This would preserve the ordering of unverified inserts to mongodb without the need to call GLE after every update, even if a version change occurred resulting in the updates being redirected via the WritebackListener.

Optimizations could also be added so that GLE would only be called once-per-migration-per-mongos (this could also be done for setShardVersion as well), and inserts or updates in the same ns not impacted by the migration could be allowed to continue in the meantime.

conn.waitForShardVersionIfNeeded( update )