code: def _get_mapreduce_spec(cls, mr_id):
      
      key = 'GAE-MR-spec: %s' % mr_id
      spec_json = memcache.get(key)
      if spec_json:
        return cls.from_json(spec_json)
      state = MapreduceState.get_by_job_id(mr_id)
      spec = state.mapreduce_spec
      spec_json = spec.to_json()
      memcache.set(key, spec_json)
      return spec
docs: Get Mapreduce spec from mr id.