find below some example folder structures for different types of projects following the Onion Architecture:

Java Spring Boot project example

com.example.project
│
├── core
│   └── domain
│       └── User.java
├── application
│   └── service
│       └── UserService.java
├── infrastructure
│   └── repository
│       └── UserRepositoryImpl.java
└── presentation
    └── controller
        └── UserController.java

Node.js backend project example

├── core
│   └── models
│       └── user.js
├── application
│   └── services
│       └── userService.js
├── infrastructure
│   └── repositories
│       └── userRepository.js
└── presentation
    └── routes
        └── userRoutes.js

React frontend project example

src/
├── core
│   └── models
│       └── User.js
├── application
│   └── services
│       └── UserService.js
├── infrastructure
│   └── api
│       └── UserApi.js
└── presentation
    └── components
        └── UserComponent.js

Conclusion

Onion Architecture is a powerful design pattern that can help you build robust and maintainable software applications. By organizing your code into distinct layers, you can achieve better separation of concerns, testability, and flexibility. Give it a try in your next project and see the benefits for yourself!

Happy coding! 🚀