It solves the problem of separation of concern as there is a separation between UI, business logic, and data access logic. The Model is used to pass the data between View and Controller on which the business logic performs any operations. The Controller is used to handle the onion structure 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.

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This way developers can really learn how the onion architecture is implemented. Still not completely clear about what goes where but your article has greatly improved my understanding about this architecture. I also liked that you started with a clean slate as most out there just show it ready-made and try to explain from there. I am leaning towards the API template as I usually use SPA’s. This project can save well over200+ hoursof development time for your team. We will do a simple test to ensure that our solution works.

In this layer services interfaces are kept separate from their implementation for loose coupling and separation of concerns. Last but not least, I would want to stress that there are options to tackle integration hell but just like anything else, not every option may suit your application integration need. On top, some advanced scaling methods in this architecture heavily relies on distributed computing methods and techniques.

Language and framework independent

We will maintain all the database migrations and database context Objects in this layer. 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.

Advantages of onion architecture

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. On the other hand, Onion Architecture addresses both the separation of concern and tight coupling issues. 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.

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If we need anything from an external system or service, we can just create an interface for it and consume it. We do not have to worry about how it will be implemented. The higher layers of the Onion will take care of implementing that interface transparently.

For this article, Let’s have a WebApi that has just one entity, Product. We will perform CRUD Operations on it while using the Onion architecture. It is where you would want to add your Infrastructure.

Advantages of onion architecture

We’ve chosen MediatR to implement CQRS in the project. This approach makes it possible to create a universal business logic that is not tied to anything. The main problem with this architecture is that all layers are built on top of the Data Access Layer and are, in fact, tied to a certain type of data storage. If this type changes, it causes changes at all levels. The Entity Framework partially solves this problem, but it supports a limited number of database types.

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You’d have to have a very large project before doing that made any sense, in which case moving the application into a more service-based architecture might well be a better choice. Interestingly, per your own admission, “EFCORE is already built on Repository pattern and UOW” and so why have IApplicationDbContext at all if all it does is expose EFCore classes and functions? Database Independent – Since we have a clean separation of data access, it is quite easy to switch between different database providers. Now, let’s work on the Core Layers starting from the Domain Project. It basically has the models/entities, Exception, validation rules, Settings, and anything that is quite common throughout the solution.

Advantages of onion architecture

Onion Architecture is more appealing for C# programmers than Java programmers. However, it’s up to the architect community to consider and argue in the discussion on whether or not to apply the architecture. Not easy to understand for beginners, learning curve involved. Architects mostly mess up splitting responsibilities between layers.


With the CRUD logic out of the way, let’s set up EFCore in the Persistence Layer and try to generate a database. Install the following packages to the Persistence Project. We will have to register Swager within the application service container.

  • I also find it very helpful to include a bootstrapper module which handles all the wiring and dependencies amongst the interfaces and their respective implementations.
  • Bounded context — each microservice is built around some business function and uses bounded context as a design pattern.
  • In onion architecture, we have the domain layer, repository layer, service layer, and presentation layer.
  • This project holds both repository and data, context classes.
  • If you have been using web forms to develop web application then adopting MVC architecture even fits better, because it provides the required framework to build web apps MVC way.

Now, we create an index view, as per the code snippet, mentioned below under the User folder of views. This is the first view when the application is accessed or the entry point of the application is executed. The user data is displayed in a tabular format and on this view, it has linked to add a new user, edit a user and delete a user. And finally, we saw how our Presentation layer is implemented as a separate project by decoupling the controllers from the main Web application. Then we saw how the Service layer was created, where we are encapsulating our business logic.

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Since this is a very basic controller that calls the mediator object, I will not go in deep. However, I have previously written a detailed article on CQRS implementation in ASP.NET Core 3.1 API. You could go through that article which covers the same scenario. We will have to register IApplicationDBContext and bind it to ApplicationDbContext, right? Similar to the Application layer, we will have to create a new class just to register the dependencies and services of this layer to the service container.

Advantages of onion architecture

As you see, the UI is talking to business logic and business logic is talking to data layer and all the layers are mixed up and depend heavily on each other. None of the layer stand independent, which raises separation of concerns. In the case of Onion Architecture, I see things pretty similarly as in the Hexagonal one. I’m not sure if that’s an Onion Architecture already, it depends on your interpretation. On the other hand, we have the monoliths, which contain much more dependencies and services depend on one another on the code level.

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. The basic principle of Onion Architecture is to follow the boundaries of these layers – the inner layer can’t depend on its outer layer but can depend on layers beneath.

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Yes, you can definitely use Repository Pattern alongside Onion Architecture. It gives a better separation between your actual database of choice and your data access code. It depends on how organized you want your project to be and how comfortable you are with the patterns. All these architectures are basically saying you should split up your code into separate areas of concern, and all the normal rules about dependencies and coupling always apply, redardless. If you put your code in the wrong layer, or couple things that shouldn’t be coupled, or whatever, none of the architectures will be successful.

Add the library project in your application and give a name to that project Repository layer. A complete implementation would be provided to the application at run time. Onion architecture provides us with the batter maintainability of code because code depends on layers. You should use different projects when you need different deliverables. Layers do not necessarily need to be in different projects, especially in small applications. There is no essential difference between n-tier, onion, hexagonal, layered etc architectures.

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I will just create a new product and make a request to query all the existing products as well. Just to make our solution a bit clean, let’s also add API Versioning to the WebAPI. Remember we created an IApplicationDBContext Interface in the Application Layer? Create a new folder named Context and add a new class ApplicationDbContext.

And for this, wish to use ADO.Net without entity framework. For instance, your Commands may use a service bus or Event Sourcing to send changes on for further processing, while your Queries may use compiled SQL views for querying. Command Query Seperation splits commands and queries into different request types, where commands alter state and may introduce side effects, while queries are read-only and side-effect free.

This is another variant that I have noticed in many huge solutions. Let’s say you have around 100 interfaces and 100 implementations. Do you add all these 100 lines of code to the Startup.cs to register them in the container?

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