The Onion Architecture : part 1 Programming with Palermo

The outer layer is like a separate part of the whole application logic. It doesn’t depend much on the previous layers, so you can easily swap databases and other details. All three patterns are aligned on this principle; it emphasizes that source code dependencies should only point inward. The outer layer can only refer to the inner layer and not vice versa.

What is onion architecture

Its only connection to the layers located inside is via other interfaces. Now, it is time we moved on to the pros and cons of this type. Briefly, onion architecture is a pattern that is designed to be used at the solution or system level. It comes with several principles and layers that will be explained below. As a developer, you need to design a user related business logic, which will persist in a database.


The View is responsible for creating the user interface. At deeper layers, we define abstract interfaces, while at the top layer, we give their concrete implementation. By doing this, we can keep our attention on the domain model and lessen our concern about implementation issues. We may also use dependency injection frameworks like Spring to link interfaces with implementation at runtime.

To keep things simple but demonstrate the architecture to the fullest, we will build an ASP.NET Core Web API that is quite scalable. For this article, Let’s have a WebApi that has just one entity, Product. We will perform CRUD Operations on it while using the Onion architecture. The presentation layer is where you would Ideally want to put the Project that the User can Access. Domain and Application Layer will be at the center of the design. These layers will not depend on any other layers.


To keep things clean, what we can do is, Create a DependencyInjection static Class for every layer of the solution and only add the corresponding. One outer layer which may surprise many is Infrastructure. Is the database we use or an external dependency not part of our domain model layer?

What is onion architecture

But we will just add the BaseAPI Controller as the base class. It is where you would want to add your Infrastructure. Maybe an Entity Framework Core Layer for Accessing the DB, a Layer specifically made to generate JWT Tokens for Authentication or even a Hangfire Layer.

The answer is, as always, “it depends”, but I would argue the extra abstraction layer is definitely worth the effort for projects of sufficient size and scope. Well the notion is Data access layer technology keep changing after almost 3-4 years of life span. However, I have covered a few of these topics in other articles in my blog already.

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.

Best continuous integration tools

It develops a loosely coupled application as the outer layer of the application always communicates with the inner layer via interfaces. No direction is provided by the Onion Architecture guidelines about how the layers should be implemented. The architect should decide the implementation and is free to choose whatever level of class, package, module, or whatever else is required to add in the solution. All these types of objects together represent the business logic of the project. In some cases though, it might make more sense to use a Domain Interface to encapsulate business logic outside of the entity. The layer higher in the hierarchy (Layer N+ 1) only uses services of a layer N.

What is onion architecture

Another option is to use tools that scan your source code to ensure architectural constraints or code reviews. They usually prefer the fast implementation approach over the long lasting and you often have to do a lot of effort to convince them. Clearly defined and commonly accepted levels of abstraction enable the development of standardized tasks and interfaces. After many years of layered architecture a lot of tools and helpers have been invented to automatically map from one layer to another for example. This also comes in line with the preferences expressed by Martin Fowler. When I observe these patterns, I feel all three patterns are trying to advocate similar ideas.

Adding Swagger To WebApi Project

The code snippet, mentioned below is for the UserProfile entity. Now, we create an User entity, which onion architecture is inherited from BaseEntity class. The code snippet, mentioned below is for the User entity.

  • It is where you would want to add your Infrastructure.
  • Our customer needed a software system compatible with their hardware so that clients could buy equipment, install software and create and manage content.
  • Few of the solutions developed and maintained by Microsoft MVPs like eShopOnWeb and eShopOnContainers also follow a similar .
  • The number of layers in application will vary but domain is always at the center.

Onion architecture layers interact to each other by using the Interfaces. C# programmers are drawn to Onion Architecture due to the dependency flows. If you are interested in learning more C# while working with the Onion Architecture, visit the TechRepublic Academy. Drilling down deeper into the domain layer makes this issue more apparent. The LayerProductionApplicationService uses a set of Domain Services. Onions are a delicious vegetable and are a core ingredient in cuisines around the world.

Domain Model Layer Rules

This is also known as DIP or Dependency Inversion Principle. We have connected all of our Onion architecture implementation layers, and our application is now ready for use. We’ve shown you how to implement the Domain layer, Service layer, and Infrastructure layer. Also, we’ve shown you the Presentation layer implementation by decoupling the controllers from the main Web application.

Application specific rules

Be sure to reflect properly on that point before jumping blindly into the Onion Architecture. Let us see what Onions combined with Domain Driven Design produces. Upon first sight the layer model seems straightforward and easy to adopt. Unfortunately developers often take the layering literally. Another very good one is “Lean Architecture for agile software development” by James Coplien, and another MUST READ is “The mythical man month” by Frederick P Brooks. Another of my question is, What about Cross cutting concerns like logging, caching, … and something will use in all layers.

You could either merge services like Twilio into a single Project layer and implement the interface within each corresponding folder. This is if you think the infrastructure implementation is not very complex. Using different projects for different layers is only ever coincidentally a good idea. You’d have to have a very large project before doing that made any sense, in which case moving the application into a more service-based architecture might well be a better choice. Elsewhere you had mentioned that adding repositories increases lines of code and yes, it does; however, “number of lines of code” is useless as a quality metric.

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. Application is divided into layers where each layer has a set of responsibilities and addresses separate concerns. Each layer acts as modules/package/namespace within the application. Layers are not tightly coupled and have a separation of concerns.

Imagine if you put the saveObjectToDatabase method in domain object, then you will depend on Infrastructure layer which is aviolationof Onion Architecture. The strategy is to make interfaces defined in the domain layer and put the implementation in Infrastructure layer. Following this principle makes sure we have loose coupling between layers and a real implementation comes only at real time.

Onion Architecture In ASP.NET Core With CQRS – Detailed

This is how you can invert the dependencies to build scalable applications. This layering can help in the separation of concerns, subdividing the solution into smaller units so that each unit is responsible for a specific task and also takes advantage of abstraction. For mid to larger scaled projects where multiple teams work, layering has very obvious advantages up its sleeves.

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