Onion Architecture explained Building maintainable software Medium

Let’s consider the use case to create an order with a list of items. We first need to calculate the price including tax computation/discounts, etc., save order items and send order confirmation notification to the customer. The application services can be only invoked by Infrastructure services. 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.

Modifying the database modeling should not affect the software’s business rules. Thus, we register these as a Service in the ConfigureServices method in the StartUp class as per the following code snippet. To implement the Onion architecture, we develop an ASP.NET Core application. This application performs CRUD operations on entities. Each project represents a layer in onion architecture. Onion architecture consists of several concentric layers interacting with each other towards the core, which is the domain.

We started with the Domain layer, where we saw the definitions for our entities and repository interfaces and exceptions. We have connected all of our Onion architecture implementation layers, and our application is now ready for use. The Onion architecture is a form of layered architecture and we can visualize these layers as concentric circles. The Onion architecture was first introduced by Jeffrey Palermo, to overcome the issues of the traditional N-layered architecture approach. I’ll be writing more about the Onion Architecture as a default approach for building enterprise applications. I will stay in the enterprise system space and all discussion will reside in that context.

Infrastructure

However, there are still a couple of things to take care of. But how are we going to use the controller if it is not in the Web application? Great, we have seen how to implement the Presentation layer.

Layers are not tightly coupled and have a separation of concerns. Figure 2 — Practical Onion ModelEstimating the fare is a core business use case. The business would not functional well if it could not give it’s customers proper pricing. Hence this behaviour shall be declared in the most central layer in the interface IRiderFareCalculator. Dependency Injection is a necessary evil with this architecture. It causes us to rely heavily on something quite external that binds the entire application together and allows it to function at run-time.

Frameworks, Clients and Drivers

DeleteUser() action, then the delete user screen is shown, as below. The system is constructed around an independent application core. Visualization — use tools that allow you to visualize the collected data. Hence, when you separate these requests, onion structure you can use different technologies for handler implementation . To put it simply, every action in Web API is either a request or a command , but it shouldn’t do both. Consequently, each API method is divided into requests and commands.

• For our application End to End testing and BDD are the most appropriate testing strategies.
• The following code snippet is for the same (IUserService.cs).
• The UserController has an action method named EditUser, which returns the view to edit a user.
• It’s not so clear if this behavior should be implemented by the Account model, so you can choose to implement it in a Domain Service.
• In today’s world of request and response logic, you will need a repository interface and gateways to handle the situation.
• The higher layers of the Onion will take care of implementing that interface transparently.

Onion Architecture is comprised of multiple concentric layers interfacing each other towards the core that represents the domain. The architecture does not depend on the data layer as in classic multi-tier architectures, but on the actual domain models. The DbContext must have an instance of DbContextOptions in order to execute. We will use dependency injection, so we pass options via constructor dependency injection.

It is an ASP.NET Core Web application in this sample but it could be a Unit Test or Web API project. It is the most external part of an application by which the end-user can interact with the application. It builds loosely coupled applications with in-built dependency injection in ASP.NET Core. The circles represent different layers of responsibility.

Software Architecture — The Onion Architecture

Note that, ideally, you should always try to implement behaviors in Domain Models to avoid falling in the Anemic Domain Model pitfall. As this layer is purely logical, it should be pretty easy to test it, as you don’t have to worry about mocking IO operations. By isolating your domain logic, it becomes easy to test, and easier to maintain. When doing software development, one of the most important things to have in mind is that your software should always be evolving. It will always be maintained, evolved, receiving new features, improvements, and bug fixes.

There are two types of coupling – tight coupling and loose coupling. The foundation of Onion Architecture is based on the inversion control principle. This architecture is made up of several concentric layers interfacing towards the core that denotes the domain. The unique part of Onion Architecture is – it has zero dependencies over data layers like classic multi-tier architectures. Onion Architecture is comprised of multiple concentric layers interfacing with each other towards the core that represents the domain. The architecture does not focus on underlying technology or frameworks but the actual domain models.

I find this pattern to help greatly with Test Driven Development . I often find it easier to drive out business logic code through tests than I do integrated code. The Service layer holds interfaces with common operations, such as Add, Save, Edit, and Delete. Also, this layer is used to communicate between the UI layer and repository layer. The Service layer also could hold business logic for an entity. In this layer, service interfaces are kept separate from its implementation, keeping loose coupling and separation of concerns in mind.

We have prepared a project that follows the Onion architecture which we are going to use in the rest of the article. The Onion architecture is also commonly known as the “Clean architecture” or “Ports and adapters”. These architectural approaches are just variations of the same theme.

What is the motivation for splitting the Service layer?

The core consists of the domain layer, repositories layer and services layer. The number of layers in the application core will vary, however, the domain will always be the very center of the onion architecture. Our fare calculation depends on external services such as routing information and fare models.

Because it depends on the layers below it in the hierarchy, it can only call the methods that are exposed by the lower layers. The main idea behind the Onion architecture is the flow of dependencies, or rather how the layers interact with each other. The deeper the layer resides inside the Onion, the fewer dependencies it has. Using dependency inversion throughout the project, depending on abstractions and not the implementations, allows us to switch out the implementation at runtime transparently. We are depending on abstractions at compile-time, which gives us strict contracts to work with, and we are being provided with the implementation at runtime. Instead of each module being responsible of instantiating it’s own dependencies, it has its dependencies injected during it’s initialization.

This architecture should be used when creating services that deal with business rules. When you are creating a software that does not deal with business rules, this architecture won’t fit well. It would be really cumbersome to implement, for example, a simple gateway using Onion Architecture. It’s easy to find where are the business rules, the use cases, the code that deals with the database, the code that exposes an API, and so on. It’s easier to maintain an application that has a good separation of concerns. You can change things in the Infrastructure Layer without having to worry about breaking a business rule.

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Domain Entities are the fundamental building block of Domain-Driven Design and they’re used to model concepts of your Ubiquitous 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. Order is an entity and has attributes like OrderId, Address, UserInfo, OrderItems, PricingInfo and behaviour like AddOrderItems, GetPricingInfo, ValidateOrder, etc. Since a repository typically interacts with a database, the actual implementation of a repository is not found in this layer.

Services.Abstractions project it will only be able to call methods that are exposed by this project. We are going to see why this is very useful later on when we get to the Presentation layer. Presentation project will be the Presentation layer implementation.

Why Microservices Are Good for Our Project

The concrete implementation of a repository is implemented outside the application core, thus decoupling the actual database from the application core. This layer is the bridge between external infrastructure and the domain layers. The domain layers often need information or functionality in order to complete business functionality, however they should not directly depend on these. Instead, the application layer needs to depend on the the contracts defined in the Domain Services layer. Next, we looked at the Infrastructure layer, where the implementations of the repository interfaces are placed, as well as the EF database context.

Working with User Input in C#: Basic to Advanced

The GET request for the DeleteUser action method returns _DeleteUser partial View. The code snippet mentioned below is under the User folder of Views. In this class, we override the OnModelCreating() method. The following is the code snippet for the context class. Now, we define the configuration for the UserProfile entity that will be used when the database table will be created by the entity.

If you are interested in learning more C# while working with the Onion Architecture, visit the TechRepublic Academy. The second layer of the onion contains the Application Interfaces. The contracts defined here orchestrate the use of the Domain Services and are the entry point for all business logic. Generally speaking, the Application Interfaces orchestrate an operation on the domain entities and services. The ICheckoutService in the example above does just that. To pass the data from UI to a controller to edit a user, use same view model named UserViewModel.

Onion Architecture & Simple sample Code

ASP.NET Core is designed from the ground to support and leverage dependency injection. Thus, we create generic repository interface for the entity operations, so that we https://globalcloudteam.com/ can develop loosely coupled application. The code snippet, mentioned below is for the IRepository interface. It holds POCO classes along with configuration classes.