method talks with the map of instances of aggregated objects
            :param composite_key presents tuple, comprising of domain_name and timeperiod

def _get_aggregated_object(self, composite_key):
        """ method talks with the map of instances of aggregated objects
            :param composite_key presents tuple, comprising of domain_name and timeperiod"""
        if composite_key not in self.aggregated_objects:
            self.aggregated_objects[composite_key] = self._init_sink_object(composite_key)
        return self.aggregated_objects[composite_key]