Onion Architecture Used in Software Development DEV Community

The Domain layer defines the entities, services, and interfaces that are required by the application. The Domain layer is responsible for maintaining the state and the behavior of the application. Onion Architecture is a software architecture pattern that follows the Dependency Inversion Principle.

This way, when you want to test it, you can just inject a mock that implements the interface your code is expecting to. So, the only place in your application that actually creates objects that are capable of doing IO is the application’s entrypoint. The Infrastructure Layer uses them, but is does not create them. In Onion Architecture, the database is just a infrastructure detail.


It would be really cumbersome to implement, for example, a simple gateway using Onion Architecture. The Infrastructure Layer is the outermost layer of the Onion Architecture. They represent the business models, containing the business rules from it’s domain. Using Gradle setup as an example, one can define three modules — domain, application, and infrastructure —
in settings.gradle file. Then, in the build files corresponding to each of the modules, declare their dependencies,
clearly defining the direction of dependencies. The domain, although the most important part of the application, tends to be also the smallest in terms of code size.

onion architecture software

It allows developers to focus on the value-providing
implementation rather than thinking Hmm where should I put this class?. Additional complexity to the build setup and extra learning curve introduced by the layered approach pays back during
development. It reduces the cognitive load on the programmer by giving a more concrete structural foundation and guidance. Onion Architecture requires additional code to implement the layers of the application. This can result in increased code overhead and a larger codebase, which can make the application more difficult to maintain.

The Onion has Layers

Furthermore, the added complexity of defining contracts / interfaces and religiously enforcing them requires a strong understanding of the pattern. If executed well, the benefits will supercharge productivity and greatly increase the flexibility of the applications being developed. Previously, we used Microsoft’s data access stack as an example of onion-based architecture.

  • The change in paradigm is not so straightforward, so you will need to invest some time in learning the architecture before you can use it effortlessly.
  • It holds all the logic related to the Business requirements.
  • BusinessAPIs will be proprietary APIs and access to these APIs will be subscription based (paid access).
  • An application written to help manage a Library would most probably have classes like Book,
    Reader, Copy and so on.
  • With this approach, if we don’t provide an actual CancellationToken value a CancellationToken.None will be provided for us.
  • For a Web application, it represents the Web API or Unit Test project.

The clear separation of concerns and decoupling of dependencies enable easier maintenance and modification of code, making it more adaptable to changing requirements. To demonstrate a common folder structure based on Onion Architecture, let’s consider a hypothetical e-commerce application. Having created a domain model and a web API, we needed to seamlessly connect them. It greatly depends on the complexity of the application and the size of the project to divide source code into multiple modules.

Turn web pages into structured data

In cases where there is both a physical and logical separation of concerns, it is often referred to as n-tiered application where n is the number of separations. Performance overhead may result from the Onion Architecture’s additional layers and interfaces, particularly in applications that demand high-performance or real-time processing. Using contracts onion architecture software allows each layer to set its expectations onto the next and couples it to only what it requires to be. This is a simple use-case but the real question being asked is why. Onion Architecture builds on the Ports & Adapters Architecture to add some internal organisation to the business logic of the application based on a few Domain Driven Design concepts.

onion architecture software

This layer contains the implementation of the behaviour contracts defined in the Model layer. The primary proposition of this architecture is good coupling. The higher the coupling, the lower the ability to change and evolve the system. Instead of each module being responsible of instantiating it’s own dependencies, it has its dependencies injected during it’s initialization.

Deploy and update your applications in a declarative manner with the option to roll back if things don’t go as planned

Each layer has a specific responsibility, and there is a clear separation of concerns between the layers. This makes the application more modular, easier to understand, and maintainable. This is another variant that I have noticed in many huge solutions. Let’s say you have around 100 interfaces and 100 implementations.

onion architecture software

The rider selects their destination, then are presented with an estimated price for their trip. Trip estimation is a business use-case, and it’s the one I’ve selected for our implementation. Figure 2 below outlines the domain within the application structure. It can be hard to implement a service using Onion Architecture when you have a database-centric background. The change in paradigm is not so straightforward, so you will need to invest some time in learning the architecture before you can use it effortlessly.

SOLID principles: building robust and maintainable code with Typescript

We have connected all of our Onion architecture implementation layers, and our application is now ready for use. However, in the OnModelCreating method, we are configuring our database context based on the entity configurations from the same assembly. As you can see, we mark the service implementations with the internal keyword, which means they will not be publicly available outside of the Services project. As we can see, it consists of the Web project, which is our ASP.NET Core application, and six class libraries. The Domain project will hold the Domain layer implementation.

We will do a simple test to ensure that our solution works. I will just create a new product and make a request to query all the existing products as well. You can see that we are adding the API Versioning data to the route attribute and also creating an IMediator object. Just to make our solution a bit clean, let’s also add API Versioning to the WebAPI.

Architecture Overview

In this article, we will delve into the key concepts of Onion Architecture and provide an example folder structure that illustrates its implementation. The Service layer holds interfaces with common operations, such as Add, Save, Edit, and Delete. Also, this layer is used to communicate between the UI layer and repository layer.


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