code: def rollback(self):
          
          self._check_state()
          database = self._session._database
          api = database.spanner_api
          metadata = _metadata_with_prefix(database.name)
          api.rollback(self._session.name, self._transaction_id, metadata=metadata)
          self._rolled_back = True
          del self._session._transaction
docs: Roll back a transaction on the database.