Software Architecture The Onion Architecture by Shivendra Odean

Filip18 września, 2021

This means that in the Domain layer, we are not concerning ourselves with infrastructure details such as the database or external services. Onion architecture is also applicable to microservices when viewing each microservice in isolation. Each microservice has its own model, its own use cases and defines its own external interfaces for retrieving or modifying the data. These interfaces can be implemented with an adapter that connects to another microservice by exposing HTTP Rest, GRPC, Thrift Endpoints, etc. Domain services are responsible for holding domain logic and business rules. All the business logic should be implemented as a part of domain services.

They all do it by splitting code on separate layers. The whole difference is what components are defined and what dependencies exist between those layers. You can add as many layers as you need at any level you want.

Infrastructure services

There are some cases where it’s hard to fit a behavior into a single domain model. The domain layer is the innermost layer of the architecture. Domain-driven design is the concept that developers and domain experts should use the same names both in code and business domain. Something that Steve smith mentioned in this post, All interfaces like services interfaces and repositories interfaces placed in inner center area of domain. These days we have a different idea of what should go in a layer than 30 years ago, and we are more careful about where we put domain logic, presentation logic, and so on. But the architectural principles are all essentially the same.

What is onion architecture

The outer layers are all allowed to reference the layers that are directly below them in the hierarchy. Divide the application into different modules/projects each responsible for a layer in onion architecture. Naturally, maybe you want to start the development by the database, but it’s a mistake! When working with Onion Architecture, you should always start developing the inner layers before the outer ones. An Anemic Domain Model is a domain model that has no behavior, just data.

Department Service

Having created a domain model and a web API, we needed to seamlessly connect them. In addition, the onion architecture itself introduced certain problems. It took us some time to distribute functional parts between appropriate layers. But eventually, this problem was practically eliminated. To address your question directly “Isn’t all of that achieved by merely adding façades to my traditional N-layered architecture?”.

Any developer, familiar with the domain, should be able to understand the code, and easily know where to change things. So, we can see that it’s important to build maintainable software. We should be able to build a software that can be maintained by future developers.

Onion Architecture Layers

OData API – This API will interact with database and will perform database related operations only . I want this API to be only platform to access and manipulate data. It would provide the functionality to retrieve data through different means . I am making the structure of a .Net based application. Here are details of different components of the system.

What is onion architecture

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 . The Domain layer does not have any direct dependencies on the outside layers.

Benefits and Drawbacks of Onion Architecture

In order to access the Database, we introduce a Data Access Layer. This layer usually holds ORMs for ASP.NET to fetch/write to the database. The main problem with this architecture is that all layers are built on top of the Data Access Layer and are, in fact, tied to a certain type of data storage.

  • It has access to the interfaces of the assembly as well.
  • PS, I use Visual Studio 2019 Comunity which is completely FREE.
  • Yes, EFCore is installed in the Application layer as well.
  • It took us some time to distribute functional parts between appropriate layers.
  • The Service layer also could hold business logic for an entity.
  • To submit a letter to the editor for publication, write to

Using this approach, we can encapsulate all of the rich business logic in the Domain and Service layers without ever having to know any implementation details. In the Service layer, we are going to depend only on the interfaces that are defined by the layer below, which is the Domain layer. Outer layer data formats should not be used by inner layers. Data formats used in an API can vary from those used in a DB for persistence. Whenever data crosses layers/boundaries, it should be in a form that is convenient for that layer. 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.

What is the Onion Architecture?

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What is onion architecture

As you can see in the picture, the three inner layers i.e. domain model, domain services, and application services are parts of the application core. Application core contains all logic necessary to run and test the application as long as necessary dependencies are provided at runtime. This is possible thanks to the dependency rule that we introduced in the previous paragraph. Since no code in the application core depends on outer layers, we can just swap out the UI or the database for the testing purposes. In onion architecture, we have the domain layer, repository layer, service layer, and presentation layer. Onion architecture solves the problem that we face during the enterprise applications like coupling and separations of concerns.

Ubiquitous language between domain experts and developers

Similar to other architectural styles like Hexagonal, Layered, Clean Architecture, etc. it provides a solution for common problems. Domain Entities are the fundamental building block of Domain-Driven Design and they’re used to model concepts of your Ubiquitous onion software architecture Language in code. Entities are Domain concepts that have a unique identity in the problem domain. Domain entities encapsulate attributes and entity behaviour. It is supposed to be independent of specific technologies like databases or web APIs.