DAO, Repositories and Services in DDD

Question

After reading several articles, I am starting to understand the difference between DAO and Repositories, but I find myself in trouble trying to understand the difference between

Answer 1

I'm not sure what "DAO" even is. Repositories are an abstraction for loading entities. You should be able to Get an entity and Save one, that is it. No querying. If you want to query some data, write a query (maybe even within an MVC action method, or with the simplest of simple abstractions allowing some SQL to be executed and some DTOs returned that can be rendered straight into the HTML).

Services on the other hand are tricky. For a start the term is overloaded. "Application Services" as defined by the DDD book by Eric Evans exist because objects in the Domain Model are not allowed to access infrastructure concerns like databases, messaging, caching etc. They need all of that stuff done for them and handed to them on a plate, and Application Services do just that. Application Services, for their part do not contain any logic. I would not expect to see ICustomerService.ChangeAddress() do anything other than:

1. Load the Customer entity.
2. Call Customer.ChangeAddress(newAddress) <- this encapsulates the domain logic
3. Save the customer.
4. Perhaps publish some events.

If you have a service that is loading a customer, setting it's Address property and saving it, then that service is actually a Transaction Script and the Customer is a DTO. Meaning you definitely have a leaky abstraction and likely have an anaemic domain model. Domain model objects should not have public setters, and when DDD is combined with CQRS, your domain model may not even have any public state at all beyond the main entity ID values.