code: public static String getCluster(Configuration jobConf) {
      String jobtracker = jobConf.get(RESOURCE_MANAGER_KEY);
      if (jobtracker == null) {
        jobtracker = jobConf.get(JOBTRACKER_KEY);
      }
      String cluster = null;
      if (jobtracker != null) {
        // strip any port number
        int portIdx = jobtracker.indexOf(':');
        if (portIdx > -1) {
          jobtracker = jobtracker.substring(0, portIdx);
        }
        // An ExceptionInInitializerError may be thrown to indicate that an exception occurred during
        // evaluation of Cluster class' static initialization
        cluster = Cluster.getIdentifier(jobtracker);
      }
      return cluster;
    }
docs: Returns the cluster that a give job was run on by mapping the jobtracker hostname to an
  identifier.
  @param jobConf
  @return