Currently there is no way to pause the firehose of documents returned by asynchronous iteration of a cursor.

One possibility is to extend MongoFuture with MongoCursorFuture, and add appropriate methods there to implement back pressure:

public interface MongoCursorFuture extends MongoFuture<Void> {
   void requestMore(int howMany);
}

*Update*
Should follow the http://www.reactive-streams.org/ spec once 0.4 is out of milestone