When establishing a change stream cursor on a sharded collection, mongos will try to establish cursors on all shards it knows about. To protect against a shard being added during this process, mongos will first establish cursors on every shard it knows about from its cache of the shard registry, then after all cursors have been established, it will force the shard registry to reload from the config servers and ensure that no new shards have been added. If a new shard is found, it will need to establish a cursor on any new shards, then redo the shard registry reload and recheck for new shards.