How does the Repository Pattern Differ from a Simp

问题:

I've been confused by what I've been reading during my research on the repository pattern. I'm wondering if folks are (incorrectly?) using that word when they simply mean a data access layer.

Since "repository" is not found in the index of Design Patterns (GoF), I've turned to Patterns of Enterprise Application Architecture (Fowler). Fowler seems pretty clear (page 323) when he states that clients create a criteria object and pass it to the repository to get the results. It looks something like this:

public class Person
{
    public List<Person> Dependents()
    {
        Repository repository = Registry.personRepository();
        Criteria criteria = new Criteria();
        criteria.equal(Person.BENEFACTOR, this);
        return repository.matching(criteria);
    }
}

Is the criteria object what makes the repository a repository? If not, what does? If abstracting the persistence mechanism (and therefore constructing queries) is the goal, in what way does the repository differ from a simpe DAL/ORM call like this:

public class PersonLogic
{
    public List<Person> GetDependents()
    {
        IPersonData personData = DependencyContainer.Resolve<IPersonData>();
        return personData.GetDependents();
    }
}

To me, the difference looks like this:
* With the repository pattern, the client constructs different criteria objects and calls the Matching() method on it.
* With the simple DAL, clients just call different methods based on what they want.

Is there more to it than this? Are programmers mistakenly using the term "repository" when they really mean DAL?

EDIT

David Osborne sent this link to Persistence Patterns. It states:

Basically, the Repository pattern just means putting a façade over your persistence system so that you can shield the rest of your application code from having to know how persistence works.

That's really what a data access layer is. It really appears to me that a repository and a DAL are the same thing, and maybe a "true" repository uses the criteria object.

回答1:

Take a look at the "Using the IQueryable interface" section and beyond at Extending and Enhancing the Orders and Registrations Bounded Context. It provides an insightful and balanced discussion of DAO/Repository implementations.

As subsequently highlighted by Bob Horn, the Persistence Patterns articles summarises that:

Basically, the Repository pattern just means putting a façade over your persistence system so that you can shield the rest of your application code from having to know how persistence works.

回答2:

In general I agree with author's statements, but I'd like to add some details

Difference between Repository and DAL/ORM that first not only abstracts the persistence mechanism, but also provides collection-like interface for accessing domain objects … and isolates domain objects from details of the database access code:

Differences

For external layers, such as Business Logic:

• Helps to avoid leaky abstraction. External layers depend on abstraction of Repository, rather than a specific implementation of DAL/ORM. Thus you could avoid all infrastructure and logical dependencies while working with Repository.
• operates with domain objects, rather then a instances of POJO/POCO/DTO
• CRUD operations applied to collection-like interface provided by Repository, rather then specific DAL/ORM methods. For example .net: working with collection that implements IEnumerable, rather then entity-framework context or nhibernate session

Similarities

Repository contains DAL/ORM underneath and serves same purpose