The outer layers, in contrast, are concerned with technical details such as user interface and database access. Domain entities represent core business concepts, encapsulating state and behavior. They remain persistent-ignorant and independent of infrastructure.

In this article, we are going to learn about Onion architecture and what are its advantages. We will build a RESTful API that follows the Onion architecture, with ASP.NET Core and .NET 5. No, Onion Architecture is a language- and framework-agnostic pattern that can be implemented using any programming language or framework. I am Computer Science Engineer, with an interest in AI and have worked on research projects in startups. ASP.NET Core offers Health Checks Middleware and libraries for reporting the health of app infrastructure components. But it does not quite solve the validation problem, especially if you need to take information from a database or from another microservice.

Dependency

It’s the outer-most layer, and keeps peripheral concerns like UI and tests. For a Web application, it represents the Web API or Unit Test project. This layer has an implementation of the dependency injection principle so that the application builds a loosely coupled structure and can communicate to the internal layer via interfaces. 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 addresses the challenges faced with 3-tier and n-tier architectures, and to provide a solution for common problems.

Then, we are modifying the response HTTP status code depending on what the specific exception type is. Now we only have one more layer left to complete our Onion architecture implementation. With this approach, we are being very explicit about what the higher layers of the Onion can and can not do. It is easy to miss here that the Services.Abstractions project does not have a reference to the Domain project. These exceptions will be handled by the higher layers of our architecture.

Separation of concerns

We now know that Onion Architecture has a significant role in implementing a domain-driven design. It refers to the business knowledge that our programme is attempting to model. Another difference between the two architectures is the way they handle technical concerns. By applying Onion Architecture, this e-commerce platform achieves modularity, testability, and maintainability while providing a seamless and secure shopping experience for users. Remember that we have two abstract exception classes BadRequestException and NotFoundException inside of the Domain layer?

Clean Architecture is a software design pattern introduced by Robert C. Martin(Uncle Bob) in 2012 in the post. The architecture emphasizes the separation of concerns, with each circle responsible for a distinct set of responsibilities. The architecture places a particular emphasis on the use of interfaces to decouple components. Onion Architecture is a software architecture pattern that follows the Dependency Inversion Principle. The architecture is named Onion Architecture because it has several layers around the core of the application, just like the layers of an onion.

Domain services

The layers of Onion Architecture include the Domain layer, Application layer, Infrastructure layer, and User Interface layer. By organizing the codebase according to this folder structure, developers can easily navigate and modify different components of the application. The folder structure promotes separation of concerns, with dependencies flowing inward, adhering to the dependency rule of Onion Architecture.

They can change often and thus are separate from our core application logic. The idea of the Onion Architecture is based on the inversion of control principle, i.e. placing the domain and services layers at the center of your application, externalizing the infrastructure. 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.

Observability-Driven Development (ODD)

Software architecture is an essential aspect of any software development project. It defines the structure of the software system and helps in achieving the desired qualities of the system. There are several software architecture patterns, and one of them is the Onion Architecture. When designing the architecture of a building there are many aspects you need to consider. Will our base provide enough support for every floor, what if the support beams A and B collapse, will floor C remain standing? These same questions can apply to software architecture as well.

The outer circle’s classes, methods, variables, and source code generally depend on the inner circle but not the other way around. This architecture enables greater application testability, maintainability, and dependability on infrastructures such as databases and services. The domain model is at the center of Domain-Driven Design or development, which thoroughly understands a domain’s procedures and regulations. It creates software for complicated requirements by closely connecting the implementation to a changing model of fundamental business ideas.

Performance Disadvantages of Clean Architecture: A Closer Look

Furthermore, the added complexity of defining contracts / interfaces and religiously enforcing them requires a strong understanding of the pattern. If executed well, the benefits will supercharge productivity and greatly increase the flexibility of the applications being developed. The adaptable Onion Architecture enables developers to modify an application without affecting other system components. Developers can replace or update components without having to change other system components since each layer is autonomous and only communicates with other levels through well defined interfaces. The “Onion Architecture,” a well-known software design, has a number of benefits for both businesses and developers. Some of the main advantages of onion architecture are listed below.

• The code samples are taken from an example repository, which you can find
  on GitHub.
• In conclusion, each software development effort must start with writing maintainable, clean code.
• The Infrastructure Layer is the outermost layer of the Onion Architecture.
• To make it straightforward to download the application code and be able to run the application locally we are using Docker.
• Domain entities represent core business concepts, encapsulating state and behavior.
• This layer has an implementation of the dependency injection principle so that the application builds a loosely coupled structure and can communicate to the internal layer via interfaces.

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. It refers to the business knowledge that our software is trying to model. Domain-Driven Design centres on the domain model that has a rich understanding of the processes and rules of a domain. Onion architecture implements this concept and dramatically increases code quality, reduces complexity and enables evolutionary enterprise systems. When I observe these patterns, I feel all three patterns are trying to advocate similar ideas.

Can Onion Architecture be combined with other architectural patterns?​

Different layers of onion architecture have a different set of responsibilities and accordingly, there are different testing strategies. The testing pyramid is a great framework that lays out the different types of tests. Business rules that belong to the domain model, domain what is onion architecture services and application services should be tested via Unit Testing. As we move to the outer layer, it makes more sense to have integration tests in infrastructure services. For our application End to End testing and BDD are the most appropriate testing strategies.

The Architecture of a Modern Startup

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. 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. The higher layers of the Onion will take care of implementing that interface transparently. Developing a system core that is both stable and efficient is essential when basing a system’s architecture on that of an onion.

Clean Architecture

Onion Architecture is a software design pattern that Jeffrey Palermo introduced in 2008 in the post. It is based on the concept of layers, each representing a distinct set of responsibilities. The architecture consists of an innermost core layer, surrounded by one or more layers of increasing abstraction. The innermost core layer contains the application’s most essential business logic and is isolated from the outside world.