Onion Architecture layers are connected through interfaces. Note that inside a region, inputs and outputs flow in any direction. However, once a boundary is reached, green and red arrows only flow in one direction. Inputs go INTO a region, and outputs come out of a region. It’s very easy and tempting to have the repository be an input to the generatePaySlip UseCase, and just grab the data directly.
If onion based architecture is set up properly, it is intended to provide insurance against the evolution of technology that can make Rapid application development products obsolete not long after they are developed. It includes adapters for databases, UI, external services like RESTful, etc.
These are just some of the examples of what we could define in the Domain layer. We can be more or less strict, depending on our needs.
Finally, around that is your interaction layer, which talks with the outside world, including the database, web requests, api endpoints, and the UI. The biggest offender is the coupling of the UI and business logic to the data access. Did I just say that the UI is coupled to the data access?
The repository mediates between the data source layer and the business layers of the application. It queries the data source for the data, maps the data from the data source to a business entity, and persists changes in the business entity to the data source. A repository separates the business logic from the interactions with the underlying data source. One of the advantages of functional programming is that this isolation of business domain from infrastructure occurs naturally. You don’t need to encourage (or nag!) developers to use an onion architecture; it happens automatically as a side effect of the FP approach. Many articles on functional programming, or FP, focus on low-level coding practices and FP-specific patterns . They don’t, however, touch on high-level design and architecture.
Onion Architecture combined with a guerilla style TDD approach is an extremely effective pattern designed to manage change. That is, something changes in the outside world—a customer clicks a button, an email arrives, an alert pops up. This is captured in the form of a business event—for example, “order placed” or “email received.” In an FP architecture, business events like these trigger workflows. As software designers and architects, our next challenge is to decide how to group these workflows or pipelines into logical units.
Instead of building a highly decoupled structure, we often end up with several layers that are depending on each other. This is something really bad in building scalable applications and may pose issues with the growth of the codebase. To keep it clear, in the above diagram we can see that the presentation layer depends on the logics layer, which in turn depends on the data access and so on. Though these architectures all vary somewhat in their details, they are very similar. They all have the same objective, which is the separation of concerns.
In general, the further in you go, the higher level the software becomes. The diagram at the top of this article is an attempt at integrating all these architectures into a single actionable idea. Another of my question is, What about Cross cutting concerns like logging, caching, … and something will use in all layers. It also gives the application the pleasant ability to be able to run without the need for real infrastructure nor delivery mechanisms as they can be swapped by mocks, which is great for testing. // DB interface sets out the operations allowed on our database. We could create an initialization script, connect to the Docker container while it is running the database server, and execute the script. But this is a lot of manual work, and it is error-prone.
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.
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. If you are interested in learning more C# while working with the Onion Architecture, visit the TechRepublic Academy. This layer is the bridge between external infrastructure and the domain layers.
Using this diagram alone, the naïve and simple implementation of this is to create a folder structure matching the high-level layers. In fact, some tutorials online present the following simplistic folder structure as how to implement Clean Architecture. The flow of control in the corner shows how each onion architecture pros and cons object within the system has 3 parts. It has the input port, the output port, and the interactor that converts the two. These parts can be interfaces or objects or properties, depending on the complexity, language, and level of abstraction, but all cross-boundary communication happens in these ports.
To achieve this level of maintainability, there is significant work involved in firstly setting up the structure, and secondly maintaining it along the life of the system. Implementation of features may be slower, because there are multiple layers to get through. That’s why Jeffery Palermo recommends it for Enterprise Systems, and not smaller systems non-complex systems.
The Onion architecture was first introduced by Jeffrey Palermo, to overcome the issues of the traditional N-layered architecture approach. According to some authors, the hexagonal architecture is at the origin of the microservices architecture. Each component is connected to the others through a number of exposed “ports”. Communication through these ports follow a given protocol depending on their purpose. Ports and protocols define an abstract API that can be implemented by any suitable technical means (e.g. method invocation in an object-oriented language, remote procedure calls, or Web services). If we apply the principles of the Onion Architecture to the layered architecture, we need to turn the layer diagram upside down. Bounded context is a good fit for a microservices architecture.
Since no code in the application core depends on outer layers, we can just swap out the UI or the database for the testing purposes. To illustrate the layer architecture, we will analyze a Layer.Factory code sample that is available on github . The Layer.Factory sample is a very simple Domain Driven Design sample application which follows the layered architecture pattern. The idea is that the domain model behind it represents a factory which produces layers . In order to be able to create layers a factory responsible for creating layers must be created first.
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. Elsewhere you had mentioned that adding repositories increases lines of code and yes, it does; however, “number of lines of code” is useless as a quality metric. Coupling between components and between layers is a much better quality heuristic, and onion architecture is all about managing that coupling. Interestingly, per your own admission, “EFCORE is already built on Repository pattern and UOW” and so why have IApplicationDbContext at all if all it does is expose EFCore classes and functions? In the case of Onion Architecture, I see things pretty similarly as in the Hexagonal one.