Onion Architecture explained Building maintainable software Medium

Interfaces for these are defined in the Domain Services layer — IFareRepostory and IRouteService. RiderFareCalculator is implemented in this layer also, and it depends on the fare repository and route service interfaces declared in the same layer. Note that with this approach, we do not depend on the external service, rather the external service depends on our declared contracts. According to traditional architecture, all the layers are interconnected and significantly dependent on one another. For example, the UI layer communicates with business logic, which communicates with the data layer.

what is onion architecture

Therefore, if there are corrections to be made, do leave a comment below and I will update the information here. This way, all of us get to benefit and improve the general quality of software. These rules serve as guidelines for software developers when they need to work with onion architecture and are by no means exhaustive. And they are only effective if the developers themselves are disciplined enough when it comes to applying the rules in their work. Use a dependency injection framework to implement the onion architecture. Jeffery Parker is passionate about architecture and construction.

Domain services

We are going to use them in a global exception handler that will return the proper HTTP status code based on the type of exception that was thrown. 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.

Each method or function in a service class typically represents one use case. Use a facade to provide a common entry point for multiple endpoints (RESTful API, SOAP and direct function call) if they need to consume a service provided by the application layer. Domain-Driven Design gives us a more realistic approach to defining what really has business value.

What are the typical layers in an onion architecture?

Also, it results in shorter development periods since the code is simpler to understand and has fewer defects. As a result, changing business needs can be accommodated more easily without having to completely rewrite the application’s software. I hope you’ll find my experience useful for your projects. You can check my github repository for technical details. CQRS is a development principle claiming that a method must be either a command that performs an action or a request that returns data. The main issues we faced were related to maintaining the low connectivity of microservices.

  • It’s a common choice for developers during their work on the design of web, mobile, or desktop applications.
  • The testing pyramid is a great framework that lays out the different types of tests.
  • You might also have domain interfaces in addition to domain objects.
  • We can also use dependency injection frameworks, like Spring, to connect interfaces with implementation at runtime.
  • Each of these layers represent a specific duty within the overall function of a service.
  • Meldekhanov also recommends admiring the walls of the Chillakhana, a dome-shaped brick structure that once held sacred rituals and can only be accessed with a local guide.
  • By doing dependency injection in all the code, everything becomes easier to test.

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. Application services also referred to as “Use Cases”, are services responsible for just orchestrating steps for requests and should not have any business logic. Application Services interact with other services to fulfil the client’s request. 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.

Onion Architecture In ASP.NET Core 6 Web API

A Repository is a pattern for a collection of domain objects. DTOs are well suited as objects with really specific formats and data. The application layer implements Application rules (sometimes called use cases) instead of Business rules. If the Mausoleum of Khoja Ahmed Yasawi has history buffs itching for more, the surrounding Azret Sultan Reserve-Museum won’t disappoint.

what is onion architecture

Now, let’s work on the Core Layers starting from the Domain Project. It basically has the models/entities, Exception, validation rules, Settings, and anything that is quite common throughout the solution. When there is just a logical separation in your application, we can term it as layers or N Layers. In cases where there is both a physical and logical separation of concerns, it is often referred to as n-tiered application where n is the number of separations. Services in the domain layer exists only if and only if there is a need for operations that does not quite fit into an aggregate root or entity.

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The internal implementation of external layers does not need to be a concern for all internal levels. Implementing Domain Driven Design (DDD) through onion architecture significantly improves code quality, lowers complexity, and enables the development of evolving business systems. 2.infrastructure.payment contains adapters to a payment system of our organization but it is in another bounded context. We use MakePaymentService (a domain service) to decouple the payment system from other part of this system. 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.

what is onion architecture

When doing software development, one of the most important things to have in mind is that your software should always be evolving. We should be able to build a software that can be maintained by future developers. what is onion architecture Jeffrey Palermo coined the term “Onion Architecture” in 2008. This architecture enables greater application testability, maintainability, and dependability on infrastructures such as databases and services.


Domain and Application Layer will be at the center of the design. In N Layer Architecture, the Database is usually the Core of the Entire Application, i.e It is the only layer that doesn’t have to depend on anything else. Any small change in the Business Logics layer or Data access layer may prove dangerous to the integrity of the entire application. The great thing about this approach is that the migrations will be automatically applied when we create new migrations, further down the road. To learn more about migrations and how to seed data with EF Core in both .NET 5 and .NET 6 check out this article Migrations and Seed Data with Entity Framework Core.

what is onion architecture

Its intention is to isolate the core domain logic of an application from the surrounding concerns, such as the user interface or data access. 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. Onion Architecture is based on the inversion of control principle. 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.

Understanding the difference between ONION and N-Layered architecture

The rider selects their destination, then are presented with an estimated price for their trip. Trip estimation is a business use-case, and it’s the one I’ve selected for our implementation. Figure 2 below outlines the domain within the application structure. Low coupling occurs when one module interacts with another without worrying about the internals of the other module.

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