'Register a handler for doing a background update.
The handler should take two arguments:
* A dict of the current progress
* An integer count of the number of items to update in this batch.
The handler should return a deferred integer count of items updated.
The hander is responsible for updating the progress of the update.
Args:
update_name(str): The name of the update that this code handles.
update_handler(function): The function that does the update.'

def register_background_update_handler(self, update_name, update_handler):

    self._background_update_handlers[update_name] = update_handler
