def _get_mapreduce_spec(cls, mr_id):
    """Get Mapreduce spec from 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