Software development

Onion Architecture: Going Beyond Layers NDepend

It’s how your application communicates to the outside world, whether it’s the data access logic or API calls. Palermo showed three main types, though there may be more. Since onion architecture the domain changes the most — here is the place where you put all the new features, and business requirements — it
should be as easy as possible to modify and test.

What are the Layers of the Onion Architecture

Entities are Domain concepts that have a unique identity in the problem domain. Domain entities encapsulate attributes and entity behaviour. It is supposed to be independent of specific technologies like databases or web APIs. Order is an entity and has attributes like OrderId, Address, UserInfo, OrderItems, PricingInfo and behaviour like AddOrderItems, GetPricingInfo, ValidateOrder, etc. The Infrastructure Layer is the outermost layer of the Onion Architecture. It is, in essence, no different from how you would get a reference to your self-built UOW if you had implemented an UOW/repo layer.

Layers of Onion Architecture

Domain services are responsible for holding domain logic and business rules. All the business logic should be implemented as a part of domain services. Domain services are orchestrated by application services to serve business use-case.

What are the Layers of the Onion Architecture

However, we are not doing anything useful at the moment with the transaction so from this and following the layers of Onion Architecture we need to define our Domain Services layer. The presentation layer is our final layer that presents the data to the front-end user on every HTTP request. This line of code will find all of the controllers inside of the Presentation project and configure them with the framework. They are going to be treated the same as if they were defined conventionally.

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She’s currently focused on design practices that the whole team can own, understand, and evolve over time. This Architecture style does have some learning curve for developers in the project, but once mastered, pays back many
times. Finally, as with every solution in the IT industry, it is not a one-size-fits-all, and you should always consider
if the architectural style matches your needs. 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. However, I have covered a few of these topics in other articles in my blog already.

This layer is unique, because it provides to the application in a supporting manner. The Onion Architecture was first introduced by Jeffrey Palermo around 2008. At its very essence, the architecture focused on modeling business logic and entities at the application’s core, without any notion of the outside world.


Maintaining a resilient software architecture is a constant battle. Luckily, there are a few design methods that can help teams instill the reliability they desperately need. It’s possible that the greatest insult you could hurl at an application architecture is to call it monolithic. After all, confining the totality of application logic to a single software element makes distribution, scaling and redeployment markedly difficult, if not impossible. This library provides almost limitless opportunities for setting data validation rules.

  • The application uses the behaviour expressed by the interface, the details of how the behaviour is executed lie in the
    infrastructure layer.
  • As we can see, it consists of the Web project, which is our ASP.NET Core application, and six class libraries.
  • It has its
    learning curve and is best suited for services with a clear domain definition.
  • 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.
  • The center of the allegorical onion contains the foundational domain entities and objects, which are the elements of the software that have no dependencies.

That’s the reason why I am not going to explain software architectures as a war, trying to convince you which one is the best and why you should use it alone. Instead, I am going to focus this on how to use different architectures in a combined way, trying to show the benefits of it and when I would use them. The first layer around the Domain Model is typically where we would find interfaces that provide object saving and retrieving behavior, called Repository Interfaces.

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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. Based on the DDD model, we’ve created onion architecture (aka hexagonal or clean architecture).

I have added the XML file to the root of the API Project. Similarly, create another .NET Standard Library Project in the Core Folder. Do not forget to change the target version here as well. We will start off by creating a Blank Solution on Visual Studio.

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We can write business logic without concern about any of the implementation details. 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. Naturally, maybe you want to start the development by the database, but it’s a mistake!

We will follow the same process for adding the library project in our application, but here we need some extra work after adding the project we need to add the reference of the Repository Layer. All of the layers interact with each other strictly through the interfaces defined in the layers below. The flow of dependencies is towards the core of the Onion. We will explain why this is important in the next section. Onion Architecture is a software architecture pattern that separates the application into layers based on their responsibilities.

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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. This layer is used to communicate with the presentation and repository layer.

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