Onion Architecture – Software Design Patterns Explained

Onion Architecture is a design approach that aims to create a separation of concerns within the layers of an application. It is called “onion” architecture because the design is layered, with the core domain logic at the center and the outer layers representing services and infrastructure. The layers of an onion architecture are typically organized around the business domain, with the core domain logic at the center and the outer layers representing infrastructure and service concerns.

Clean Architecture is a software design approach that aims to create a separation of concerns within the layers of an application. It is based on the idea of separating the business logic of an application from the infrastructure concerns, such as the user interface, database, and network.

Hexagonal Architecture, also known as Ports and Adapters Architecture, is a design approach that aims to create a separation of concerns between the business logic of an application and its infrastructure. It is based on the idea of creating a “hexagon” of abstractions that represent the business logic, with “ports” representing the interface to the outside world and “adapters” connecting the hexagon to the external infrastructure.

Have you heard of Onion Architecture, Clean Architecture, and Hexagonal Architecture and need the 10-minute intro to what it is and how it works? Watch here for the pattern to be clearly and visually explained.

Chapters:
0:00 Intro
0:20 Background
2:16 Use Cases
2:55 The Domain Model
4:00 The Application Layer
5:06 Leaving Holes for Infrastructure
6:08 Adding a User Interface
7:25 Filling in Infrastructure
9:37 Sections of the Outer Layer
10:09 Naming (Onion, Clean, Hexagonal)

Software Design Patterns in 10 Minutes

Onion Architecture promotes decoupling application layers, making the system more maintainable, scalable, and testable.

Here’s a breakdown of Onion Architecture:

Core:

• Entities (or Domain Models): These are the primary business objects of the application. They encapsulate the business rules and data.
• Domain Services: These are services that perform operations on the domain models. They contain business logic that doesn’t naturally fit within a domain model.

Application Core:

• Application Interfaces: These define the outer layers’ operations on the core layer, such as data access operations or communication with external services.
• Application Services: These are the entry points to the core layer from the outer layers. They coordinate the application’s activity but don’t contain business logic. They call the domain services and domain models to perform operations.

Infrastructure Layer:

• This layer supports communication between the application core and external systems, such as databases, web services, messaging systems, etc. It implements the application interfaces defined in the application core.
• It’s important to note that the infrastructure layer depends on the application core, not vice versa. This inversion of dependencies is a crucial principle of Onion Architecture.

User Interface (UI) and Test Layers:

• These are the outermost layers of the architecture. The UI layer could be a web interface, a desktop application, or any other type of user interface.
• The test layer contains unit and integration tests, ensuring the core and application layers work correctly.

Critical Principles of Onion Architecture:

• Dependency Inversion: Inner layers define interfaces, and outer layers implement these interfaces. The direction of dependency is toward the center. This means that the core layer doesn’t depend on other layers, but the different layers rely on the core.
• Separation of Concerns: Each layer has a distinct responsibility. This makes the system more maintainable and modular.
• Isolation: Keeping the core independent of external concerns (like databases or third-party services), the business logic remains isolated and can be tested without external elements.
• Infrastructure and UI Agnosticism: The application’s core doesn’t know or care about databases, external services, or UI frameworks. This allows for flexibility in choosing databases and frameworks or even changing them in the future without affecting the core business logic.

Onion Architecture is trendy in Domain-Driven Design (DDD) circles, but it can be applied in various contexts to create a clean, maintainable, and scalable application structure.