Question

I have an entity called Order, and an aggregate root OrderManager that updates order state and some other information based on requests from application layer (AppLayer calls OrderManager and OrderManager manages internal state including Orders).

Each order has expiration time, so I want to schedule an action to handle the expiration. I dont know where to put it. I think of two approaches:

1. Define an interface IScheduler in domain model. So OrderManager uses this interface for tasks scheduling.
2. Dont define interface but schedule the expitation handling at the application level. This means that app layer calls some method like OrderManager.HandleExpiration

Personally I prefer the first approach, but maybe anyone has another idea?

Answer 1

First off, the Order entity should be the aggregate root of the Order aggregate. It should encapsulate state changing behavior such that there is no need for a manager class. The application service would then delegate directly to the Order entity.

As far as handling order expiration, a few things have to be considered. Does any of this need to be persistent? In other words, will there be orders that are persisted to the database and not loaded in the application, for which expiration will need to be handled? If so, then you need to implement a workflow which goes outside of application boundaries. One way to do this is to have a continuously running background service which polls the database for orders that expire at the current time. It then sends a command to a handled which handles the order expiration event. This handler would delegate back to your domain, ultimately to the HandleExpiration method on the Order entity. An order expiration is just an event, handled like any other domain event, and the background service is just a part of infrastructure that makes this event possible. This seems to fit best with your approach #2.