Comment by mike_hearn

If you have an ability to force web-style updates on the client apps (which a tool like Conveyor supports, see my other posts or profile), then all you need is a database that supports many connections. You can then atomically upgrade all the clients by e.g. writing a version number to a db table that's checked as part of each transaction, if the version doesn't match the app proceeds to upgrade itself before continuing (unless it's marked as a soft update, i.e. not worth interrupting the user for). The versioning issues aren't much different to the ones you face once you decide to have multiple web servers and rolling upgrades.