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What is Onion Architecture | Jeffrey Palermo | Software architectural pattern
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Sun , Oct 05 2025

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What is Onion Architecture ?

Onion Architecture, introduced by Jeffrey Palermo, is a software architectural pattern emphasizing the separation of concerns and the independence of the application's core business logic from external concerns like databases, user interfaces, or frameworks. It is often visualized as concentric layers, resembling an onion, where dependencies flow inward. 


    Key Layers in Onion Architecture (and how they relate to C# projects):

      • Domain Layer (Core):
        • This is the innermost layer and contains the application's most essential business logic, entities, and value objects.
        • It is independent of all other layers.
        • In C#, this typically maps to a class library project containing your core business entities and interfaces defining domain services.
      • Application Layer (Services/Use Cases):
        • This layer orchestrates the flow of data and interaction between the Domain Layer and the outer layers.
        • It defines application-specific services and use cases that operate on the domain entities.
        • It depends on the Domain Layer.
        • In C#, this is typically another class library project containing your application services and DTOs (Data Transfer Objects).
      • Infrastructure Layer:
        • This layer handles external concerns like database access (e.g., Entity Framework Core implementations of repositories), external service integrations, and other technical details.
        • It depends on the Domain Layer and potentially the Application Layer (for interfaces defined there).
        • In C#, this is often a separate class library project containing concrete implementations of interfaces defined in the Domain and Application layers.
      • Presentation Layer (UI/API):
              • This is the outermost layer and represents the user interface or API endpoints (e.g., ASP.NET Core Web API, MVC application).
              • It interacts with the Application Layer to perform operations and display data.
              • It depends on the Application Layer.
                • In C#, this would be your ASP.NET Core project or a console application.                                                                                

                • Key Principles and Benefits in C#:
                    • Dependency Inversion Principle: 
                      Outer layers depend on abstractions (interfaces) defined by inner layers, and inner layers are independent of outer layer implementations. This is heavily facilitated by C# interfaces and dependency injection.
                    • Separation of Concerns: 
                      Each layer has a distinct responsibility, leading to cleaner, more maintainable code.
                    • Testability: 
                      The core business logic in the Domain Layer can be tested in isolation without needing to set up databases or UI components, as it has no external dependencies.
                    • Flexibility: 
                      Changes in infrastructure (e.g., switching databases) or presentation technology have minimal impact on the core business logic.


                      Thanks for Reading.

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I am an engineer with over 10 years of experience, passionate about using my knowledge and skills to make a difference in the world. By writing on LifeDB, I aim to share my thoughts, ideas, and insights to inspire positive change and contribute to society. I believe that even small ideas can create big impacts, and I am committed to giving back to the community in every way I can.

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