def list_processed_parameter_groups(self):
        """
        Returns the existing parameter groups.

        :rtype: ~collections.Iterable[str]
        """
        # Server does not do pagination on listings of this resource.
        # Return an iterator anyway for similarity with other API methods
        path = '/archive/{}/parameter-groups'.format(self._instance)
        response = self._client.get_proto(path=path)
        message = archive_pb2.ParameterGroupInfo()
        message.ParseFromString(response.content)
        groups = getattr(message, 'group')
        return iter(groups)