What is the difference between Data Access Objects (DAO) and Repository patterns? I am developing an application using Enterprise Java Beans (EJB3), Hibernate ORM as infrastructure, and Domain-Driven Design (DDD) and Test-Driven Development (TDD) as design techniques.
DAO is an abstraction of data persistence.
Repository is an abstraction of a collection of objects.
DAO would be considered closer to the database, often table-centric.
Repository would be considered closer to the Domain, dealing only in Aggregate Roots.
Repository could be implemented using DAO's, but you wouldn't do the opposite.
Also, a Repository is generally a narrower interface. It should be simply a collection of objects, with a Get(id), Find(ISpecification), Add(Entity).
A method like Update is appropriate on a DAO, but not a Repository - when using a Repository, changes to entities would usually be tracked by separate UnitOfWork.
It does seem common to see implementations called a Repository that is really more of a DAO, and hence I think there is some confusion about the difference between them.