code: public void registerApp(String appId, String shuffleSecret) {
      // Always put the new secret information to make sure it's the most up to date.
      // Otherwise we have to specifically look at the application attempt in addition
      // to the applicationId since the secrets change between application attempts on yarn.
      shuffleSecretMap.put(appId, shuffleSecret);
      logger.info("Registered shuffle secret for application {}", appId);
    }
docs: Register an application with its secret.
  Executors need to first authenticate themselves with the same secret before
  fetching shuffle files written by other executors in this application.