Sanne Grinovero, I basically agree with all you said above, except:

The sharding extension point is a user-provided implementation; I am sure people would be able to use JdbcServices as well, but I think they expect to be able to use a Session since we're using Hibernate and the query would target a mapped entity.

I think for the particular use case of index sharding we can be a bit smarter than exposing the session to the sharding strategy implementor. We should be able to do it for him provided we get a bit configuration information.

Either way, I still see this work as a prerequisite, so I am going to complete it and we can take it from there.