How is consistency sacrificed with HRD? Do you mean that, with HRD, you could potentially see the effects of some write W to entity group A, write some value dependent on that write to entity group B, then immediately look at entity group A and see a pre-W value?
No, the scenario you describe could not occur -- it would involve a consistency violation within entity group A. Once you see the effects of write W to entity group A, all operations on entity group A (across all servers) are guaranteed to see W.
The reduced consistency guarantees in HRD involve indexes, because indexes span entity groups. An example: suppose that you set name=foo in record A1, which is part of entity group A. If you then retrieve record A1, that's an operation on entity group A, so you're guaranteed to see name=foo. But if you perform a global query for all records with name == foo, that's an operation on an index, which is outside the entity group and so is not mediated by the entity group's Paxos log. Therefore, your query might not return A1. The index is "eventually" consistent -- it's guaranteed that, eventually, all indexes will be updated. But AFAIK there's no guaranteed upper bound on "eventually". In practice, it should usually be very quick, but only usually.
