[bri3d]

MongoDB's relatively simple approach involves continuing to use the old shard as an authoritative source (and committing updates to it) while shipping data in the background, then pushing the additional changes across and marking the new shard as "master."

Such an approach wouldn't be horribly difficult to implement in SQL using a copy table and write triggers - almost identically to how SoundCloud's Large Hadron Migrator allows writes to occur over a MySQL InnoDB table that's locked for migration (but even simpler because the table schema can't conflict afterwords).

The entire problem is admittedly nontrivial, since if the application happens to be writing data to the shard under migration too quickly (or the shard being migrated to dies), the server can end up in a situation where the new shard is never able to catch up and become a master. However, the easy solution (give up and retry later) is Good Enough for most situations (and is pretty much how MongoDB works).