How should I implement my Repository (DDD) in C# to handle multiple calls for the same Aggregate Root

Question

What class in my project should responsible for keeping track of which Aggregate Roots have already been created so as not to create two instances for the same Entity. Should my repository keep a list of all aggregates it has created? If yes, how do i do that? Do I use a singleton repository (doesn't sound right to me)? Would another options be to encapsulate this "caching" somewhere else is some other class? What would this class look like and in what pattern would it fit?

I'm not using an O/R mapper, so if there is a technology out there that handles it, i'd need to know how it does it (as in what pattern it uses) to be able to use it

Thanks!

Answer 1

I believe you are thinking about the Identity Map pattern, as described by Martin Fowler.

In his full description of the pattern (in his book), Fowler discusses implementation concerns for read/write entities (those that participate in transactions) and read-only (reference data, which ideally should be read only once and subsequently cached in memory).

I suggest obtaining his excellent book, but the excerpt describing this pattern is readable on Google Books (look for "fowler identity map").

Basically, an identity map is an object that stores, for example in a hashtable, the entity objects loaded from the database. The map itself is stored in the context of the current session (request), preferably in a Unit of Work (for read/write entities). For read-only entities, the map need not be tied to the session and may be stored in the context of the process (global state).

Answer 2

I consider caching to be something that happens at the Service level, rather than in a repository. Repositories should be "dumb" and just do basic CRUD operations. Services can be smart enough to work with caching as necessary (which is probably more of a business rule than a low-level data access rule).

To put it simply, I don't let any code use Repositories directly - only Services can do that. Then everything else consumes the relevant services as interfaces. That gives you a nice wrapper for putting in biz logic, caching, etc.

Answer 3

I would say, if this "caching" managed anywhere other than the Repository then you are letting concerns leak out.

The repository is your collection of items. No code consuming the repository should have to decide whether to retrieve the object from the repository or from somewhere else.

Singleton sounds like the wrong lifetime; it likely should be per-request. This is easy to manage if you are using an IoC/DI container.

It seems like the fact that you even have consider multiple calls for the same aggregate evidences a architecture/design problem. I would be interested to hear an example of what those first and second calls might be, and why they require the same exact instance of your AR.