Out on the edges we see UI, Infrastructure, and Tests. The outer layer is reserved for things that change often. These things should be intentionally isolated from the application core.

In today’s world of request and response logic, you will need a repository interface and gateways to handle the situation. This facilitates by protecting your business from undesired dependencies. On the other hand, we can see dependencies and services relied upon each other on the code level.

Understanding Onion Architecture

There are more examples, but hopefully, you get the idea. We are hiding all the implementation details in the Infrastructure layer because it is at the top of the Onion architecture, while all of the lower layers depend on the interfaces . The Domain layer does not have any direct dependencies on the outside layers. The outer layers are all allowed to reference the layers that are directly below them in the hierarchy. It can receive objects that implement some known interfaces , and it’s allowed to import entities from the Domain Layer. No direction is provided by the Onion Architecture guidelines about how the layers should be implemented.

In such cases, you will find numerous dependencies to switch with interfaces. There is no standard process for implementing Onion Architecture. We have a free hand on how we want to implement the layers. You have full liberty to choose whatever level you want. In core, you will have repositories talking to DAL and domain models, and services dealing with repositories and domain models.

What is the motivation for splitting the Service layer?

I’m proud of the professional and personal relationships that grow with each project. Year after year, as past clients recommend us to new clients, we celebrate the rewards of excellence.” Onion architecture has proven effective to lower coupling and enhancing cohesion. This overall helps to improve the performance, maintenance and testability of the system. The inner layers can be run and separated from the infrastructure.

The higher the coupling, the lower the ability to change and evolve the system. We could create an initialization script, connect to the Docker onion structure container while it is running the database server, and execute the script. But this is a lot of manual work, and it is error-prone.

Easy to maintain

It is well compatible with CQRS due to pipeline behaviors. 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. At times, we had to move a particular functionality into a separate microservice if it appeared in many places in the system. On the contrary, if some functionalities were tightly connected, we had to combine microservices into one.

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. Divide the application into different modules/projects each responsible for a layer in onion architecture. 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.

onion architecture

All these traditional architectures have some fundamental issues, such as – tight coupling and separation of concerns. The Model-View-Controller is the most commonly used web application architecture, these days. It solves the problem of separation of concern as there is a separation between UI, business logic, and data access logic.

Service Layer

This layer should interact with the Application Core services using the abstractions. Let’s understand different layers of the architecture and their responsibilities with an order creation use case. The onion architecture was embraced by the software industry and is a widely used architectural pattern, especially in enterprise software.

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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. We create a controller named UserController under the Controllers folder of the application.

We should be able to build a software that can be maintained by future developers. Onion Architecture layers are connected through interfaces. Onion Architecture uses the concept of layers, but they are different from 3-tier and n-tier architecture layers. Let’s see what each of these layers represents and should contain. Our approach to excellence begins with carefully listening to each client’s requirements and aesthetic sensibilities.

Anemic Domain Models

Externalizing the database can be quite a change for some people used to thinking about applications as “database applications”. With Onion Architecture, there are no database applications. Decoupling the application from the database, file system, etc, lowers the cost of maintenance for the life of the application. Onion Architecture solved these problem by defining layers from the core to the Infrastructure. It applies the fundamental rule by moving all coupling towards the center.

The rest of your code shouldn’t worry if you are storing your data in a database, in a file, or just in memory. Repositories, external APIs, Event listeners, and all other code that deal with IO in some way should be implemented in this layer. This layer is also allowed to know about everything contained in the inner layers, being able to import entities from the Application and Domain layers. DTOs are well suited as objects with really specific formats and data. For example, let’s say you are developing a banking system.

onion architecture

Small size — smaller microservices are easier to work with. Onion architecture became obvious to me once I understood DDD and necessary design patterns such as MVC, Dependency injection, Repository/Service, ORM. But in my opinion, organizing projects can be different and trivial when there is full understanding of the architecture. 1 It looks OK but I am not sure it’s a good idea to insert dependency resolution into the diagram.

We will use the business domain of a Ride Sharing/Taxi Booking Application. It’s quite straight-forward, just think of Uber or Lyft. 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. This layer contains the implementation of the behaviour contracts defined in the Model layer. Now we only have one more layer left to complete our Onion architecture implementation.

Domain Layer

The following code snippet is for the same (IUserService.cs). Now, let’s create a repository class to perform database operations on the entity, which implements IRepository. The code snippet is mentioned below for the Repository class under OA.Repo project. This Application uses the Entity Framework Code First approach, so the project OA.Data contains entities that are required in the application’s database.

Out on the edge, we would find a class that implements a repository interface. This class is coupled to a particular method of data access, and that is why it resides outside the application core. This class implements the repository interface and is thereby coupled to it. Each layer/circle encapsulates or hides internal implementation details and exposes an interface to the outer layer. All layers also need to provide information that is conveniently consumed by inner layers. The goal is to minimize coupling between layers and maximize coupling within a vertical slice across layers.

Can be quickly tested because the application core does not depend on anything. The firm founded by architect Rodney Leon, specializes in Cultural, Religious, Residential, and Urban Planning projects in the U.S. and abroad. By now you’ll have noticed that my onion is slightly different from other explanations that can be found online. Some of this is my own interpretation of the rules, and some of it is my pragmatic way of implementing it.

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. Please check my article about the inversion of control.

External Services

This way, when you want to test it, you can just inject a mock that implements the interface your code is expecting to. The Onion Architecture does not depend on any specific language or framework. You can implement it in basically any language that supports dependency injection. Example main()So, the only place in your application that actually creates objects that are capable of doing IO is the application’s entrypoint. The Infrastructure Layer uses them, but is does not create them. If you have a repository that expects a PostgreSQL client, the main should instantiate it and pass it to the repository during its initialization.

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