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