Mongodb游标不一致的行为

Question

I have a collection with over a billion documents. When I iterate over them with the cursor and do some processing, I don't get all the docs. However, if I just iterate over the collection and count, it does return the right count.

double c = 0;
while(cursor.hasNext){
      DBObject entity = entities.next();
      //do something
      c+=1
    }
 assert(c==446642690.0)

The assertion fails unless I don't do anything and simply count.

Answer 1

Are you doing updates or deletes during your processing? If that is the case you are running into the issue that a mongo cursor is not isolated from updates/inserts during its lifetime. You can partially avoid that behaviour with the snapshot mode but it's really limited. See the mongo documentation under cursor isolation for more information about this issue. As mongo is designed to work this way I think you need to find a way to deal with the situation.