Onion Architecture In ASP NET Core MVC

In this article, we will cover the https://globalcloudteam.com/ using the ASP.Net 6 Web API. Onion architecture term is introduced by Jeffrey Palermo in 2008. Onion architecture solves common problems like coupling and separation of concerns. At the center part of the Onion Architecture, the domain layer exists; this layer represents the business and behavior objects. The idea is to have all of your domain objects at this core. Besides the domain objects, you also could have domain interfaces. Domain objects are also flat as they should be, without any heavy code or dependencies.

In the Application Core, we have the Domain and Service. Remember, onion architecture can have multiple layers. Here, for simplicity, we just have two layers in the Application Core. This layer should interact with the Application Core services using the abstractions.

Since a repository typically interacts with a database, the actual implementation of a repository is not found in this layer. The concrete implementation of a repository is implemented outside the application core, thus decoupling the actual database from the application core. Please check my article about the inversion of control. Another big advantage, it’s very easy to write automated tests for everything in the Application Core layer, because the infrastructure is outside that layer.

That’s why Jeffery Palermo recommends it for Enterprise Systems, and not smaller systems non-complex systems. For example, a Notification microservice that sends an email or something like that. And that’s why I included it inside my template because it can be useful for anyone. I didn’t add any library like AutoMapper or similar because I wanted to remain agnostic about that. If you want to use AutoMapper or any similar NuGet package feel free.

Observability-Driven Development (ODD)

The services layer is a layer providing additional services needed for an application, example a message service, notification service, encoder service, etc. Our task is to extract the common code – models and interfaces, which can be used in both web and mobile applications. In this layer, we normally put the repository access, migrations, and all the infrastructure services. The advantage of having this layer separately is, you can swap the technology without touch the other layers. A way of structuring your code for a long-life application with low maintenance costs. When the application runs and calls the index() action method from UserController with a HttpGet request, it gets all the users listed in the UI, as shown in Figure 4.

Write click on the project and then click the Add button after that we will add the project references in the Repository layer. 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. First, you need to create the Asp.net Core web API project using visual studio.

Benefits of Onion Architecture

The GET request for the EditUser action method returns _EditUser partial view, where code snippet follows under the User folder of views. The GET request for the AddUser action method returns _AddUser partial view; the code snippet follows under the User folder of views. To implement the Onion architecture, we develop an ASP.NET Core application. This application performs CRUD operations on entities. Each project represents a layer in onion architecture.

In fact, while there are numerous definitions of microservices, there is no single clear and unified definition. Broadly speaking, microservices are web services that create a type of service-oriented architecture. 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. But here we need to add the project reference of the Domain layer in the repository layer.

But it does not quite solve the validation problem, especially if you need to take information from a database or from another microservice. Therefore, we built a validation mechanism into the MediatR pipeline using Fluent Validation. Hence, when you separate these requests, you can use different technologies for handler implementation . The practice has shown that 90 percent of requests concern get operations; as a rule, they are small and quick.

It holds business logic for an entity so it’s called the business logic layer as well. The outermost layer of the onion is the layer that is directly outside the application core. The purpose of this layer is to externalize infrastructure components, example SQL Database, Azure Cosmos Database, Azure Service Bus etc. Tests and user interfaces are also found in this layer. The drawback of 3-tier and n-tier architectures is unnecessary coupling. Onion Architecture solved this problem by defining layers from the core to the Infrastructure.

Domain Model

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. Figure 2 below outlines the domain within the application structure. 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.

Now, we define the configuration for the User entity that will be used when the database table will be created by the entity. The following is a code snippet for the User mapping entity (UserMap.cs). This architecture relies heavily on the Dependency Inversion Principle. The UI communicates to business logic through interfaces. It can have access to both the database and UI layers. In my implementation, I intend to demonstrate some of the key layers of this architecture and how they work together.

Onion vs Clean vs Hexagonal Architecture

Such systems are very hard to understand and maintain. The drawback of this traditional architecture is unnecessary coupling. This Application uses the Entity Framework Code First approach, so the project OA.Data contains entities that are required in the application’s database. The OA.Data project holds three entities, one is the BaseEntity class that has common properties that will be inherited by each entity.

• Is a application service or if in clean architecture is a interactor are small details.
• We will use dependency injection, so we pass options via constructor dependency injection.
• We’ve chosen MediatR to implement CQRS in the project.
• At times, we had to move a particular functionality into a separate microservice if it appeared in many places in the system.
• One of the most important parts of each application is architecture.
• It is the level of dependency of one thing upon another.

It is much easier to build a microservice around a bounded context. In the case of the API presentation layer that 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. 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.

An Overview of C# Encapsulation

A Web UI could be replaced with a console UI, for example, without changing the business rules. The architecture does not depend on the existence of some library of feature laden software. This allows you to use such frameworks as tools, rather than having to cram your system into their limited constraints.

Command-Query Request System (CQRS)

There is a Rider — someone who needs to travel from point A to point B, and a Driver — the car driver who will pick-up and drop-off the rider in their vehicle. 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. This layer contains the implementation of the behaviour contracts defined in the Model layer. Business Logic behaviour is declared as contracts with the use of interfaces in a Object-Oriented context.

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. This layer communicates to both Web applications and repository projects.

Since all coupling in an onion architecture is toward the center, the domain is only coupled to itself. In the late 2000s Jeffrey Palermo, presented an architectural pattern called the Onion Architecture. The main purpose of this architectural pattern was to control coupling in a software system, to improve separation of concerns and to force the externalization of infrastructure.

The Entities Domain layer is a core and central part of the architecture. So first, we create “OA.Data” project to implement this layer. This project holds POCO class and fluent API configuration for this POCO classes. To implement the Onion Architecture in the ASP.NET Core application, create four projects as described in the above section. These four projects represent four layers of the onion architecture. It develops a loosely coupled application as the outer layer of the application always communicates with the inner layer via interfaces.