By doing this, your Infrastructure code can expect to receive an object that implements an interface, and the main can create the clients and pass them to the infrastructure. So, when you need to test your infrastructure code, you can make a mock that implements the interface (libs like Python’s MagicMock and Go’s gomock are perfect for this). A Domain Service contains behavior that is not attached to a specific domain model. It could be basically a function, instead of a method.

Application interface methods will typically be called from a controller in the WebApi Infrastructure layer to perform an action on its behalf. The name Onion Architecture was originally coined by Jeff Palermo, but has since gone under lots of other pseudonyms. Ports and Adapters, Hexagonal Architecture and Clean Architecture are all different names for effectively the same thing. They represent a way to structure the code that clearly separates the domain of the problem from the underlying technologies that implement the solution.

Infrastructure Layer

This approach makes it possible to create a universal business logic that is not tied to anything. 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 .

Domain services are responsible for holding domain logic and business rules. All the business logic should be implemented as a part of domain services. Domain services are orchestrated by application services to serve business use-case. They are NOT typically CRUD services and are usually standalone services. The biggest offender is the coupling of UI and business logic to data access.

A tightly coupled object is dependent on another object; that means changing one object in a tightly coupled application, often requires changes to a number of other objects. It is not difficult when an application is small but in an enterprise-level application, it is too difficult to make the changes. The popularity of microservices is growing due to the range of benefits they offer to developers and businesses. In this article, I will tell you about my experience of using onion architecture with a harmonized combination of DDD, ASP.NET Core Web API and CQRS for building microservices.

The business domain of the app gravitates around the pricing and purchase of items available at the supermarket. The business rules are based on the popular CodeKata of the same name. To keep things simple, I only implemented the pricing rules for items with a cost that varies based on quantity or weight. The GET request for the EditUser action method returns _EditUser partial view, where code snippet follows under the User folder of views.

I’ve found that it leads to more maintainable applications since it emphasizes separation of concerns throughout the system. I must set the context for the use of this architecture before proceeding. This architecture is not appropriate for small websites. It is appropriate for long-lived business applications as well as applications with complex behavior. It emphasizes the use of interfaces for behavior contracts, and it forces the externalization of infrastructure.

External Services

That being said, it’s not a big deal and it does not outweigh the pros. Services.Abstractions project does not reference any other project, we have imposed a very strict set of methods that we can call inside https://globalcloudteam.com/ of our controllers. OnModelCreating method, we are configuring our database context based on the entity configurations from the same assembly. Lazy class to ensure the lazy initialization of our services.

This approach is biased towards Object Oriented Programming . However it’s principles can still be applied in a wider sense. The very centre of the Model, this layer can have dependencies only on itself. It represents the Entities of the Business and the Behaviour of these Entities. Business Logic behaviour is declared as contracts with the use of interfaces in a Object-Oriented context. It is the level of dependency of one thing upon another.

Why Onion Architecture?

The Contracts folder represents the Domain Interfaces. These interfaces act directly or indirectly on Domain Entities to perform business logic. The Contracts folder also includes the interfaces for the Infrastructure Layer.

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. Onion architecture layers interact to each other by using the Interfaces. C# programmers are drawn to Onion Architecture due to the dependency flows.

Onion Architecture mainly aims at shielding your business fundamentals, and the dependency rule. 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. 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.

• It represents the Domain Entities layer of the onion architecture.
• The application’s entrypoint should be responsible for instantiating all necessary dependencies and injecting them into your code.
• Each layer/circle encapsulates or hides internal implementation details and exposes an interface to the outer layer.
• Onion architecture might seem hard in beginning but is widely accepted in the industry.
• The domain layers often need information or functionality in order to complete business functionality, however they should not directly depend on these.
• This approach is biased towards Object Oriented Programming .

The obvious advantage of the Onion architecture is that our controller’s methods become very thin. We moved all of the important business logic into the Service layer. The Onion Architecture relies heavily on the Dependency Inversion principle. An Anemic Domain Model is a domain model that has no behavior, just data. It acts just like a bag of data, while the behavior itself is implemented in a service.

Application Structure & Layers

These services just communicate with external resources and don’t have any logic. External notification Service, GRPC Server endpoint, Kafka event stream adapter, database adapters. 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.

There are some cases where it’s hard to fit a behavior into a single domain model. Application architecture is built on top of a domain model. A key tenet of Onion Architecture is that dependencies flow inwards or laterally toward the core. Concretely this means a project that is in an outer layer can refer to a project in an inner layer or the same layer. A project should never refer to a project that is farther from the core than itself.

Dependency

Onion Architecture is more appealing for C# programmers than Java programmers. However, it’s up to the architect community to consider and argue in the discussion on whether or not to apply the architecture. Not easy to understand for beginners, learning curve involved. Architects mostly mess up splitting responsibilities between layers.

These objects have no behavior, being just bags of data used alongside your models. Imagine that you are modeling a banking system, where you have the Account domain model. Then, you need to implement the Transfer feature, which involves two Accounts. Onion Architecture is an architectural pattern which proposes that software should be made in layers, each layer with it’s own concern.

This gets even more interesting when there are multiple processes making up a single software system. So, you should start by modeling your domain layer, instead of the database layer. Also, the code is easier to test due to dependency injection, which also contributes to making the software more maintainable. A Repository is a pattern for a collection of domain objects.

UI Layer

Another big advantage, it’s very easy to write automated tests for everything in the Application Core layer, because the infrastructure is outside that layer. UI layer doesn’t have any direct dependency so it’s easy to swap out implementations. onion structure might seem hard in beginning but is widely accepted in the industry. It is a powerful architecture and enables easy evolution of software.

Layers

In 3-tier and n-tier architectures, none of the layers are independent; this fact raises a separation of concerns. Such systems are very hard to understand and maintain. The drawback of this traditional architecture is unnecessary coupling. Most of the traditional architectures raise fundamental issues of tight coupling and separation of concerns.

In general, the deeper we dive, the closer we get to the domain and business rules. The outer circles represent mechanisms and the inner circles represent core domain logic. The outer layers depend on inner layers and the inner layers are completely unaware of outer circles. Classes, methods, variables, and source code in general belonging to the outer circle depends on the inner circle but not vice versa.