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 Asp.net core these entities are used to create the tables in the database. The domain models and services will be inside this layer, containing all the business rules of the software. It should be purely logical, not performing any IO operations at all. The Onion.Factory sample is a very simple Domain Driven Design application which follows the onion architecture pattern. The idea is that the domain model behind it represents a factory which produces onions (what a coincidence).

onion layer architecture

So, I thought let’s see if I can find a way to structure my application avoiding the repositories and using the DbContext only. It’s highly recommended to use a multi-projects solution. This lack of organization at the project level leads to spaghetti code. To resolve this issue, we should design a multi-project solution, each layer should be a separate project.

Onion Architecture: A Guide to Clean and Sustainable Code

And that’s why I included it inside my template because it can be useful for anyone. Also, I didn’t want to introduce CQRS and Event Sourcing with difficult examples with different databases and propagating changes from the Write DB to the Read DB. It is just a simple “DataBase” (there is no database access, just creating mocked data inside the Repository implementation). It contains the Controller classes which are injected with Service interfaces and/or Repository interfaces. (This project also has a dependency on Infrastructure, but that is solely for DI configuration purpose). Some recommendations I’ve found suggests use of the DbContext directly in my Controllers, which I didn’t like.

onion layer architecture

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. In Onion Architecture, the database is just a infrastructure detail.

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Contains the Service classes which implements the Service interfaces. Service classes are injected with Repository interfaces. Honestly, it’s not completely new, but I’m proposing it as a named, architectural pattern.

For a bigger example, check out the NorthwindTraders github repository. The author of this repo also has presentations on Youtube where he elaborates on the how (and why). So, if Service Layer have a reference to the Infrastructure, the Application Core is directly dependent on the Data Access Layer, and I don’t think we can call it an Onion anymore. I highly recommend you to read the article from Jeffrey Palermo about Onion Architecture. The same happens to IApplicationSettings, using the abstraction we are not tied to the way how we read the configs from the settings. Exceptional product leaders unite teams, drive innovation, and create outstanding products with visionary thinking and data-driven decisions.

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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.

onion layer architecture

If you have very complex business logic, it would make sense to encapsulate it inside of our domain entities. But for most applications, it is usually easier to start with a simpler domain model, and only introduce complexity if it is required by the project. We can use lower layers of the Onion architecture to define contracts or interfaces. The outer layers of the architecture implement these interfaces. This means that in the Domain layer, we are not concerning ourselves with infrastructure details such as the database or external services. Onion Architecture solved these problem by defining layers from the core to the Infrastructure.

Onion architecture vs 3 layered architecture

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. The Onion architecture was first introduced by Jeffrey Palermo, to overcome the issues of the traditional N-layered architecture approach which we discussed above. In Onion Architecture, dependencies flow from the outer layers toward the innermost core layer. This means the core layer is entirely decoupled from the outside world and can be tested independently of other components. 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. You register your db context in your DI framework and then your service layer is able to have this context injected.

onion layer architecture

In order to get rid of this dependency we would need to introduce a small abstraction (interface) defined by the needs of the Core. The application is built around an independent object model. The whole application core is independent because it cannot reference any external libraries and therefore has no technology specific code. These interfaces should be focusing on the business meaning of that interface and not on the technical aspects. So the shape of the interface is directly related to the scenario it is used in the business logic. Outer rings implement interfaces, meaning all technology related code remains in the outer rings.

The Essence of Onion Architecture

Within the application, each layer functions as a module/package/namespace. 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. Sylvia Fronczak is a software developer that has worked in various industries with various software methodologies. She’s currently focused on design practices that the whole team can own, understand, and evolve over time.

Onion Architecture has great practical value, particularly for creating expansive, intricate software systems. It is simpler to test, maintain, and upgrade the codebase over time when an application is built in layers, which isolates the business logic from the display layer and infrastructure. Throughout my developer career, every rails-style application (.net MVC, spring MVC, whatever) has a folder/project called “Utils”. Or some other very generic name with unknown purpose – up until I open this project and look onto the code. Sometimes there are two or more projects like that, written by different generations/teams of software developers. Application might be formally split into layers following layered or onion architecture, but these folders are referenced from everywhere.

How the Dependencies are mapped in a Clean Architecture?

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 onion layer architecture with the presentation and repository layer. The service layer holds all the business logic of the entity. In this layer services interfaces are kept separate from their implementation for loose coupling and separation of concerns.

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