async build(outputOperand) {
    try {
      console.log("try to load graph...");
      this.graph_ = await this.context_.loadGraph(this.modelCacheKey_);
      console.log("load graph succeed!");
    } catch (e) {
      console.log("failed to load graph: ", e.message, " try to build graph...");
      this.graph_ = await this.builder_.build({'output': outputOperand});
      await this.context_.saveGraph(this.modelCacheKey_, this.graph_);
    }
  }