The Key Differences Between Onion Architecture and Clean Architecture

Onion Architecture and Clean Architecture both are used to provide a standard architecture to the software. These architectures play a key role when we build large and complex projects which require regular enhancements.
In this article, we will know about Onion architecture and Clean architecture in detail. Also, we will see a basic example of .Net Core-based application to understand onion and clean architecture.
Before we proceed, we should understand what was the concern with prior architecture i.e. N-Layer Architecture.

One more point, this article is very important from an interview perspective.

Overview of N-Layer Architecture

It is one of the most popular Architectures in ASP.NET Core Applications. In this architecture, the Presentation Layer generally holds the part that the User can collaborate with, i.e, WebApi, MVC, Webforms, etc. Business Logic is likely the main piece of this whole arrangement. It holds all the rationale connected with the Business prerequisites. Presently, every application in a perfect world has its own committed Database. To access the Database, we present a Data Access Layer. This layer normally holds ORMs for ASP.NET to read or write to a database.

Disadvantages of N-Layer Architecture

To clearly understand the advantages of Onion Architecture in ASP.NET Core Applications, we will need to study the problems with N-Layer Architecture. It is one of the most commonly used Solution Architectures amongst .NET developers.

The N-Layer architecture was introduced to make sure that expert developers can work on their core expertise layers. For eg – a front-end developer can work on presentation layers and back-end developers can work on business layer and data access layers.

Although, it is good for small-scale applications. But not good for large and complex software applications. Also, if we will work with Microservice architecture it will get more complex to work with N-Layer architecture, this is one of the major problems with n-tier architecture.

One other problem with n-tier architecture is its nature to create tightly coupled layers and this may lead to deployment complexity.

Clean Architecture

Clean Architecture was introduced by Robert “Uncle Bob”. Some of the forms of clean architecture are Hexagonal Architecture, Ports and Adapter, and Onion Architecture. It builds on the concepts of Onion Architecture with some refinement. Instead of “Domain Model”, it refers to the core as “Entities”.
The main purpose of Clean Architecture is to allow the business to adapt to changing technology and interfaces.

In this article, we will see Onion architecture in detail.

Onion Architecture

The Onion architecture was first introduced by Jeffrey Palermo, to overcome the issues of the traditional N-layered architecture approach which we discussed above. It is based on Domain Driven model.

The onion layer solves the problem of tight coupling that we faced with n-tier architecture. This architecture has Domain Layer, Repository Layer, Service Layer and Presentation Layer. Domain Layer will be at the core of the architecture.

Layers in Onion Architecture

Onion Architecture divides the complete projects into 4 different layers.

• Domain Layer – It is the core of Onion architecture. This layer contains application entities and DB model classes which work independently.
• Repository Layer – This layer works as a middle layer between Domain and Service layers.
• Service Layer – The service layer is used to hold the business logic of the application and also used to set communication between the presentation and repository layers.
• Presentation Layer – This layer presents the UI of the application.

Advantages of Onion Architecture in ASP.NET Core

The below factors shows the major advantages to use Onion Architecture

• Better Testability – As the layers are more independent, it is easy to write test cases for each of the layers.
• Loosely Coupled – Onion architecture helps to write loosely coupled code which helps software developers to provide better maintainability.
• Good for Microservice
• Better flexibility, portability

Apart from that, it provides much cleaner code with well-structured Projects for better understanding with teams.

Onion Architecture Project Structure

I have created one small project in Visual Studio which explains the structure of Onion Architecture.

You may require to add the reference of Domain Layer in your repository or other projects.

Summary

Onion, and Clean Architecture all centre around the idea of externalizing the database and other dependencies instead of building around them. They build on each other and use different terminology. Onion architecture is a form of clean architecture

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