Domain Driven Design: Domain Services or Method on an Entity?
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There has been much discussion on the DDD list regarding where to put the business logic control, whether in a service or entity. Being more specific, in order to ship an order, the following things should happen:
- Validate that the order can be shipped
- Update quantity
- Set the status to shipped
- Save the order
- Send an email to the customer that the order has been shipped
So, the following Java code snippets were suggested:
Everything in the domain:
public class Order
private IOrderShippedNotificationPolicy notificationPolicy;
private IOrderRepository orderRepository;
public void ship()
throw new InvalidObjectException();
private class OrderShippedNotifyByEmailPolicy implements INotificationPolicy
// The object that gets injected is implemented
// in the infrastructure layer
private IEmailGateway emailGateway;
public void send(Order this)
// Create email here
2. Or have an application service coordinate:
public class OrderService
// orderRepository and orderShippedNotificationPolicy
// are injected dependencies
public void shipOrder(Order order)
order.ship(); // in this case it only validates and updates quantity
There's been a lot of replies also. I highlighted the interesting ones:
"The advantage of the latter scenario is that you're calling _orderRepository. Save in the application layer, which I prefer since it's easier to see the transactional control. The problem with the latter scenario is that it seems it's putting things in the application layer which don't need to be there. The action to
Ship()seems to me an atomic, domain centered action and should therefore sit in the domain. I consider the application layer to be like a thin domain facade as defined by Fowler. That is, it is only there to direct/coordinate domain activities. Like I said,
Ship()seems like it should be considered one activity, and therefore coordination from a service layer shouldn't be necessary."
"The way I look at it - what needs to go into
Ship()is the stuff that _must_ happen before shipping can happen. And shipping can happen without the notification part. You only have a rule that says "send a notification to the customer upon shipping the order". You don't have a rule that says "make sure the customer gets the notification or there are no shipments".
"So, perhaps as a rough, preliminary rule we can say anything which affects the state of the domain should go be placed in the domain. (Of course, the application layer can affect the state of the domain, but only by using domain items to do so maybe think of it as the Law of Demeter among layers.) The email doesn't have any meaning within the domain - it's simply a reflection of the domain."
"Notificitation of an order and the order itself is two separate concepts."
"This could as easily be implemented using AOP."
IMHO, I wouldn't have designed it on the domain layer, because I want transactional control on the application service layer. I think the domain has to deal with its particularities, not with sending email or adding things to a repository, even being decoupled of theirs implementations (the domain has a reference only to interfaces). So I prefer the latter approach. And you? What are yout thoughts about this design? How would you have designed it? Everything on the domain or have an application service coordinating the activities?From http://rnaufal.livejournal.com
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