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Onion Architecture: Definition, Principles & Benefits

Nevertheless, repository is a kind of adapter between domain and data model. The main difference I’ve found in the implementations of Hexagonal Architecture and Onion Architecture lies mostly in the overall, more structured approach to the code layout of the latter. Making the concept a first-class citizen represented in the code guides implementation and gives more clear overall structure to the codebase.

Any developer, familiar with the domain, should be able to understand the code, and easily know where to change things. So, we can see that it’s important to build maintainable software. We should be able to build a software that can be maintained by future developers.

Domain Layer

So the UI and database layers can depend on your domain logic and business rules, but not the other way around. All the coupling and dependencies flow down toward the center layer of the onion. If you have very complex business logic, it would make sense to encapsulate it inside of our domain entities. But for most applications, it is usually easier to start with a simpler domain model, and only introduce complexity if it is required by the project.

This separation makes the application more modular and easier to understand. The presentation layer is the default Asp.net core web API project Now we need to add the project references of all the layers as we did before. Now we need to add a new project to our solution that will be the service layer. ‘and so on’ – this is the most important statement here.

Advantages of Onion Architecture

You could go through that article which covers the same scenario. Basically, these 5 Classes would cover our CRUD Operations implementation. Make sure that you have gone through my article about CQRS for ASP.NET Core before proceeding. I have already written a detailed article on MediatR and CQRS patterns in ASP.NET Core 3.1 WebApi Project. You can follow that article and add the Required Commands and Handlers to the Application Layer.

Jeffrey Palermo describes layered architecture in his initial article introducing term ‘onion’. Low coupling in which one module interacts with another module and does not need to be concerned with the other module’s onion software architecture internals. All the internal layers need not be concerned about internal implementation of external layers. When you are creating a software that does not deal with business rules, this architecture won’t fit well.

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One of such choices is the decision about the code, highly reused by whole application including the domain objects. Divide the application into different modules/projects each responsible for a layer in onion architecture. Let’s understand different layers of the architecture and their responsibilities with an order creation use case. This architecture should be used when creating services that deal with business rules. It’s easy to find where are the business rules, the use cases, the code that deals with the database, the code that exposes an API, and so on.

The object saving behavior is not in the application core, however, because it typically involves a database. Out on the edges we see UI, Infrastructure, and Tests. The outer layer is reserved for things that change often. These things should be intentionally isolated from the application core. Out on the edge, we would find a class that implements a repository interface.

Brief Overview of N-Layer Architecture

However, I prefer implementing read operations through use cases as well. This allows for implementing aggregations or incorporating security within the core layer. Thus, the domain layer contains both read and write data models, and the use cases cover operations such as creation, editing, deletion, and reading. In both cases, you would have to create a new version of your DAL. But how would you account for these two different layers in your business layer? In layered architecture, the abstractions are in the DAL, so in order for the business layer to compile, it has to include a reference to the layer in which the abstractions exist.

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  • The Domain layer, which contains the business logic, can be easily tested without the need for the Infrastructure layer or the User Interface layer.
  • It’s quite simple and distributes the responsibilities for 3 layers.
  • At the center part of the Onion Architecture, the domain layer exists; this layer represents the business and behavior objects.
  • Using the code first approach in the application development using Asp.net core these entities are used to create the tables in the database.
  • In this layer is where the majority of our business logic lives, it carries out the operations to turn A into B, input into output, egg into chicken.

This architecture is not appropriate for small websites. It is appropriate for long-lived business applications as well as applications with complex behavior. It emphasizes the use of interfaces for behavior contracts, and it forces the externalization of infrastructure. The diagram you see here is a representation of traditional layered architecture. This is the basic architecture I see most frequently used. Each subsequent layer depends on the layers beneath it, and then every layer normally will depend on some common infrastructure and utility services.

Discussing the benefits of segregating business and technical code and tackling common misconceptions

The main issues we faced were related to maintaining the low connectivity of microservices. That’s why it was difficult to immediately divide the functionality into the necessary microservices. Automation — microservices should be deployed and updated automatically and independently from each other.

Domain-Driven Design centres on the domain model that has a rich understanding of the processes and rules of a domain. Onion architecture implements this concept and dramatically increases code quality, reduces complexity and enables evolutionary enterprise systems. Modifying the view layer should not break any domain logic. Modifying the database modeling should not affect the software’s https://www.globalcloudteam.com/ business rules. Even though you said application layer doesn’t depend on any other layers, you immediately leak entity Framework and asp.net core into it by the means of dbset and iservicecollection. This is effectively the same as leaking infrastructure and presentation layer into application layer, you’re bound to the technologies and cannot replace them with anything else.

I’ll propose a testing strategy for the hexagonal architecture.

We started with the Domain layer, where we saw the definitions for our entities and repository interfaces and exceptions. We have connected all of our Onion architecture implementation layers, and our application is now ready for use. The obvious advantage of the Onion architecture is that our controller’s methods become very thin. We moved all of the important business logic into the Service layer. We are using a Web API built with ASP.NET Core to create a set of RESTful API endpoints for modifying the domain entities and allowing consumers to get back the data. There are more examples, but hopefully, you get the idea.