(1) if there are any assumptions on compatibility between coordination between say, mongod <-> mongos, then the system must verify the user has compatible versions running of both. otherwise users will mix them and there will be very confusing situations.

(2) likewise with replication

(3) also it would be good if mongos reports if versions are different even if they are compatible. this would help with support. if talking to a mongod, it could log once if the version of the mongos and mongod vary.

note the other node may upgrade while the one is still up, so checking just once per run, if the mongos runs for months, is probably not enough