Is It Okay for an Application Layer with a multi-step process to not use the domain layer

Question

I am working on an N-Layered .NET application with the following layers:

• Presentation
• Application
• Domain
• Infrastructure (Contains Persistence and common utility functions like email)

At some point in my application, a request is approved. Once approved, there are 4 steps which must be performed as follows:

1. Assign a product tag code to the issued product in the Request.
2. Update the Request status from “Being Processed” to “Order Completed”
3. Send an email to the requester stating their product is ready for pickup
4. Send an email to the requester’s manager informing them of what their employee has been given and a copy of the request they approved for the product

The above steps must be part of an atomic operation meaning they must all be done or the action is cancelled. I am thinking of having the Application Layer directing the Request Repository and Persistence Layer to carry out the tasks as follows:

1. Application Layer requests for the Request Repository to perform steps 1 and 2 as a unit of work transaction
2. Application Layer requests for the Infrastructure layer to carry out steps 3 and 4.

I'm not exactly sure if the Application Layer is supposed to do this. I think it is because the first 2 actions require contacting a repository which I have avoided doing in the Domain Layer. The last 2 steps involve the Infrastructure Layer which the domain layer may talk to via a dependency injected instance. Following this logic, the Application Layer is not asking the Domain Layer to do anything. Is this typically how it is for a multi-step process like this when you have an Application Layer? Or did I miss something here?

I am aware of a framework called Windows Workflow but I'm not sure if that would help in this case as this multi-step process does not involve human interaction along the processing steps where things could be waiting for a few days. I also don't want to make the application overly complex if I don't have to.

Thanks in advance.

Answer 1

Steps 1 and 2 in your question sound like domain logic to me. So no, it is not okay. These Two steps should be carried out in the domain. You can do the following:

1. In the application service, load the relevant aggregate (probably the Request).

2. Either invoke the operation that does step 1 and 2 on the Request, or (if the operation does not really belong to the Request) invoke a domain service that performs these two steps.

3. Once the domain operation is completed, the application service can save the modified Request back to the DB.

4. If the above steps are successful, you can send now the emails. You should have some kind of retry mechanism in place, so that the email gets sent eventually, even if there is a temporary problem.

If the changes in step two modify more than one aggregate, the process becomes slightly more complex. Sagas as mentioned in the comments are one solution to this problem. Still, the individual changes are domain operations, so model them accordingly.

As suggested by plalx in the comment, the sending of the emails can be offloaded to a domain event consumer if you have an eventing infrastructure in place. This usually solves the retry mechanism for free.