code: def rollback(self, project_id, transaction):
          
          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,
          )
docs: 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.