It is a very difficult problem. I worked for a short time for a company that was trying to solve consistency, distributed redundancy, and scalability. It succeded consistency and redundancy, but failed because transaction chatter caused it to fail at scaling. I think that within an application or enterprise schemas will be partitioned among types of database by whether the data must always be consistent(RDB's), are mostly readonly and can be updated leisurely (contact-lists, personnel records), and those that can afford to be inconsistent (archives, web chatter).