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The core layer, being the central layer, doesn’t even know that your domain, API, and infrastructure exist. Ubiquitous Language, which should be used in all forms of communication, from meetings and documentation all the way to source code, becoming the domain model implemented in the code. In my implementation, I intend to demonstrate some of the key layers of this architecture and how they work together. In addition, the onion architecture itself introduced certain problems. It took us some time to distribute functional parts between appropriate layers. The outermost layer is generally composed of frameworks and tools such as the Database, the Web Framework, etc.

Onion architecture in development

If we return mutable objects through the API, people using the code could gain access to domain components we might not intend to expose. Although the API has access to the Domain and Core, it doesn’t know anything about the Infrastructure. They may be accounting managers, marketing specialists, cooks, waiters, etc. Dependency Injection is a necessary evil with this architecture. It causes us to rely heavily on something quite external that binds the entire application together and allows it to function at run-time. That being said, it’s not a big deal and it does not outweigh the pros.


Domain-Driven Design or DDD is an approach to software development that centers the development on programming a domain model that has a rich understanding of the processes and rules of a domain. Today, most Agile methodologies are encouraging rapid development to implement or enhance features as quickly as possible. To embrace this, the software onion structure architecture of the system needs to be simple and easy to extend. The onion architecture is a simple, robust, maintainable and extensible software architecture that fits perfectly in an Agile world. The domain layer is in the very center of the onion architecture. It represents the state, processes, rules and behavior of an organization.

In the custom service folder, we will create the custom service class that inherits the ICustomService interface code of the custom service class is given below. Now our service layer contains the reference of the repository layer. First, you need to create the Core web API project using visual studio. After creating the project, we will add our layer to the project.

  • This rule also exists in other similar architectures, such as Clean Architecture.
  • Concepts and technological details are important, but they are secondary.
  • Since infrastructure or things that change often are pushed to the outer edges of the onion, the onion architecture relies heavily on the Dependency Inversion Principle.
  • When we use Onion Architecture, we start with the central layer, the core.
  • To embrace this, the software architecture of the system needs to be simple and easy to extend.
  • Both software developers and domain experts should be able to talk in a Ubiquitous Language.

Domain objects are also flat as they should be, without any heavy code or dependencies. This layer lies in the center of the architecture where we have application entities which are the application model classes or database model classes. Using the code first approach in the application development using core these entities are used to create the tables in the database. Note that Anemic Domain Models is an anti-pattern when working in OOP languages, because, when executing a business rule, you are expected to change the current state of a domain object. When working in a FP language, because of immutability, you are expected to return a new domain object, instead of modifying the current one.

Since all coupling in an onion architecture is toward the center, the domain is only coupled to itself. Ward Cunningham and Martin Fowler have also been instrumental in architectural patterns. This was created to be used as a non-mainstream approach, not a breakthrough in new technique.

Layer separation

The application layer is where all our application features or “use cases” live. A complete implementation would be provided to the application at run time. Any specific implementation will be provided to the application at runtime. The code, which is specific for the platform, we’ll move to the Infrastructure and UI. It can be hard to implement a service using Onion Architecture when you have a database-centric background. By doing dependency injection in all the code, everything becomes easier to test.

Onion architecture in development

Aliaksandr is a Senior .NET developer at SaM Solutions with 13 years of experience. Being a Microsoft certified engineer, he specializes in web development and has experience in creating desktop and mobile solutions. Aliaksandr is fond of learning new technologies, conducting meetups and teaching newbies at internal company courses. The challenge was to create a cloud software solution for a digital signage hardware manufacturer. At SaM Solutions, we’ve developed a kind of platform that allows you to automate the development and deployment of systems that use Docker. At the lower right of the diagram is an example of how we cross the circle boundaries.

The domain layers often need information or functionality in order to complete business functionality, however they should not directly depend on these. Instead, the application layer needs to depend on the the contracts defined in the Domain Services layer. The Infrastructure Layer will implement interfaces from the Application Layer to provide functionality to access external systems. These will be hooked up by the IoC container, usually in the Presentation Layer. Now i can see all my business logic concentrating on Services layer.

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Note that, ideally, you should always try to implement behaviors in Domain Models to avoid falling in the Anemic Domain Model pitfall. 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. This rule also exists in other similar architectures, such as Clean Architecture. It’s very powerful and closely connected to two other architectural styles—Layered and Hexagonal. Onion Architecture is more appealing for C# programmers than Java programmers.

Onion Architecture was introduced by Jeffrey Palermo to provide a better way to build applications in perspective of better testability, maintainability, and dependability. 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. If you are interested in learning more C# while working with the Onion Architecture, visit the TechRepublic Academy.

Onion Architecture In ASP.NET Core 6 Web API

The architecture is intended to address the challenges faced with traditional architectures and the common problems likecouplingandseparation of concerns. Further, the biggest drawback of this architecture is unnecessary coupling that it creates. The obvious advantage of the Onion architecture is that our controller’s methods become very thin. In the following sections, we’re going to briefly talk about each high-level element of this architecture. The most common type involves division into Presentation Layer, Logic Layer / Domain Layer, and Data Layer.

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It would be really cumbersome to implement, for example, a simple gateway using Onion Architecture. Adapters are the glue between components and the outside world. They tailor the exchanges between the external world and the ports that represent the requirements of the inside of the application component. There can be several adapters for one port, for example, data can be provided by a user through a GUI or a command-line interface, by an automated data source, or by test scripts. Service Interface Layer– common operations like Add, Save, Delete should go in here within interfaces. You can define your transactional interfaces like (IOrderService.SubmitOrder) here.

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. We are hiding all the implementation details in the Infrastructure layer because it is at the top of the Onion architecture, while all of the lower layers depend on the interfaces .

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Onion architecture in development

We will add the interfaces that consist the of data access pattern for reading and writing operations with the database. We will add the interfaces that consist of the data access pattern for reading and writing operations with the database. In onion architecture, we have the domain layer, repository layer, service layer, and presentation layer. Onion architecture solves the problem that we face during the enterprise applications like coupling and separations of concerns. Onion architecture also solves the problem that we confronted in three-tier architecture and N-Layer architecture. In Onion architecture, our layer communicates with each other using interfaces.

ICustom Service

They are typically used via Use Case Request and Response value objects. These are features and rules that are not necessarily part of the Domain Model, but that define the app’s business. This layer is also called “Domain Rules” or “Domain Services”. In the JavaScript world it’s not so easy unless you use TypeScript and interfaces and do the dependency injections manually or with a library. In the Onion Architecture, the dependencies are always pointing inwards. The inner layer is the Domain Model and the outer one is the Infrastructure layer, which takes care of communicating with the external world.

The domain layer is the innermost layer of the architecture. When doing software development, one of the most important things to have in mind is that your software should always be evolving. It will always be maintained, evolved, receiving new features, improvements, and bug fixes. It works well for applications in professional DevOps environments, and the model demonstrates how DevOps assets are organized in relation to the rest of the code.

One of the main issues with the code in Listing 1 is that it takes the single responsibility principle and throws it completely out of the window. I encourage you to use the term “Onion Architecture” when speaking about architectures that adhere to the above four tenets. The term “Onion Architecture” was first coined by Jeffry Palermo back in 2008 in a series of blog posts.

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In the future I’d like to explore and write about similar architectures applied to other programming paradigms such as Functional Programming. This approach is biased towards Object Oriented Programming . However it’s principles can still be applied in a wider sense.

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