code: private void validateRestServiceConfiguration() {
          boolean isRestEnabled = voldemortConfig.isRestServiceEnabled();
          boolean isRestPortDefined = (identityNode.getRestPort() != -1) ? true : false;
          if(isRestEnabled != isRestPortDefined) {
              if(isRestEnabled) {
                  String errorMessage = "Rest Service is enabled without defining \"rest-port\" in cluster.xml .  "
                                        + this.identityNode.getStateString();
                  logger.error(errorMessage);
                  throw new VoldemortApplicationException(errorMessage);
              } else {
                  logger.warn("\"rest-port\" is defined in cluster.xml but Rest service is not enabled.");
              }
          }
      }
docs: To start Rest Service two parameters need to be set: 1) set
  "enable.rest=true" in server.properties 2) set "<rest-port>" in
  cluster.xml. If rest Service is enabled without setting <rest-port>, the
  system exits with an error log.