Onion Architecture in ASP NET Core

On the other hand, Onion Architecture addresses both the separation of concern and tight coupling issues. This architecture follows the Dependency Inversion Principle as well as Domain-Driven Design DDD. Data model and business logic at the center, instead of infrastructure concerns . What we have is infrastructure depending on abstraction . A center layer never depends on the outside, always the inverse. In other words, all coupling is toward the center, and this is achived using inversion of control.

To achieve this level of maintainability, there is significant work involved in firstly setting up the structure, and secondly maintaining it along the life of the system. Implementation of features may be slower, because there are multiple layers to get through. That’s why Jeffery Palermo recommends it for Enterprise Systems, and not smaller systems non-complex systems. Then we saw how the Service layer was created, where we are encapsulating our business logic.

• Hence, it solves the problem of separation of concern while the Controller is still used to database access logic.
• At times, we had to move a particular functionality into a separate microservice if it appeared in many places in the system.
• The unique part of Onion Architecture is – it has zero dependencies over data layers like classic multi-tier architectures.
• No direction is provided by the Onion Architecture guidelines about how the layers should be implemented.
• It represents the Entities of the Business and the Behaviour of these Entities.
• Now we create the third layer of the onion architecture which is a service layer.

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. https://globalcloudteam.com/ is built on a domain model in which layers are connected through interfaces. The idea is to keep external dependencies as far outward as possible where domain entities and business rules form the core part of the architecture.

Taking Care of Database Migrations

Based on the DDD model, we’ve created onion architecture . Various technologies — microservices within a project can be written in various programming languages and technologies. Automation — microservices should be deployed and updated automatically and independently from each other.

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. It relies on dependency injection for doing it’s layer’s abstraction, so you can isolate your business rules from your infrastructure code, like repositories and views. Modifying the view layer should not break any domain logic.

In addition, the onion architecture itself introduced certain problems. It took us some time to distribute functional parts between appropriate layers. But eventually, this problem was practically eliminated.

Benefits of Onion Architecture

The purpose of the Presentation layer is to represent the entry point to our system so that consumers can interact with the data. We can implement this layer in many ways, for example creating a REST API, gRPC, etc. These are just some of the examples of what we could define in the Domain layer. We can be more or less strict, depending on our needs. We have to realize that everything is a tradeoff in software engineering.

This lets the Domain Entities and Interfaces exploit the underlying Infrastructure of the application without knowing their implementation. We can notice that Controller takes both IUserService and IUserProfileService as a constructor parameters. The ASP.NET Core dependency injection will take care of passing an instance of these services into UserController. The controller is developed to handle operations requests for both User and UserProfile entities. Now, let’s develop the user interface for the User Listing, Add User, Edit User and Delete User. The layer holds interfaces which are used to communicate between the UI layer and repository layer.

Since a Domain Interface acts on an Entity, it should be considered pure business logic. In some cases though, it might make more sense to use a Domain Interface to encapsulate business logic outside of the entity. The CartItem class in the Supermarket.Core project is such a class. It happens to be an abstract class that has two implementations, QuantityBasedItem and WeightBasedItem, which are also considered Domain Entities. At the core of the onion, you’ll find the Domain Layer, which is composed of Entities and Interfaces. Together they represent the core business logic of the application.

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 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. Domain model, domain services, and application services together make the application core.

Language and framework independent

At SaM Solutions, we’ve developed a kind of platform that allows you to automate the development and deployment of systems that use Docker. Visualisation — use tools that allow you to visualise the collected data. It consists of algorithms that are essential to its purpose and implement the use cases that are the heart of the application. Interfaces define behaviour contracts and stand as foundations amongst the layers.

Inside the UI you would use the DAL or Service implementations through the defined interfaces. The correct implementation would be resolved through the help of the Dependency Resolution component (have a look at the concept of “Inversion Of Control” or “Dependency Injection”). It’s highly recommended to use a multi-projects solution.

By separating the application into layers, the system becomes more testable, maintainable and portable. It helps easy adoption of new frameworks/technologies when old frameworks become obsolete. Similar to other architectural styles like Hexagonal, Layered, Clean Architecture, etc. it provides a solution for common problems. Low coupling in which one module interacts with another module and does not need to be concerned with the other module’s internals.

Frameworks, Clients and Drivers

The layer is intended to create an abstraction layer between the Domain entities layer and the Business Logic layer of an application. It is a data access pattern that prompts a more loosely coupled approach to data access. If an application is developed with the ORM entity framework then this layer holds POCO classes or Edmx with entities. The basic principle of Onion Architecture solely depends on how the dependency inversion principle is applied with architecturally defined priorities between layers.

The key proposition of Onion Architecture is a good coupling. In simple words, it is a dependency of one thing upon another. Always, these dependencies should be inward and never outward.

Then, you are implementing a use case which lets the user check her or his account balance. The application layer implements Application rules instead of Business rules. 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. The domain layer is the innermost layer of the architecture. Both software developers and domain experts should be able to talk in a Ubiquitous Language. So, we can see that it’s important to build maintainable software.

Separation of concerns

Now we create a second layer of the onion structure which is a repository layer. To build this layer, we create one more class library project named OA.Repo. This project holds both repository and data, context classes.

I have followed his guidelines, however I need some verification on the structure of the project so far. This layer can reference any project from the inner layer. All the technologies are completed isolated and are easy to swap to another. This is just my point of view about the Onion Architecture. I respect all the other implementations and perspectives. In the Application layer, the FareRepository is able to retrieve data from external sources and transform it into meaningful Business Entities.

The Onion Architecture

Today, most Agile methodologies are encouraging rapid development to implement or enhance features as quickly as possible. To embrace this, the software architecture of the system needs to be simple and easy to extend. The onion architecture is a simple, robust, maintainable and extensible software architecture that fits perfectly in an Agile world. 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.

Business logic can’t function if data access isn’t there. I’m intentionally ignoring infrastructure here because this typically varies from system to system. We often don’t keep systems up-to-date because it’s impossible to do. If coupling prevents easily upgrading parts of the system, then the business has no choice but to let the system fall behind into a state of disrepair. This is how legacy systems become stale, and eventually they are rewritten. When working in a FP language, because of immutability, you are expected to return a new domain object, instead of modifying the current one.

Observability-Driven Development (ODD)

Each layer bounds together concepts that will have a similar rate of change. Code should depend only on the same layer or layers more central to itself. The primary proposition of this architecture is good coupling. The Onion Architecture is an Architectural Pattern that enables maintainable and evolutionary enterprise systems. Then, we explained how we can connect all of the layers using an ASP.NET Core Web API. Great, we saw how we wired up all of the dependencies of our application.