Onion Architecture: Business Services Interfaces and Implementation

Question

I am learning about the Onion Architecture. I have a confusion about the service layer, because I see some people saying that the core layer should only contain:

Models
Repository interfaces
Services interfaces

But others express that it also should implement the services interfaces. So what layer should implement the services interfaces?

I thought that the infrastructure layer should implement:

Repository Interfaces
Services Interfaces

And inject them to UI layer and Test layer when requested.

Thank you!

According to Jeffrey Palermo: "The overall philosophy of the Onion Architecture is to keep your business logic and model in the middle (Core) of your application and push your dependencies as far outward as possible. The problem domain is then completely independent of the required infrastructure (testing, databases, security, etc.). For example, this means that testing database accesses can be done thoroughly without a real database." — Robert Harvey
That means that if a class WorkOrderService that implements a service interface IWorkOrderService, need a repository IOrderRepository then it would need a class OrderRepository that implements that repository interface and by design that class is outside(Infraestructure layer), so the service class WorkOrderService can not use it. Do you see the problem? In the other way: if you implement the service interface IWorkOrderService outside the core,you can use DI/IoC for getting repository IOrderRepository instances and get the job done. There is my consufion. — reliasr

MaxSC MaxSC · Accepted Answer · 2013-06-06T09:56:50

The core layer should contain:

Models/Entities/POCOs/Whatever_the_name... it's all about domain objects
ALL the interfaces (including repositories and services)
Your core business services implementation (*)

(*) If your business is about handling orders, then the implementation of your IWorkOrderService should be in the core layer. If your WorkOrderService needs to access let's say a ShippingService (which is not your business), then it will only manipulate the IShippingService defined in the core layer and the IShippingService implementation will be somewhere in the infrastructure layer.

If your WorkOrderService needs an OrderRepository it will be done excatly the same way.

Here's a code example:

namespace MyBusiness.Core.Services
{
  internal class WorkOrderService: IWorkOrderService
  {
    public WorkOrderService(IOrderRepository orderRepository, IShippingService shippingService)
    {
      _orderRepository = orderRepository;
      _shippingService = shippingService;
    }

    ...
  }
}

This will be up to the outermost layer of your onion architecture - the Dependency Resolution Layer - to bound all your interfaces with the right service implementation at run time.

For<IWorkOrderService>().Use<Core.Services.WorkOrderService>();
For<IShippingService>().Use<Infrastructure.Services.ShippingService>();
For<IOrderRepository>().Use<Infrastructure.Data.OrderRepository>();

Onion Architecture: Business Services Interfaces and Implementation

2 Answers