code: static String generateDagId(Dag<JobExecutionPlan> dag) {
      Config jobConfig = dag.getStartNodes().get(0).getValue().getJobSpec().getConfig();
      String flowGroup = jobConfig.getString(ConfigurationKeys.FLOW_GROUP_KEY);
      String flowName = jobConfig.getString(ConfigurationKeys.FLOW_NAME_KEY);
      Long flowExecutionId = jobConfig.getLong(ConfigurationKeys.FLOW_EXECUTION_ID_KEY);
      return Joiner.on("_").join(flowGroup, flowName, flowExecutionId);
    }
docs: Generate a dagId from the given {@link Dag} instance.
  @param dag instance of a {@link Dag}.
  @return a String id associated corresponding to the {@link Dag} instance.