code: def _save_states(self, state, serialized_readers_entity):
      
      mr_id = state.key().id_or_name()
      fresh_state = model.MapreduceState.get_by_job_id(mr_id)
      if not self._check_mr_state(fresh_state, mr_id):
        return False
      if fresh_state.active_shards != 0:
        logging.warning(
            "Mapreduce %s already has active shards. Looks like spurious task "
            "execution.", mr_id)
        return None
      config = util.create_datastore_write_config(state.mapreduce_spec)
      db.put([state, serialized_readers_entity], config=config)
      return True
docs: Run transaction to save state.
  
      Args:
        state: a model.MapreduceState entity.
        serialized_readers_entity: a model._HugeTaskPayload entity containing
          json serialized input readers.
  
      Returns:
        False if a fatal error is encountered and this task should be dropped
      immediately. True if transaction is successful. None if a previous
      attempt of this same transaction has already succeeded.