code: public static void overrideConfiguration(JobConf conf, int instance) {
      final String CONFIG_KEYS[] =
          new String[]{"mapred.job.tracker", "mapred.local.dir",
                       "mapred.fairscheduler.server.address"};
      for (String configKey : CONFIG_KEYS) {
        String value = conf.get(configKey + "-" + instance);
        if (value != null) {
          conf.set(configKey, value);
        } else {
          LOG.warn("Configuration " + configKey + "-" + instance + " not found.");
        }
      }
    }
docs: Replce the jobtracker configuration with the configuration of 0 or 1
  instance. This allows switching two sets of configurations in the
  command line option.
  @param conf The jobConf to be overwritten
  @param instance 0 or 1 instance of the jobtracker