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So metimes different design approaches try to achieve similar objectives. Dependencies flow inward, with inner layers having onion architecture no knowledge of outer layers. This ensures that high-level modules do not depend on low-level modules directly.

What are the Layers of the Onion Architecture

API’s can have DTO’s, DB layer can have Entity Objects depending on how objects stored in a database vary from the domain model. It refers to the business knowledge that our software is trying to model. Domain-Driven Design centres on the domain model that has a rich understanding of the processes and rules of a domain. Onion architecture implements this concept and dramatically increases code quality, reduces complexity and enables evolutionary enterprise systems. So, I thought let’s see if I can find a way to structure my application avoiding the repositories and using the DbContext only. Your solution/project structure is not dictated by the onion architecture.

How we observe, monitor and improve the operational performance of our applications

First, you need to add the Models folder that will be used to create the database entities. In the Models folder, we will create the following database entities. 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. And finally, we saw how our Presentation layer is implemented as a separate project by decoupling the controllers from the main Web application.

  • This can be a Command Line application used by technical support staff, or a full-blown nifty Web App used by the company’s end-users.
  • One may split Domain model into Domain Entities/Domain Services as well.
  • Onion Architecture is a software architecture pattern that separates the application into layers based on their responsibilities.
  • We will explain why this is important in the next section.
  • However, we are going to do something different from what you are normally used to when creating Web APIs.

They are NOT typically CRUD services and are usually standalone services. The Service layer holds interfaces with common operations, such as Add, Save, Edit, and Delete. Also, this layer is used to communicate between the UI layer and repository layer. The Service layer also could hold business logic for an entity. In this layer, service interfaces are kept separate from its implementation, keeping loose coupling and separation of concerns in mind. So our implementation of that repository interface should live in the infrastructure layer , isolating everything from the core.

Separation of concerns

I hope you’ll find my experience useful for your projects. You can check my github repository for technical details. CQRS is a development principle claiming that a method must be either a command that performs an action or a request that returns data.

Honestly, it’s not completely new, but I’m proposing it as a named, architectural pattern. Again, both Clean and Onion Architecture point in similar directions; they suggest that there should be a layer where you manage application specific logic sitting next to enterprise rules. As we have seen, all three architectural styles share the principles of loose coupling and attempt to minimize moving parts by properly layering the application. To handle this, DDD requires that each language belongs to one application context. It defines a scope where a ubiquitous language can be used freely.

Creating a scalable and maintainable architecture for your Web API project

The obvious advantage of the Onion architecture is that our controller’s methods become very thin. We moved all of the important business logic into the Service layer. 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 (abstractions). The interesting part with the ServiceManager implementation is that we are leveraging the power of the Lazy class to ensure the lazy initialization of our services.

What are the Layers of the Onion Architecture

This means that our service instances are only going to be created when we access them for the first time, and not before that. Using dependency inversion throughout the project, depending on abstractions (interfaces) and not the implementations, allows us to switch out the implementation at runtime transparently. We are depending on abstractions at compile-time, which gives us strict contracts to work with, and we are being provided with the implementation at runtime. The Domain layer, which contains the business logic, can be easily scaled by adding more instances of the application. The Infrastructure layer can also be easily scaled by adding more servers or resources to handle the increased load.

ASP.NET Core Metrics with Prometheus

The drawback of this traditional architecture is unnecessary coupling. If you are wanting to inject your DbContext instead of repositories, this rule stays true. Include all your DbSets and include any repository methods you want access to (SaveChangesAsync, FindAsync, etc…). This interface lives in your Domain layer, whereas the implementation of it lives in the Infrastructure layer (on one of the outer layers of your onion).

Honestly, it’s not completely new, but I’m proposing it as a named, architectural pattern. Patterns are useful because it gives software professionals a common vocabulary with which to communicate. There are a lot of aspects to the Onion Architecture, and if we have a common term to describe this approach, we can communicate more effectively. Last but not least, this layer is the only one that is responsible and that knows how to read and write data in the data source. It knows to establish a connection to the data source, convert the entity objects to DTO (Data Transfer Object) to send it to the above layer. It greatly depends on the complexity of the application and the size of the project to divide source code into multiple modules.

Setting up the Solution Structure

You can see that we are adding the API Versioning data to the route attribute and also creating an IMediator object. Then, run the following commands to add migrations and to generate/update the database. In the Application Layer, Create a New Folder called Features. This will have all the logic related to each Feature / Entity.

What are the Layers of the Onion Architecture

Hence, when you separate these requests, you can use different technologies for handler implementation (Dapper, Entity Framework). The practice has shown that 90 percent of requests concern get operations; as a rule, they are small and quick. 10 percent of requests concern put operations; these operations are usually complicated due to a range of transactions and validations.

Understanding Real-Time Application Monitoring

The main difference between “the classic” three-tier architecture
and the Onion, is that every outer layer sees classes from all inner layers, not only the one directly below. Moreover,
the dependency direction always goes from the outside to the inside, never the other way around. It does so
with ideas similar to Hexagonal Architecture,
Clean Architecture and
other related architecture styles.