This project represents the Service layer of the onion architecture. There are several traditional architectures, like 3-tier architecture and n-tier architecture, all having their own pros and cons. All these traditional architectures have some fundamental issues, such as – tight coupling and separation of concerns. The Model-View-Controller is the most commonly used web application architecture, these days.

  • Then finally, the last layer is this interaction layer.
  • You could go through them to understand the core concepts and to learn how everything works.
  • This results in bad coupling and changing the library or database provider could hamper overall business importance.
  • I’m going to turn it just as an exercise to explain it.
  • Good to know where we all started and how things are shaping in terms of application architecture.

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. Externalizing the database can be quite a change for some people used to thinking about applications as “database applications”. With Onion Architecture, there are no database applications.

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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.

What is onion architecture

If it could just be an imperative, “Fetch this, look at the response. If it’s good enough, send it. Otherwise, add more images.” That, to me, is not even that much code to write that. When you do architecture, you’re taking on…there’s a trade-off. This is how we’re going to do our architecture. That’s how we’re going to structure our work, our code.

Onion Architecture Layers

Around the domain layer are other layers with more behavior. The number of layers in application will vary but domain is always at the center. The first layer around the domain is typically we would place interfaces that provides saving and retrieving behaviors, called repository interfaces. Only the interface is in the application core.

What is onion architecture

And there’s my framework dilemma, which I keep coming back to, mostly because of Uncle Bob’s writings that suggest that Onion and Hexagonal Architectures avoid framework dependencies. Hence, I’d say that to some extent all of us should take from Onion Architecture, but that extent should be decided on a case-by-case basis. Now, these business rules are also pure functions with immutable data.

Layer Vs Tiers

In this article, we will deal with Layered Architecture. 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. This gets even more interesting when there are multiple processes making up a single software system. Now, we create action method, which returns an index view with the data. The code snippet of Index action method in UserController is mentioned below.

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The AuthorController has an action method named AddUser which returns the view to add a user. The code snippet mentioned below is for the same action method for both GET and Post requests. The DbContext must have an instance of DbContextOptions in order to execute. We will use dependency injection, so we pass options via constructor dependency injection.

The primary proposition of this architecture is good coupling. Onion Architecture is widely accepted in the industry. It’s very powerful and closely connected to two other architectural styles—Layered and Hexagonal. 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.

How to Build Microservices Using Onion Architecture: Hands-On Experience

Now, this is convoluting it because the rules aren’t that complicated, but if you did have a lot of rules that you needed to apply, this thing could tell you all of the problems. This is an example of something that, on the surface, if this were the real thing that I was implementing, I probably would put all of that logic right in the outer layer. 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. The business rules decide what gets stored, but they don’t do the storing. This is an example of a pattern that I’m going to have, actually, I have this on my to-do list, an episode about this.

Great, we are done with the Infrastructure layer. Now we only have one more layer left to complete our Onion architecture implementation. Presentation project will be the Presentation layer implementation. 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.

There are two types of coupling – tight coupling and loose coupling. The Service layer holds interfaces with common operations, such as Add, Save, Edit, and Delete. Also, this layer is used to communicate between the UI layer and repository layer. The Service layer also could hold business logic for an entity. In this layer, service interfaces are kept separate from its implementation, keeping loose coupling and separation of concerns in mind.

Can you please make articles on Logging, Validation and Transactional Behavior. I Know u have explaned the validator Behavior in MediatoR pattern But to keep things for tracking Can you please make an artcle on it. For instance, your Commands may use a service bus or Event Sourcing to send changes on for further processing, while your Queries may use compiled SQL views for querying. The two types of requests can/do share a common model and data store.

Create and Configure Azure Network Watcher

These domain entities don’t have any dependencies. On the other hand, Onion Architecture addresses both the separation of concern and tight coupling issues. 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.

By separating the application into layers, the system becomes more testable, maintainable and portable. It helps easy adoption of new frameworks/technologies when old frameworks become obsolete. Similar to other architectural styles like Hexagonal, Layered, Clean Architecture, etc. it provides onion architecture a solution for common problems. It greatly depends on the complexity of the application and the size of the project to divide source code into multiple modules. In a microservice architecture, modularisation may or may not make sense depending upon the complexity and use-case.


As mentioned earlier, the Core Layers will never depend on any other layer. Therefore what we do is that we create interfaces in the Application Layer and these interfaces get implemented in the external layers. This is also known as DIP or Dependency Inversion Principle. Let’s look at one of the most popular Architecture in ASP.NET Core Applications.

Back then Onion architecture was seems just like any other architecture that tries to solve problem around loose coupling and high cohesion. Till this date this architecture design is so relevant that we still follow it in our application design. I hope you’ll find my experience useful for your projects.

E.g. for smaller applications that don’t have a lot of business logic, it might not make sense to have domain services. Regardless of layers, dependencies should always be from outer layers to inner layers. The most prominent classes in the Core are Visitor and VisitorProcessor, members of domain model and application services layers respectively. Different layers of onion architecture have a different set of responsibilities and accordingly, there are different testing strategies. The testing pyramid is a great framework that lays out the different types of tests.

There is no standard process for implementing Onion Architecture. We have a free hand on how we want to implement the layers. You have full liberty to choose whatever level you want. Whenever dependency rules come in the picture, you have the leverage to implement the dependency inversion principle whenever an inner layer class wants to interact with outer layer class.

At the center part of the Onion Architecture, the domain layer exists; this layer represents the business and behavior objects. The idea is to have all of your domain objects at this core. Besides the domain objects, you also could have domain interfaces. These domain entities don’t have any dependencies. Domain objects are also flat as they should be, without any heavy code or dependencies. Most of the traditional architectures raise fundamental issues of tight coupling and separation of concerns.

The Onion Architecture : part 1

These are rules that businesses like banks and accounting firms have that aren’t really accounting, but they’re part of their business. These are things that change a lot, a lot more than accounting, that is. Yes, Infact you should use a Repository pattern always to keep a nice abstraction over the database. This also depends on the project requirement. I just wonder and want to know why all the samples we found with Entity framework?