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

Repositories are - like you say - an abstraction. They originate from Martin Fowler's Object Query Pattern. Both Repositories and DTOs can simplify database persistence by mapping persisted data to equivalent collection of entity objects. However, Repositories are more coarse-grained than DAOs by providing control of an entire Aggregate Root (AG) often hiding a lot of internal state from the client. DAO's on the other hand can be as fine-grained as being dedicated to a single entity object. For both Repositories and DAOs it is common to use Hibernate or other Object/Relational Mapping (ORM) Frameworks instead of writing your own implementation.

Typically, services can reside in a Service Layer and can act both as a functionality facade, anti-corruption layer and coordinator for caching & transaction. They are often a good place to conduct logging. Services coarse-grained and usecase-oriented, e.g. Service.updateCustomerAdress() or Service.sendOrder(). Repositories can be too fine-grained for clients to consume, e.g. Customer.add(…), Order.modify(…).

Repositories and DAOs have the same purpose - to persist data permanently. Services on the other hand should be ignorant of persistence and have no knowledge about your database. They usually work tightly together with domain services, repositories, domain core.