Onion Architecture: A Guide to Clean and Sustainable Code

MVC is a flexible pattern that can be adapted to fit the needs of any project. It’s also easy to learn and understand, which makes it a good choice for beginner and experienced developers alike. The Onion Architecture’s testability is one of its main advantages. It is simpler to test each layer independently since the architecture encourages the separation of concerns. The program can easily be expanded with additional features and capabilities because of its modular architecture without affecting the primary domain layer. These guidelines are crucial because they free developers from the burden of sifting through a maze of disorganized code in order to swiftly add new features and solve errors.

The deeper the layer resides inside the Onion, the fewer dependencies it has. Instead of each module being responsible of instantiating it’s own dependencies, it has its dependencies injected during it’s initialization. This way, when you want to test it, you can just inject a mock that implements the interface your code is expecting to. Also, the code is easier to test due to dependency injection, which also contributes to making the software more maintainable.

Unfolding infrastructure in the Onion architecture

However, in the OnModelCreating method, we are configuring our database context based on the entity configurations from the same assembly. The interesting part with the ServiceManager implementation is that we are leveraging the power of the Lazy class to ensure the lazy initialization of our services. This means that our service instances are only going to be created when we access them for the first time, and not before that. Using dependency inversion throughout the project, depending on abstractions (interfaces) and not the implementations, allows us to switch out the implementation at runtime transparently. We are depending on abstractions at compile-time, which gives us strict contracts to work with, and we are being provided with the implementation at runtime. All of the layers interact with each other strictly through the interfaces defined in the layers below.

The abstractions can be easily mocked with a mocking library such as Moq. To learn more about unit testing your projects in ASP.NET Core check out this article Testing MVC Controllers in ASP.NET Core. Let us take a look at what are the advantages of Onion architecture, and why we would want to implement it in our projects. Repositories, external APIs, Event listeners, and all other code that deal with IO in some way should be implemented in this layer. Your Domain models can have Value objects in their attributes, but the opposite is not allowed.

ICustom Service

Therefore, if there are corrections to be made, do leave a comment below and I will update the information here. This way, all of us get to benefit and improve the general quality of software. Services in the domain layer exists only if and only if there is a need for operations that does not quite fit into an aggregate root or entity.

  • In the very center we see the Domain Model, which represents the state and behavior combination that models truth for the organization.
  • We have already discussed the separation of concerns as one of the principles in Onion Architecture, but we must understand the differences in couplings.
  • We are also able to write Unit Tests for our business logic whilst not coupling our tests to implementation either.
  • It’s a software that any developer should be able to do improvements and fixes without worrying about breaking something under the hood.
  • Much Cleaner Codebase with well-structured Projects for better understanding with teams.
  • Now, let’s work on the Core Layers starting from the Domain Project.

The most prominent classes in the Core are Visitor and VisitorProcessor, members of domain model and application services layers respectively. Different layers of onion architecture have a different set of responsibilities and accordingly, there are different testing strategies. The testing pyramid is a great framework that lays out the different types of tests.

Benefits of Onion Architecture:

Onion Architecture addresses the challenges faced with 3-tier and n-tier architectures, and to provide a solution for common problems. Onion architecture layers interact to each other by using the Interfaces. C# programmers are drawn to Onion Architecture due to the dependency flows.

what is onion architecture

Onion architecture, also known as clean architecture, is a software design principle that suggests separating an application into layers. In the context of a web application, this would typically include a presentation layer, a business logic layer, and a data access layer. The goal of this separation is to increase code reusability and decrease coupling between the different parts of the application. As you can see in the picture, the three inner layers i.e. domain model, domain services, and application services are parts of the application core. Application core contains all logic necessary to run and test the application as long as necessary dependencies are provided at runtime. This is possible thanks to the dependency rule that we introduced in the previous paragraph.

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The Domain layer is responsible for maintaining the state and the behavior of the application. The domain layer lies in the heart of what is onion architecture the Onion Architecture, representing the business and behavioral objects. All of your domain objects should be located at this core.

what is onion architecture

The service layer holds all the business logic of the entity. In this layer services interfaces are kept separate from their implementation for loose coupling and separation of concerns. In an application following the Onion Architecture, the business logic is typically stored in the Domain layer. It represents the core of the application and is independent of the infrastructure and the user interface. The Domain layer defines the entities, services, and interfaces that are required by the application.

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It’s easier to maintain an application that has a good separation of concerns. It’s the outer-most layer, and keeps peripheral concerns like UI and tests. For a Web application, it represents the Web API or Unit Test project. This layer has an implementation of the dependency injection principle so that the application builds a loosely coupled structure and can communicate to the internal layer via interfaces. Onion Architecture solved these problem by defining layers from the core to the Infrastructure. It applies the fundamental rule by moving all coupling towards the center.

As this layer is purely logical, it should be pretty easy to test it, as you don’t have to worry about mocking IO operations. Nishi is an inquisitive architect based in Nepal who sees architecture as a powerful means of storytelling. With a genuine belief that just like every person, every building has a unique story to tell, she is passionate about unraveling these narratives and expressing them through her words. The complex puzzle box invitation that was mailed to each guest was reverse-engineered to serve as a miniature model of the dome.

Solution Structure

They handle injection of repositories into domain services that need them to function. Use a facade to provide a common entry point for multiple endpoints (RESTful API, SOAP and direct function call) if they need to consume a service provided by the application layer. Domain-Driven Design gives us a more realistic approach to defining what really has business value.

Ubiquitous language between domain experts and developers

Mark Seeman in the “Dependency Injection in .NET”, chapter 2, draw layers without something called “infrastructure”, effectively bypassing this piece of the software as well. He only focusing on analysis of data access as a crucial piece of infrastructure. The Domain layer, which contains the business logic, can be easily tested without the need for the Infrastructure layer or the User Interface layer.

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We are going to use them in a global exception handler that will return the proper HTTP status code based on the type of exception that was thrown. The flow of dependencies dictates what a certain layer in the Onion architecture can do. Because it depends on the layers below it in the hierarchy, it can only call the methods that are exposed by the lower layers. The main idea behind the Onion architecture is the flow of dependencies, or rather how the layers interact with each other.

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