Perform a ``rollback`` request.

        :type project_id: str
        :param project_id: The project to connect to. This is
                           usually your project name in the cloud console.

        :type transaction: bytes
        :param transaction: The transaction ID to rollback.

        :rtype: :class:`.datastore_pb2.RollbackResponse`
        :returns: The returned protobuf response object.

def rollback(self, project_id, transaction):
        """Perform a ``rollback`` request.

        :type project_id: str
        :param project_id: The project to connect to. This is
                           usually your project name in the cloud console.

        :type transaction: bytes
        :param transaction: The transaction ID to rollback.

        :rtype: :class:`.datastore_pb2.RollbackResponse`
        :returns: The returned protobuf response object.
        """
        request_pb = _datastore_pb2.RollbackRequest(
            project_id=project_id, transaction=transaction
        )
        # Response is empty (i.e. no fields) but we return it anyway.
        return _rpc(
            self.client._http,
            project_id,
            "rollback",
            self.client._base_url,
            request_pb,
            _datastore_pb2.RollbackResponse,
        )