Onion Architecture explained Building maintainable software Medium

Imagine that you are modeling a banking system, where you have the Account domain model. Then, you need to implement the Transfer feature, which involves two Accounts. There are some cases where it’s hard to fit a behavior into a single domain model. Domain-driven design is the concept that developers and domain experts should use the same names both in code and business domain.

What is onion architecture

This time we arranged _isFinancialInstitutionExistBusinessOperation to return true.Because we want to test our exception, in order to do that the data must already exist in database. This could be calling a function or method, calling a REST API, or interacting with a web page. By now you’ll have noticed that my onion is slightly different from other explanations that can be found online.

Data Folder

For example, you can easily replace SQL with MongoDB without affecting the core. Because all the code depends on deeper layers or the center, it offers superior maintainability. Layers have a separation of concerns and are not tightly coupled. Let’s create the table in SQL using the migration commands. Open the package manager console and switch the default project to Repositorylayer and execute the below commands one after another. It can be hard to implement a service using Onion Architecture when you have a database-centric background.

What is onion architecture

I designed the application using the Onion Architecture, which I’d been wanting to get more familiar with in the context of a real-world application. This article chronicles the journey of building that application. In this class, we override the OnModelCreating() method. The following is the code snippet for the context class. Now, we define the configuration for the UserProfile entity that will be used when the database table will be created by the entity.

Briefly about Microservices

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. Now we need to add the student controller that will interact will our service layer and display the data to the users. Using the onion architecture our application is loosely coupled because our layers communicate with each other using the interface.

Thus, we create generic repository interface for the entity operations, so that we can develop loosely coupled application. The code snippet, mentioned below is for the IRepository interface. Now we create a second layer of the onion architecture which is a repository layer. To build this layer, we create one more class library project named OA.Repo.

I’ve included the Solution Explorer view to see the relationship between the logical and physical layers side-by-side. I’ll explain onion structure each of the layers by working my way from the inside out. There are several advantages of the Onion Architecture, as listed below.

What is onion architecture

At deeper layers, we define abstract interfaces, while at the top layer, we give their concrete implementation. By doing this, we can keep our attention on the domain model and lessen our concern about implementation issues. We may also use dependency injection frameworks like Spring to link interfaces with implementation at runtime. For Example, Infrastructure layer implementations include external services used in Application Services and repositories used in the domain. I find this pattern to help greatly with Test Driven Development .

This means that in the Domain layer, we are not concerning ourselves with infrastructure details such as the database or external services. This layer is used to communicate between the Repository layer https://globalcloudteam.com/ and Main Project where it consists of exposable API’s. In this layer, the service interfaces are kept separate from their implementation for loose coupling and also the separation of concerns.

Project Structure.

This architecture relies heavily on the Dependency Inversion Principle. The UI communicates to business logic through interfaces. It looks very similar to an onion with layers wrapping around a central core. Each of these layers represent a specific duty within the overall function of a service.

  • This contains the Core Business Logic as part of our project which acts as a layer between the Repositorylayer and Controller.
  • It solves the problem of separation of concern as there is a separation between UI, business logic, and data access logic.
  • The onion architecture is a specific application of the concepts explained in the article.
  • Infrastructure is visually broken into pieces, all of those are application boundaries.
  • It develops a loosely coupled application as the outer layer of the application always communicates with the inner layer via interfaces.

Now we create the third layer of the onion architecture which is a service layer. To build this layer, we create one more class library project named OA.Service. This project holds interfaces and classes which have an implementation of interfaces. This layer is intended to build loosely coupled applications.

The Service layer also could hold business logic for an entity. In this layer, service interfaces are kept separate from its implementation, keeping loose coupling and separation of concerns in mind. On the other hand, the Onion Architecture tackles the problems of tight coupling and separation of concerns. The layer is intended to act as an abstraction layer between an application’s Domain Entities layer and its Business Logic layer.

External Services

At the center of Onion Architecture is the domain model, which represents the business and behavior objects. Around the domain layer are other layers, with more behaviors. To pass the data from UI to a controller to edit a user, use same view model named UserViewModel. The UserController has an action method named EditUser, which returns the view to edit a user. Now, let’s create a repository class to perform database operations on the entity, which implements IRepository.

What is onion architecture

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. Low coupling occurs when one module interacts with another without worrying about the internals of the other module. The internal implementation of external layers does not need to be a concern for all internal levels. 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.

Ubiquitous language between domain experts and developers

Using dependency inversion throughout the project, depending on abstractions 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. In essence, MVC resolves the separation of concerns problem, but the tight coupling problem remains. Loose Coupling — It denotes the independence of two objects and the fact that one object can use another without becoming dependent on it. Tight Coupling — A class is said to be tightly coupled to another type if it concretely depends on another class.

Domain services

You might also have domain interfaces in addition to domain objects. Additionally, domain objects are flat and free of cumbersome dependencies and code. It holds a generic repository class with its interface implementation. The Entity Framework Code First data access approach needs to create a data access context class that inherits from the DbContext class.

Unfolding infrastructure in the Onion architecture

The Controller is used to handle the web request by action methods and returns View accordingly. Hence, it solves the problem of separation of concern while the Controller is still used to database access logic. In essence, MVC solves the separation of concern issue but the tight coupling issue still remains. This layer creates an abstraction between the domain entities and business logic of an application. In this layer, we typically add interfaces that provide object saving and retrieving behavior typically by involving a database. This layer consists of the data access pattern, which is a more loosely coupled approach to data access.

It’s become even harder because our logical dependencies are being messed up with the language patches, only needed to add an essential feature to the language itself. One may replace Application Services with Use Cases/Ports if it better suites the application. One may split Domain model into Domain Entities/Domain Services as well. Now we can see when we hit the GetAllStudent Endpoint we can see the data of students from the database in the form of JSON projects. We will follow the same project as we did for the Domain layer. Add the library project in your application and give a name to that project Repository layer.