During testing we found that setting read Preference to "secondaryPreferred" always fetched records from primary even if the tag was invalid . But the online documentation suggests an error should be thrown " http://docs.mongodb.org/manual/reference/read-preference/#secondaryPreferred " . This leads me to believe Mongos/Java Driver is not respecting tags or behaving otherwise and always reading from primary . Since our actual deployment is large and geographically we need multiple secondaries