docs: public final String shardKey(int stage) {
      if (shardStageKeys == null) {
        return null;
      } else {
        shardKey = shardStageKeys.get(stage);
        if (shardKey == null) {
          shardKey = UUID.randomUUID().toString();
          shardStageKeys.put(stage, shardKey);
        }
        return shardKey;
      }
    }