Onion Architecture Is Interesting

The main disadvantage of the Onion architecture is that it is more complex than the traditional N-layered architecture approach. This is because there are more layers and each layer has a more specific responsibility. Software architecture is a key factor in software development that helps create maintainable, scalable, and easily understood systems. Two popular software architectures often discussed in software development are Clean Architecture and Onion Architecture. Using contracts allows each layer to set its expectations onto the next and couples it to only what it requires to be. In this layer is where the majority of our business logic lives, it carries out the operations to turn A into B, input into output, egg into chicken.

  • This layer consists of the data access pattern, which is a more loosely coupled approach to data access.
  • The Controller handles web requests via action methods and returns the appropriate View.
  • Onion Architecture solved these problem by defining layers from the core to the Infrastructure.
  • Then, in the build files corresponding to each of the modules, declare their dependencies,
    clearly defining the direction of dependencies.
  • A classic example is Microsoft’s data access stack, which tends to change every few years.
  • Furthermore, the added complexity of defining contracts / interfaces and religiously enforcing them requires a strong understanding of the pattern.

It is the outermost layer and contains peripheral aspects such as UI and tests. It represents the Web API or Unit Test project in a Web application. This layer implements the dependency injection principle, allowing the application to design a loosely linked structure and communicate with the internal layer using interfaces. The Domain entities in the center represent the business and behavior objects. These layers can change, but the domain entities layer is always in the middle.

Application Structure & Layers

These concerns shouldn’t dictate the business rules and shouldn’t creep into all the layers. Hence, all infrastructure components are represents via abstract interface in Application Core. Architecture patterns are the core of how we design our applications. In my  experience, understanding these rules has helped me to create extendable, testable and comprehensive software systems. The application layer implements Application rules instead of Business rules.

For example, as in the example above, a user interface adapter and a REST adapter can both be connected to the port to control the application. And a port for sending notifications could have an email adapter, an SMS adapter, and a WhatsApp adapter connected to it. Then you have your business rules, which are https://www.globalcloudteam.com/ things like, «Ah, we want to return five images in our response.» I’m trying to come up with business rules that make sense in this domain. If you apply a change to the core, it will affect all the outer layers. That’s because the whole application focuses on logic, so any wrong action could break it down.


The clear separation of concerns between the layers makes it easier to modify and maintain the application. Changes in one layer do not affect the other layers, which reduces the risk of introducing bugs into the system. And finally, we saw how our Presentation layer is implemented as a separate project by decoupling the controllers from the main Web application. Then, we explained how we can connect all of the layers using an ASP.NET Core Web API.

what is onion architecture

The clear separation of concerns and decoupling of dependencies enable easier maintenance and modification of code, making it more adaptable to changing requirements. To demonstrate a common folder structure based on Onion Architecture, let’s consider a hypothetical e-commerce application. 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.

SOLID principles: building robust and maintainable code with Typescript

We are creating a project called Presentation and giving it a reference to the Microsoft.AspNetCore.Mvc.Core NuGet package so that it has access to the ControllerBase class. Then we can create onion architecture our controllers inside this project. To learn how to implement the repository pattern with Entity Framework Core you can check out this article ASP.NET Core Web API – Repository Pattern.

what is onion architecture

The separation of concerns between the domain and infrastructure layers allows for a more modular and testable software design. This layer creates an abstraction between the domain entities and business logic of an application. In this layer, we typically add interfaces that provide object saving and retrieving behavior typically by involving a database. This layer consists of the data access pattern, which is a more loosely coupled approach to data access. The “domain model” defines the “enterprise business rules” and thus corresponds to the “entities” ring – i.e., the innermost circle – of the clean architecture.

RSC implementation

We can write business logic without concern about any of the implementation details. If we need anything from an external system or service, we can just create an interface for it and consume it. 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. 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.

what is onion architecture

They are going to be treated the same as if they were defined conventionally. Now we only have one more layer left to complete our Onion architecture implementation. These exceptions will be handled by the higher layers of our architecture.

What do architectural patterns teach us?

This is part two of a three-part series I’m doing on functional
architecture. What does functional programming have to say about
architecture? We went over the first thing, which was the stratified
design. This is where you build layers of meaning on top of more
fundamental layers of meaning.

what is onion architecture

The application is separated into layers, each with its own duties and concerns. Within the application, each layer functions as a module/package/namespace. But precisely what is Onion Architecture, on which principle it is based, what is the essence of Onion Architecture, when to implement it, etc., will be discussed in this article. 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.

Domain-Driven Design (DDD) with Node.js: Organizing Your Application

It is well compatible with CQRS due to pipeline behaviors. Bounded context is a good fit for a microservices architecture. It is much easier to build a microservice around a bounded context. To organize business logic for our project, we used Domain-Driven Design (DDD). This layer will contain operation-specific orchestration and related logic for the application. The Global Cloud Team specializes in different application architectures.

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