I am trying to come up with a simple way of automatically handling data and schema migrations across a database that spans client and server treating both as masters. It just seems incredibly difficult to get right. Your idea of handling migration at read time sounds promising.
Yep--and just to reassure you, it's more than an idea on my end... I designed and maintained a critical system a few years ago on a large K/V store that used this method, and it worked without a hitch. We never even needed to think about the fact that the documents within the store were comprised of various schemas from several generation of the application (except when we wrote and tested the next incremental link the upgrade chain).
