We will also together build a WebApi that follows a variant of Onion Architecture so that we get to see why it is important to implement such an architecture in your upcoming projects. You can find the source code of this implementation on my GitHub. Yes, Onion Architecture can be combined with other architectural patterns, such as microservices, event-driven architecture, and domain-driven design, to create complex and scalable systems. Onion Architecture promotes maintainability, supports testing, and enables loose coupling and separation of concerns.
These interfaces can be implemented with an adapter that connects to another microservice by exposing HTTP Rest, GRPC, Thrift Endpoints, etc. It’s a good fit for microservices, where data access layer not only comprises database, but also for example an http client, to get data from another microservice, or even from an external system. In addition to promoting maintainability and testability, onion architecture also supports loose coupling and separation of concerns.
Do not forget to change the target version here as well. Interactions between domain and persistence will follow a defined standard and will be independent of persistence details. DeleteUser() action, then the delete user screen is shown, as below. Here, the DefaultConnection is connection string which defined in appsettings.json file as per following code snippet. The code samples are taken from an example repository, which you can find on GitHub.
You also need to ensure that each layer only depends on the layer beneath it, and use inversion of control and dependency injection to manage dependencies. On the other hand, the Onion Architecture tackles the problems of tight coupling and separation of concerns. Onion architecture provides several advantages over other architectural patterns, making it an ideal choice for building scalable and maintainable software systems. One of the primary benefits of onion architecture is its ability to promote maintainability.
Command Query Seperation splits commands and queries into different request types, where commands alter state and may introduce side effects, while queries are read-only and side-effect free. The two types of requests can/do share a common model and data store. This is an application level pattern to clarify intent. All these architectures are basically saying you should split up your code into separate areas of concern, and all the normal rules about dependencies and coupling always apply, redardless. If you put your code in the wrong layer, or couple things that shouldn’t be coupled, or whatever, none of the architectures will be successful.
To do so, we must expose only immutable objects, preventing misuse of the API to gain domain access. If we return mutable objects through the API, people using the code could gain access to domain components we might not intend to expose. Although the API has access to the Domain and Core, it doesn’t know anything about the Infrastructure. Domain-Driven Design also has a service concept that is slightly different from the concept of an application service.
The outer rings are mechanisms , whereas the inside circles are fundamental domain logic. The outer layers rely on the inner layers, and the inner layers are unaffected by any changes being introduced in the outer rings. Domain Entities are the fundamental building block of Domain-Driven Design and they’re used to model concepts of your Ubiquitous Language in code.
Next, we looked at the Infrastructure layer, where the implementations of the repository interfaces are placed, as well as the EF database context. Each layer/circle wraps or conceals internal implementation details while providing an interface to the outer layer. All layers must also supply information that inner layers can easily consume.
Clean Architecture was introduced by Robert “Uncle Bob”. Some of the forms of clean architecture are Hexagonal Architecture, Ports and Adapter, and Onion Architecture. It builds on the concepts of Onion Architecture with some refinement. Instead of “Domain Model”, it refers to the core as “Entities”. Although, it is good for small-scale applications.
Likewise, they’re almost not even business decisions. They’re incidental to the way you have architected your system. By being on the outside, it is the thing that is easiest to change. You pass it as an argument to some function in your domain layer, your pure layer, your core, and it returns some answer. You take that answer, and you send it back as the response that’s also an action.
The classes, relations and interactions between them describe the core of the domain of the application, i.e. what business needs it fulfils and in what way. In the Library, there would be a process of adding new titles to the catalogue, a process of borrowing and returning copies of a book, charging readers for overdue books, and many more. The main difference I’ve found in the implementations of Hexagonal Architecture and Onion Architecture lies mostly in the overall, more structured approach to the code layout of the latter. Making the concept a first-class citizen represented in the code guides implementation and gives more clear overall structure to the codebase.
Onion Architecture separates the application into layers based on their responsibilities. This separation makes the application more modular and easier to understand. Each layer has a specific responsibility, and there is a clear separation of concerns between the layers.
We’ll explore them shortly, but first let’s have a look at what each of these patterns are about individually and how they compare to one another. Onion Architecture provides several benefits, including separation of concerns, testability, maintainability, flexibility, and scalability. However, it also presents some challenges, including a learning curve, increased complexity, and increased code overhead. Onion Architecture provides flexibility in the implementation of the application.
We can find some Domain-Driven Design concepts present in the Onion Architecture domain layer, but it’s important to point out that DDD and Onion Architecture are not necessarily the same thing. Onion Architecture divides the complete projects into 4 different layers. Regarding using repository interfaces as ports, that is one way to go about it, but not the root reason i disagree with Palermo, as i hope u can see by my explanation above.
The UI can’t function if business logic isn’t there. Business logic can’t function if data access isn’t there. I’m intentionally ignoring infrastructure here because this typically varies from system to system. We often don’t keep systems up-to-date because it’s impossible to do. If coupling prevents easily upgrading parts of the system, then the business has no choice but to let the system fall behind into a state of disrepair.
You may require to add the reference of Domain Layer in your repository or other projects. These architectures play a key role when we build large and complex projects which require regular enhancements. In practice, i don’t think there is any positive or negative impact of any of the approaches, but i feel that conceptually its more correct to put them in the application layer.
You might come up with a new accounting system, that’s better. This double-entry bookkeeping system, you can encode it once. You’re pushing out all this responsibility of carrying out the actions into the interaction layer and deciding what actions to take is part of the domain layer. Like I said, on my previous episode, what’s happening there is you’re not doing functional programming. If you need more data like that’s not the right level of abstraction.
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. The diagram you see here is a representation of traditional layered architecture. This is the basic architecture I see most frequently used.