How to apply the Onion Architecture

If you are interested in learning more C# while working with the Onion Architecture, visit the TechRepublic Academy. Onion architecture is also applicable to microservices when viewing each microservice in isolation. Each microservice has its own model, its own use cases and defines its own external interfaces for retrieving or modifying the data. These interfaces can be implemented with an adapter that connects to another microservice by exposing HTTP Rest, GRPC, Thrift Endpoints, etc.

onion architecture life learning

In clean architecture these two match up, while in the standard three-tier architecture they don’t; and this can quickly lead to all kinds of logical inconsistencies and mess if you aren’t careful. You can see in the diagram that all dependencies in the standard three-tier architecture go to the database. That means that abstraction and dependency don’t match. Logically, the business layer should be the center of the app, but it isn’t, as dependencies go towards the database.

It has access all the inner layers but most operations should go through the API, one exception being domain interfaces with infrastructure implementations. Externalizing the database can be quite a change for some people used to thinking about applications as “database applications”. There are applications that might use a database as a storage service but only though some external infrastructure code that implements an interface which makes sense to the application core.

Adding The Entities to the Domain Project

Domain and Application Layer will be at the center of the design. We can refer to these layers as the Core Layers. These layers will not depend on any other layers.

The main thing being that side effects still aren’t happening in the core, so the core is still referentially transparent. I think the grandparent comment boils down to saying “there’s no known persistent data structure with O random access for read and write”. Whether you need such a structure (for a cache, histogram, frame buffer, etc.) is up to you.

onion architecture life learning

I must set the context for the use of this architecture before proceeding. This architecture is not appropriate for small websites. It is appropriate for long-lived business applications as well as applications with complex behavior.

Application Layer Rules

It basically has the models/entities, Exception, validation rules, Settings, and anything that is quite common throughout the solution. To keep things simple but demonstrate the architecture to the fullest, we will build an ASP.NET Core Web API that is quite scalable. For this article, Let’s have a WebApi that has just one entity, Product. We will perform CRUD Operations on it while using the Onion architecture.

  • I don’t know what Haxl itself does, but Fetch is kind of unsatisfactory in that regard.
  • I think the main reason for not having behavior in the domain entities is the desire to use them as DTO’s rather than true domain models.
  • Sure, let me know when u publish them and i will link to them on my posts.
  • This is if you think the infrastructure implementation is not very complex.
  • The outer circles represent mechanisms and the inner circles represent core domain logic.

This layer usually holds ORMs for ASP.NET to fetch/write to the database. The Onion Architecture relies heavily on the Dependency Inversion principle. Onion Architecture is based on the inversion of control principle. Onion Architecture is comprised of multiple concentric layers interfacing each other towards the core that represents the domain. The architecture does not depend on the data layer as in classic multi-tier architectures, but on the actual domain models.

Few Fundamental Principles of Software Development

It asks the business-rule layer, “What should I do with this? Is this good enough? Should I return it?” There’s a little logic in the interaction layer, but it’s very much coordination. Then in your business logic, you have a function called valid album response. It could be true, false or it could be something like true, then return or a list of problems.

onion architecture life learning

The infrastructure layer is a bit more tricky. It is where you would want to add your Infrastructure. Maybe an Entity Framework Core Layer for Accessing the DB, a Layer specifically made to generate JWT Tokens for Authentication or even a Hangfire Layer. You will understand onion architecture more when we start Implementing Onion Architecture in ASP.NET Core WebApi Project. Application architecture is built on top of a domain model. Low coupling in which one module interacts with another module and does not need to be concerned with the other module’s internals.

Peeling Back the Onion Architecture

This would not necessarily happen if the idea that style matters enough that when we can’t change language, we could, and would still change how we express the solution within our constraintd. Be it in any language or paradigm under the sun, we need the words from them all to be able to talk about our problems, as they are either unique or already solved. The first example is using dependency injection and correctly hides the implementation details of `getAccount` while not being referentially transparent. The parts of the software that are more subtle to change are or that we have less control about are in the most outer layers, while in the inner layers we have the most meaningful parts of our application.

onion architecture life learning

This will be an Empty API Controller which will have API Versioning enabled in the Attribute and also a MediatR object. We will not have to re-define the API Versioning route or the Mediator object. But we will just add the BaseAPI Controller as the base class.

Choose your language

It depends on the use cases and the complexity of the application. It is also possible to create more layers of abstractions depending on application needs. 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. Different layers of onion architecture have a different set of responsibilities and accordingly, there are different testing strategies.

Application services

Then the overhead of creating the session only takes place on each request rather than multiple times per request. Per request makes sense when you want to isolate transaction scope at the request level. But for reading data you could have a single session. And for static data, check out level 2 cache solutions for better performance. “If I have seen so far it is only because I have stood on the shoulders of giants” Isaac Newton.

It holds all the logic related to the Business requirements. Now, every application ideally has its own dedicated Database. In order to access the Database, we introduce a Data Access Layer.

Using different projects for different layers is only ever coincidentally a good idea. You’d have to have a very large project before doing that made any sense, in which case moving the application into a more service-based architecture might well be a better choice. The diagram to the left depicts the Onion Architecture. This architecture is unashamedly biased toward object-oriented programming, and it puts objects before all others. 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.

Domain Entities are the fundamental building block of Domain-Driven Design and they’re used to model concepts of your Ubiquitous Language in code. Entities are Domain concepts that have a unique identity in the problem domain. Domain entities encapsulate attributes and entity behaviour. It is supposed to be independent of specific technologies like databases or web APIs. Order is an entity and has attributes like OrderId, Address, UserInfo, OrderItems, PricingInfo and behaviour like AddOrderItems, GetPricingInfo, ValidateOrder, etc.

Each layer is coupled to the layers below it, and each layer is often coupled to various infrastructure concerns. However, without coupling, our systems wouldn’t do anything useful, but this architecture creates unnecessary coupling. Infrastructure Layer – this is the outermost layer of onion architecture which deals with Infrastructure needs and provides the implementation of your repositories interfaces. In other words, this is where we hook up the Data access logic or logging logic or service calls logic. Only the infrastructure layer knows about the database and data access technology (Entity framework or and other layers don’t know anything about from where the data comes and how it is being stored. Organising our application in layers helps in achieving separation of concerns.

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