def phase(self, session):
        """Determine the session phase in prepare commit.

        :param session: sqlalchemy session
        :return: phase "prepare" or "commit"
        """
        sp_key, _ = self._keygen(session)
        if self.r.sismember(sp_key, session.meepo_unique_id):
            return "prepare"
        else:
            return "commit"