How To Build Microservices With Onion Architecture

Such as Spring has grown as DI framework/IoC container . RxJava is designed to account for the reactive programming. The quick essence of that chapter is given in the Mark’s article.

Why do developers use onion architecture

Formatting of the dates to user then remains totally unaffected by the decision made by the technical team working on the API. Instead, it may be driven by aesthetic feelings of the customer as well as by necessity to display dates in a timezone of user choice. You may want to do adding/subtracting on dates, formatting the dates to human-readable form, formatting the dates to API-readable form, parsing the dates.

Adding Swagger To Webapi Project

Presently, every application in a perfect world has its own committed Database. To access the Database, we present a Data Access Layer. This layer normally holds ORMs for ASP.NET to read or write to database. In this article, we will know about Onion architecture and Clean architecture in details. Also, we will see a basic example of .Net Core based application to understand onion and clean architecture. I’m of the opinion that entity classes can at times also serve as DTO’s, provided they do not contain domain-specific logic.

An integration test, for example, would also use the DI container to get instances of interface implementations, without having to reference assemblies containing those classes. The RepositoryModule class resides in a separate DependencyResolution assembly, which references the Infrastructure.Data assembly and binds IProductRepository to ProductResository. The assembly containing the Ninject modules references the Data assembly, so that web client doesn’t have to, keeping the web client ignorant of the actual repository implementation. Once the bindings are set up, Ninject serves up the appropriate instance wherever the interface is used. Mark Seeman in the “Dependency Injection in .NET”, chapter 2, draw layers without something called “infrastructure”, effectively bypassing this piece of the software as well. He only focusing on analysis of data access as a crucial piece of infrastructure.

This is if you want a really good separation of the implementations, as each Project will be generating a DLL. There is no essential difference between n-tier, onion, hexagonal, layered etc architectures. If you were to cut from the outside to the centre of your onion diagram, then bend the edges upwards, you’d have the same diagram as for n-tier, for example.

These use cases orchestrate the flow of data to and from the entities, and direct those entities to use their enterprise wide business rules to achieve the goals of the use case. Consequently, the domain layer doesn’t have any dependencies on NuGet packages because it is used purely for business logic. It’s okay to use some NuGet packages in the core but it should be kept to the strict minimum. These interfaces act directly or indirectly on Domain Entities to perform business logic.

Abstract your DbContext and create an interface for it. Include all your DbSets and include any repository methods you want access to (SaveChangesAsync, FindAsync, etc…). This interface lives in your Domain layer, whereas the implementation of it lives in the Infrastructure layer . Beginning at the center, each layer is translated into one or more languages with lower-level semantics. Using the View interface allows the presenter to remain loosely coupled to any particular UI technology (for example, ASP.NET).

The onion architecture, introduced by Jeffrey Palermo, puts the widely known layered architecture onto its head. Get to know the onion architecture and its merits with simple and practical examples. Combined with code structuring by feature your software is easy to understand, changeable and extendable. For a very long time the standard answer to the question how components and classes should be organized in the software architecture was layers.

Visually, the levels of clean architecture are organized into an unspecified number of rings. The outer levels of the rings are lower level mechanisms and the inner, higher levels contain policies and Entities. This description of persistence and UI being “peer layers” sounds like it’s at odds with what an actual onion architecture should be.

With layered and hexagonal architectures understood, the time has come to talk about a relative – Onion Architecture, which puts a new spin on layers. Martin Fowler, in his article Inversion of Control Containers and the Dependency Injection Pattern, helps to understand how pattern works. At runtime, the IoC container will resolve the classes that implement interfaces and pass them into the SpeakerController constructor.

Why do developers use onion architecture

Also, Application layer is not depending on anything else other than the domain entities. This is a infrastructure concern and should be handled by the application infrastructure. It can be a separate repo that creates external resources or a tool that lives in the application’s repo but runs before the application itself.


You need to have one class – to take advantage of entity framework’s context management and that class has to inherit from ObjectContect – this is dependency. I Suppose those two interfaces will be implemented in AppArch.Infrastructure.Data project. I don’t think one approach is right and the other wrong – it all depends on the complexity of the system you are developing and whether it involves a WCF services layer. With ASP.NET MVC there usually is not a WCF component, while with SL/WPF there usually is. You want to put services in a separate class lib so that they can be loaded into different WCF hosts .

  • We also created two new concepts that were hidden in the old implementation and we’ve boiled them down a simple pattern, the repository pattern.
  • Recall the rise and fall of domain specific languages , which held enormous promise, but were too costly to implement and maintain.
  • I haven’t worked WCF outside of web services, but I can see the need to be flexible.
  • Kubernetes provides container orchestration in multiple hosts and is used for container lifecycle management.
  • It’s responsible for implementing all the IO operations that are required for the software.

In this series of articles, the authors reframe software architecture in terms of decisions that teams make about how their system will handle its quality attribute requirements . In their view, software architecture, reframed in terms of decisions, completes the picture of how the system works by making clear the choices that the team has made, and why. The Onion Architecture does not depend on any specific language or framework. You can implement it in basically any language that supports dependency injection. The Infrastructure Layer should not implement any business logic, as well as any use case flow.

Each layer that you create is built on top of an existing layer that doesn’t rely on any other layer but it’s own or the layers contained below it…hence the Onion Architecture. The UI Layer is your ASP.NET MVC application and references the Services layer which, in turn, uses the Data Layer, which in turn, uses the Business Entities. Again, I hate to cop out, but my answer to this is…it depends. As mentioned by Marc Andreessen, software is eating the worldand web APIs are assisting in that feast. If your website doesn’t have an API, most developers think you are behind the times and do not have a vision for your product to become scalable. Just make sure you know how many layers you are creating and which layer calls which layer.

Such stateless/dependency-less services are mixed up with stateful dependencies and other dependencies, which assumes the application boundary invocation. As soon as there are more than 5-6 dependencies in the service – its constructor become hard to read. So, it is hard to quickly understand the responsibility of the service by analyzing its dependencies. Some authors unfold the infrastructure in onion architecture and provide the more comprehensive and less layered-oriented kind of onion. The term “Onion Architecture” was first coined by Jeffry Palermo back in 2008 in a series of blog posts.

When used appropriately, FP principles can reduce complexity while increasing the testability and maintainability of an application. On the other hand when we talk about DDD we talk about Business and when we take a decision about the architecture we always have the Business model we want to solve on our minds. We could say that a DDD is a “programming abstraction” of the Business Model).

We just need a little flexibility to choose the most appropriate style for different parts of the project. Core Domain is often part of the project that stands for a competitive advantage. If it is going to outperform competitors, simple solutions may not suffice.

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The idea is that your app is then completely abstracted away from persistence concerns. Just wondering where you will put the abstraction and implemenetation of the Unit of Work pattern in your sample project, say, IUnitOfWork.cs and UnitOfWork.cs. Took me time to learn all of the comments, however I really enjoyed the article.

Why do developers use onion architecture

Inside the v1 Folder, add a new empty API Controller named ProductController. Since this is a very basic controller that calls the mediator object, I will not go in deep. However, I have previously written a detailed article on CQRS implementation in ASP.NET Core 3.1 API.

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I am a London-based technical architect who has spent more than twenty five years leading development across start-ups, digital agencies, software houses and corporates. The sample application provides a reference architecture based on these principles. To use it you’ll need to download our good old friend, the Northwind sample database. Instead of getting the best out of the benefits of the layered architecture style, we end up with several layers dependent on the layers below it. For example giving the previous layering structure the presentation layer depends on the application layer and then on the domain layer and finally on the database layer. This means that each layer is coupled to the layers below it and often those layers end up being coupled to various infrastructure concerns.


Additionally, the Onion Architecture relies heavily on the Dependency Inversion principle to provide the interface implementations at runtime. I have hooked up Castle Windsor in the sample project to achieve dependency inversion at run time. For those who are new to Castle Windsor, it is one of the best IoC container in market today. There are several others also which are in market but Castle Windsor is personally my favorite. Application ServicesLayer– the implementation of Interfaces defined in Service Interface layers comes here.

The Need To Follow An Architecture

As you move inwards the software grows more abstract, and encapsulates higher level policies. We do, however, expect that changes to the operation of the application will affect the use-cases and therefore the software in this layer. If the details of a use-case change, then some code onion structure in this layer will certainly be affected. By the same token, data formats used in an outer circle should not be used by an inner circle, especially if those formats are generate by a framework in an outer circle. We don’t want anything in an outer circle to impact the inner circles.

The fundamental rule is that all code can depend on layers more central, but code cannot depend on layers further out from the core. This architecture is undoubtedly biased toward object-oriented programming, and it puts objects before all others. At the very center is the domain model, which represents the business and behavior objects. Around the domain layer are other layers with more behavior.

Python is no different than other languages in that matter. The application service implementation as we can see is in a separate layer and the dependency finally is on the Core Domain. The Green arrows in the diagram represents the dependencies. Many enterprises are obtaining computing power through cloud services platforms via the internet and adopting a cloud-first strategy for most application development.

If all of the workflows can live in the same process, it can be a simple in-memory queue. But if workflows need to be deployed separately and independently, then an external queue, service bus, or Kafka-style event log is the preferred choice. Of course, at some point we will need to do I/O—reading and writing files, accessing a database, and so on. Functional programmers try to keep this kind of nondeterminism at the edges of the pipeline as much as possible. Some languages, like Elm and Haskell, are very strict about this and don’t allow any deviation, while others treat this more as a guideline than a rule.