The Onion Architecture : part 1 Programming with Palermo

Classes, methods, variables, and source code in general belonging to the outer circle depends on the inner circle but not vice versa. 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. It’s a software that any developer should be able to do improvements and fixes without worrying about breaking something under the hood. Any developer, familiar with the domain, should be able to understand the code, and easily know where to change things.Modifying the view layer should not break any domain logic. Modifying the database modeling should not affect the software’s business rules.

onion architecture

The architecture does not depend on the data layer as in classic multi-tier architectures, but on the actual domain models. Most of the traditional architectures raise fundamental issues of tight coupling and separation of concerns. Onion Architecture was introduced by Jeffrey Palermo to provide a better way to build applications in perspective of better testability, maintainability, and dependability.

Onion Architecture In ASP.NET Core With CQRS – Detailed

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onion architecture

Your Domain models can have Value objects in their attributes, but the opposite is not allowed. 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. There are two golf courses just to the east of Sofia — in Elin Pelin (St Sofia club) and in Ihtiman (Air Sofia club), and a horseriding club (St George club).


The Controller handles web requests via action methods and returns the appropriate View. As a result, it solves the problem of separation of concerns while still allowing the Controller to perform database access logic. It is the outermost layer and contains peripheral aspects such as UI and tests.

onion architecture

By doing dependency injection in all the code, everything becomes easier to test. If you have a repository that expects a PostgreSQL client, the main should instantiate it and pass it to the repository during its initialization. The application’s entrypoint (usually, the main) should be responsible for instantiating all necessary dependencies and injecting them into your code. The inner layers shouldn’t know if your application is being exposed through an API, through a CLI, or whatever. The parts of your code that expose your application to the outside world are also part of the Infrastructure Layer, as they deal with IO.

Onion Architecture explained — Building maintainable software

We can also use dependency injection frameworks, like Spring, to connect interfaces with implementation at runtime. Repositories used in the domain and external services used in Application Services are implemented at the infrastructure layer. In the very center we see the Domain Model, which represents the state and behavior combination that models truth for the organization.

  • Onion Architecture is more appealing for C# programmers than Java programmers.
  • In my implementation, I intend to demonstrate some of the key layers of this architecture and how they work together.
  • The testing pyramid is a great framework that lays out the different types of tests.
  • But it does not quite solve the validation problem, especially if you need to take information from a database or from another microservice.

Some of the main advantages of onion architecture are listed below. It also exchanges data with the infrastructure layer in order to read and write data. Also, this layer offers an API that the infrastructure layer can leverage to obtain business needs, and it is in charge of turning those requirements into usable code. Building clean and durable code is critical for any project’s long-term success in software development. The difference between clean and sustainable code is that the former can be updated and maintained throughout time, while the latter is simple to read, comprehend, and edit.

Data Folder

If coupling prevents easily upgrading parts of the system, then the business has no choice but to let the system fall behind into a state of disrepair. This is how legacy systems become stale, and eventually they are rewritten. We can use lower layers of the Onion architecture to define contracts or interfaces. The outer layers of the architecture implement these interfaces.

onion architecture

This means that in the Domain layer, we are not concerning ourselves with infrastructure details such as the database or external services. An approach to layering the code of an application according to its functionality and purpose is known as onion architecture. The pattern entails constructing concentric circles or layers around a central domain model, each of which is responsible for a distinct task and has dependencies flowing inward toward the core. Instead of building a highly decoupled structure, we often end up with several layers that are depending on each other. This is something really bad in building scalable applications and may pose issues with the growth of the codebase.

Observability services

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In this article, we will learn to deploy Blazor WebAssembly to AWS Amplify along with CI/CD with GitHub, providing you with a streamlined development workflow…. However, I have covered a few of these topics in other articles in my blog already. onion architecture You could go through them to understand the core concepts and to learn how everything works. I have written a detailed article on API Versioning in ASP.NET Core 3.1 WebApi. Feel feel to read it to get a complete idea of this concept.

What are Some Problems with Onion Architecture?

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.

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