Is there a better way then just polling them?
You could use tailable cursors on MongoDB’s OpLog for Publish-Subscribe type of data flow.
See also Blog: Tailing the MongoDB OpLog on Sharded Clusters for more information.
Alternatively, depending on your use case see MongoDB Stitch Push Notifications Services.
Also worth mentioning that in the upcoming MongoDB v3.6 there will be a new event notification API, see Blog: New Driver Features for MongoDB 3.6