I researched about repositories in DDD and found too much different thing. Everyone says different things about repositories and that made me confused.
I want to know:
Thanks.
For each aggregate root (AR) you should have a repository. As a minimum the repository would probably have a void Save(Aggregate aggregate) and a Aggregate Get(Guid id) method. The returned aggregate would always be fully constituted.
I sometimes add methods for specific use cases in order to update only certain bits of data. For instance, something like void Activate(Guid id) or some such. This is simply to avoid manipulating more data than is necessary.
Querying on a repository is usually problematic since you should typically avoid querying your domain. For such scenarios my recommendation is to use a query mechanism that is closer to the data and in a more raw format than a domain object or object graph. The query mechanism would more-then-likely return primitives, such as int Count(Query.Specification specification) or perhaps return a list of read model instances.
Eric Evans coined "domain driven design" in 2003; so the right starting point for any definitions in that context is his book. He defines the repository pattern in chapter 6 ("Lifecycle of a Domain Object").
A REPOSITORY represents all objects of a type as a conceptual set (usually emulated). It acts like a collection, except with more elaborate querying ability. Objects of the appropriate type are added and removed, and the machinery behind the repository inserts them or deletes them from the database.
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For each type of object that requires global access, create an object that can provide the illusion of an in-memory collection of all objects of that type.
The primary use case of a repository: given a key, return the correct root entity. The repository implementation acts as a module, which hides your choice of persistence strategy (see: Parnas 1971).
You are right, a repository has different meanings in different contexts - and many authors have their own interpretation. The way I understand them is from multiple perspectives:
So to answer your questions, repositories should contain collection related methods like add, remove, addAll, findByCriteria - instead of save, update, delete. They can return whole aggregate or parts of aggregates or some internal aggregate relation - it is dependent on your domain model and the way you want to represent objects
