This layer acts as a middleware to provide data from Infrastructure to UI. The term “Onion Architecture” was first coined by Jeffry Palermo back in 2008 in a series of blog posts. The architecture is intended to address the challenges faced with traditional architectures and the common problems like coupling and separation of concerns. In the case of Onion Architecture, I see things pretty similarly as in the Hexagonal one. I’m not sure if that’s an Onion Architecture already, it depends on your interpretation. On the other hand, we have the monoliths, which contain much more dependencies and services depend on one another on the code level.

The main premise behind onion architecture is the fundamental concept of pushing your code and having as few dependencies in your code as possible. If the code lasts more than five years, this is a significant accomplishment. The way technology is growing, it becomes increasingly more difficult for software to stay up to date. Platforms that existed ten, fifteen, and twenty years ago are becoming increasingly obsolete.

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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. This post gives a description of the ideas of Onion Architecture and discusses a sample implementation which explicitly
defines layers in the code and build setup. The modular design facilitates the introduction of new technologies or frameworks without affecting the core business logic, enhancing the scalability and future-proofing of the application. Putting business-specific rules in a centralized place is something suggested by both Clean and Onion Architecture. Although they use different names for very similar concepts, they both encourage us to think about business logic in the same way.

The Domain layer does not have any direct dependencies on the outside layers. The outer layers are all allowed to reference the layers that are directly below them in the hierarchy. All of the layers interact with each other strictly through the interfaces defined in the layers below. The flow of dependencies is towards the core of the Onion. We will explain why this is important in the next section. The Onion architecture is a form of layered architecture and we can visualize these layers as concentric circles.

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At the system’s core you will have your business logic, surrounding your core you can add your dependencies. Just like an onion, your levels are separate layers that do not intermingle, they are their own separate layers of coding. Because of the top to down coupling, you can peel layers off from the outside without ever affecting your inner layers of coding. By forcing your coding to couple with only the layer under it, you are able to place key dependencies closer to the core to reduce downtime and increase system stability. The Onion Architecture relies heavily on the Dependency Inversion principle. The modular and decoupled architecture that the Onion Architecture encourages makes it simpler to maintain the application over time.

Each layer has a distinct responsibility, ensuring that business logic remains decoupled from infrastructure or presentation concerns. This separation improves code maintainability and facilitates testing. 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.

Why does Software Architecture matter?

Adapters are the physical manifestations of these ports. They translate the data from the format used by the business logic to the format used by the external actors and vice versa. These adapters can be technology-specific adapter converts for REST APIs, SQL databases, or messaging systems, but they can also be batch scripts or user interface code. The adapters form the boundary of the application, allowing the application to be technology-agnostic. Infrastructure Layer – this is the outermost layer of onion architecture which deals with Infrastructure needs and provides the implementation of your repositories interfaces.

The View is responsible for creating the user interface. At deeper layers, we define abstract interfaces, while at the top layer, we give their concrete implementation. By doing this, we can keep our attention on the domain model and lessen our concern about implementation issues. We may also use dependency injection frameworks like Spring to link interfaces with implementation at runtime. For Example, Infrastructure layer implementations include external services used in Application Services and repositories used in the domain.

Frameworks, Clients and Drivers

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. All three patterns are aligned on this principle; it emphasizes that source code dependencies should only point inward. The outer layer can only refer to the inner layer and not vice versa. onion architecture Hexagonal Architecture is sometimes referred to as ports and adapters architecture. Alistair Cockburn introduced it in 2005, with the core idea behind it being to make applications independent of direct dependency from UI and database. This isolation is supported through the concept of ports and adapters.

  • In my  experience, understanding these rules has helped me to create extendable, testable and comprehensive software systems.
  • The outer layers depend on inner layers and the inner layers are completely unaware of outer circles.
  • Developers can make changes to one layer without impacting the other levels since each layer has a distinct function and communicates with other layers through clearly defined interfaces.
  • DDD implies that you distinguish a certain bounded context, which is a set of entities tightly connected with each other but minimally connected with other entities in your system.

In 3-tier and n-tier architectures, none of the layers are independent; this fact raises a separation of concerns. The drawback of this traditional architecture is unnecessary coupling. Onion architecture is built on a domain model in which layers are connected through interfaces. The idea is to keep external dependencies as far outward as possible where domain entities and business rules form the core part of the architecture. We can use lower layers of the Onion architecture to define contracts or interfaces. The outer layers of the architecture implement these interfaces.

User Interface Layer

Onion architecture comes with very clear rules about priorities, dependence, and layers. Due to their clarity, you will always be able to understand which part relies on which layer. If you decide to implement onion architecture in your application, you’ll get multiple benefits. The business rules will
return, well this is the new state that has to be stored, or now we have
to send an email. That interaction
layer is just going to iterate through the plan and do it. Then a lot of
systems you do the database, the queries and the SQL statements are all
smeared all over the code.

The central layer —  the domain model —  contains all business rules. At the next level are domain services, which are like contracts of repositories and other dependencies. The outermost layer contains the user interface and connectivity to external infrastructure. Onion Architecture sets a clear dependency rule between layers, making it a more restrictive variant of Layered and Hexagonal Architectures. At the very center of our application sits a domain model, surrounded by domain services and application services. Those 3 layers form the application core – no relevant application logic should reside outside of it and it should be independent of all peripheral concerns like UI or databases.

Onion-based Software Architecture

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. The domain model is at the center of Domain-Driven Design or development, which thoroughly understands a domain’s procedures and regulations.