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

With Onion Architecture, there are no database applications. Decoupling the application from the database, file system, etc, lowers the cost of maintenance for the life of the application. The biggest offender is the coupling of UI and business logic to data access. 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.

Since infrastructure or things that change often are pushed to the outer edges of the onion, the onion architecture relies heavily on the Dependency Inversion Principle. And finally, we saw how our Presentation layer is implemented as a separate project by decoupling the controllers from the main Web application. We started with the Domain layer, where we saw the definitions for our entities and repository interfaces and exceptions. We have connected all of our Onion architecture implementation layers, and our application is now ready for use.

Programming with Palermo

This layer consists of the data access pattern, which is a more loosely coupled approach to data access. 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. Business rules that belong to the domain model, domain services and application services should be tested via Unit Testing.

Next, we looked at the Infrastructure layer, where the implementations of the repository interfaces are placed, as well as the EF database context. 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. Let’s consider the use case to create an order with a list of items.

We can write business logic without concern about any of the implementation details. If we need anything from an external system or service, we can just create an interface for it and consume it. We do not have to worry about how it will be implemented.

Domain Exceptions

Great, we have seen how to implement the Presentation layer. Services.Abstractions project does not reference any other project, we have imposed a very strict set of methods that we can call inside of our controllers. Services.Abstractions project it will only be able to call methods that are exposed by this project.

Order is an entity and has attributes like OrderId, Address, UserInfo, OrderItems, PricingInfo and behaviour like AddOrderItems, GetPricingInfo, ValidateOrder, etc. With https://globalcloudteam.com/, there is only an object model at the lowest level, which does not depend on the type of database. The actual type of database and the way of storing data is determined at the upper infrastructure level. Bounded context is a good fit for a microservices architecture. It is much easier to build a microservice around a bounded context. Bounded context — each microservice is built around some business function and uses bounded context as a design pattern.

• Since a Domain Interface acts on an Entity, it should be considered pure business logic.
• This rule also exists in other similar architectures, such as Clean Architecture.
• Onion Architecture is an architectural pattern which proposes that software should be made in layers, each layer with it’s own concern.
• Bounded context is a good fit for a microservices architecture.
• The main thing is, that in Core you put the interfaces that will be implemented inside DAL and Web Services.
• Application architecture is built on top of a domain model.

What we have is infrastructure depending on abstraction . A center layer never depends on the outside, always the inverse. In other words, all coupling is toward the center, and this is achived using inversion of control. It relies on dependency injection for doing it’s layer’s abstraction, so you can isolate your business rules from your infrastructure code, like repositories and views. Modifying the view layer should not break any domain logic. Modifying the database modeling should not affect the software’s business rules.

Repository Layer

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. Each subsequent layer depends on the layers beneath it, and then every layer normally will depend on some common infrastructure and utility services. The big drawback to this top-down layered architecture is the coupling that it creates. Each layer is coupled to the layers below it, and each layer is often coupled to various infrastructure concerns.

Ports and Adapters, Hexagonal Architecture and Clean Architecture are all different names for effectively the same thing. They represent a way to structure the code that clearly separates the domain of the problem from the underlying technologies that implement the solution. A curated list of awesome articles, videos, and other resources to learn and practice software architecture, patterns, and principles. Now we only have one more layer left to complete our Onion architecture implementation. OnModelCreating method, we are configuring our database context based on the entity configurations from the same assembly.

The Essence of Onion Architecture

The challenge was to create a cloud software solution for a digital signage hardware manufacturer. This layer can reference any project from the inner layer. All the technologies are completed isolated and are easy to swap to another. When working with Scrum, you will probably want to break the development of the software into different tasks, so it can be done by different people.

I’ve included the Solution Explorer view to see the relationship between the logical and physical layers side-by-side. I’ll explain each of the layers by working my way from the inside out. Then, we explained how we can connect all of the layers using an ASP.NET Core Web API.

The higher layers of the Onion will take care of implementing that interface transparently. Using this approach, we can encapsulate all of the rich business logic in the Domain and Service layers without ever having to know any implementation details. In the Service layer, we are going to depend only on the interfaces that are defined by the layer below, which is the Domain layer. Onion architecture consists of several concentric layers interacting with each other towards the core, which is the domain.

Observability-Driven Development (ODD)

This article chronicles the journey of building that application. Inside the UI you would use the DAL or Service implementations through the defined interfaces. The correct implementation would be resolved through the help of the Dependency Resolution component (have a look at the concept of “Inversion Of Control” or “Dependency Injection”). The onion architecture was embraced by the software industry and is a widely used architectural pattern, especially in enterprise software. The services layer is a layer providing additional services needed for an application, example a message service, notification service, encoder service, etc.

CodeGuru covers topics related to Microsoft-related software development, mobile development, database management, and web application programming. Cloud services such as Microsoft Azure and database options including SQL Server and MSSQL are also frequently covered. Most of the traditional architectures raise fundamental issues of tight coupling and separation of concerns. 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.

Working with User Input in C#: Basic to Advanced

The onion architecture is a simple, robust, maintainable and extensible software architecture that fits perfectly in an Agile world. In the late 2000s Jeffrey Palermo, presented an architectural pattern called the Onion Architecture. The main purpose of this architectural pattern was to control coupling in a software system, to improve separation of concerns and to force the externalization of infrastructure. There are more examples, but hopefully, you get the idea. We are hiding all the implementation details in the Infrastructure layer because it is at the top of the Onion architecture, while all of the lower layers depend on the interfaces . The Domain layer does not have any direct dependencies on the outside layers.

External Services

As per traditional architecture, the UI layer interacts to business logic, and business logic talks to the data layer, and all the layers are mixed up and depend heavily on each other. 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. You will see the the Domain Model/Core layer is referenced across multiple layers, and that’s fine, to a certain degree.

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. This is how legacy systems become stale, and eventually they are rewritten. Note that Anemic Domain Models is an anti-pattern when working in OOP languages, because, when executing a business rule, you are expected to change the current state of a domain object. When working in a FP language, because of immutability, you are expected to return a new domain object, instead of modifying the current one.

Create and Configure Azure Network Watcher

In the very center we see the Domain Model, which represents the state and behavior combination that models truth for the organization. Around the Domain Model are other layers with more behavior. The first layer around the Domain Model is typically where we would find interfaces that provide object saving and retrieving behavior, called repository interfaces. The object saving behavior is not in the application core, however, because it typically involves a database. The outer layer is reserved for things that change often. These things should be intentionally isolated from the application core.

We are going to see why this is very useful later on when we get to the Presentation layer. These are just some of the examples of what we could define in the Domain layer. We have to realize that everything is a tradeoff in software engineering. Conceptually, we can consider that the onion structure Infrastructure and Presentation layers are on the same level of the hierarchy. Visualisation — use tools that allow you to visualise the collected data. It consists of algorithms that are essential to its purpose and implement the use cases that are the heart of the application.

So, you should start by modeling your domain layer, instead of the database layer. Instead of each module being responsible of instantiating it’s own dependencies, it has its dependencies injected during it’s initialization. This way, when you want to test it, you can just inject a mock that implements the interface your code is expecting to.