[20after4]

It's not something that you can easily abstract away from the application's concerns without a significant performance penalty. For the most basic CRUD operations, an ORM could probably do a decent job. Surely someone has already written an ORM that supports sharding?

Beyond CRUD, I'm not so sure, and any application that's big enough to use sharding probably has some pretty complicated queries. At that point you really don't want to be depending on a library to handle the low level details. You need to think about the performance implications of the access patterns and the trade-offs inherent in different design choices. If a library makes these choices for you then you are unlikely to get optimal results.