So, we should stick to writing services that deal with encapsulating repository calls as well as handling business logic. This way, we define the purpose of each layer perfectly, and in the long run, this architecture will be able to handle future maintenance elegantly. This layer creates an abstraction between the domain entities and the business logic of an application.

  • I’m not sure if that’s an Onion Architecture already; it depends on your interpretation.
  • This layer is intended to build loosely coupled applications.
  • Still not completely clear about what goes where, but your article has greatly improved my understanding of this architecture.
  • Different layers of onion architecture have a different set of responsibilities, and accordingly, there are different testing strategies.
  • It is much easier to build a microservice around a bounded context.

An Application Service is a piece of code that implements a use case. The Application Layer is the second-most inner layer of the architecture. A Value Object is an object that has no identity and is immutable. It’s not so clear if this behavior should be implemented by the account model, so you can choose to implement it in a Domain Service. This rule also exists in other similar architectures, such as Clean Architecture. Then, we should start thinking about separating different concerns into different units of code.

ICustom Service

Bounded context: each microservice is built around some business function and uses bounded context as a design pattern. So, you should start by modeling your domain layer instead of the database layer. Naturally, maybe you want to start the development with the database, but it’s a mistake! When working with Onion Architecture, you should always start developing the inner layers before the outer ones.

onion architecture explained

To maintain structural sanity in mid- to large-scale solutions, it is always recommended to follow some kind of architecture. You must have seen most of the Open Sourced Projects having multiple layers within a complex folder structure. It provides us with better testability for unit tests; we can write separate test cases in layers without affecting the other modules in the application. To handle this, DDD requires that each language belong to one application context. It defines a scope where a ubiquitous language can be used freely. When using ubiquitous language, each term should have only one meaning.

Onion Architecture: A Guide to Clean and Sustainable Code

Organizing our application in layers helps in achieving the separation of concerns. It depends on the use cases and the complexity of the application. It is also possible to create more layers of abstractions depending on application needs. E.g. for smaller applications that don’t have a lot of business logic, it might not make sense to have domain services. Regardless of layers, dependencies should always be from outer layers to inner layers.

To pass the data from UI to a controller to edit a user, use the same view model named UserViewModel. The UserController has an action method named EditUser, which returns the view to edit a user. We can notice that the Controller takes both IUserService and IUserProfileService as a constructor parameter.

The Power of Hexagonal Architecture

This gets even more interesting when there are multiple processes making up a single software system. There are more examples, but hopefully, you get the idea. 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. With onion architecture, there is only an object model at the lowest level, which does not depend on the type of database. The actual type of database and the way of storing data is determined at the upper infrastructure level.

onion architecture explained

It gives a nice abstraction over the DB Provider as well. Meaning you can switch to another database technology with ease. I salute your time and efforts to share articles.

Infrastructure Layer

This means nothing in an inner circle can know anything at all about something in an outer circle. I.e. the inner circle shouldn’t depend on anything in the outer circle. The Black arrows represented in the diagram show the dependency rule. Each circle represents different areas of the software. The outermost layer is the lowest level of the software and as we move in deeper, the level will be higher. In general, as we move in deeper, the layer is less prone to change.

onion architecture explained

It is a data access pattern that prompts a more loosely coupled approach to data access. We create a generic repository, which queries the data source for the data, maps the data from the data source to a business entity, and persists changes in the business entity to the data source. 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 the repository layer. The Service layer also could hold business logic for an entity. In this layer, service interfaces are kept separate from their implementation, keeping loose coupling and separation of concerns in mind.


As you see, the UI is talking to business logic and business logic is talking to the data layer, and all the layers are mixed up and depend heavily on each other. None of the layers stand independent, which raises a separation of concerns. In the case of the API presentation layer, it presents us the object data from the database using the HTTP request in the form of JSON Object. But in the case of front-end applications, we present the data using the UI by consuming the APIS. As the concept of dependency injection is central to the ASP.NET Core application, we register context, repository, and service to the dependency injection during the application start-up.

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