private void start() {
      restRequest.setArg(RestUtils.InternalKeys.KEEP_ALIVE_ON_ERROR_HINT, true);
      securityService.processRequest(restRequest, securityProcessRequestCallback());
    }
Start the chain by calling {@link SecurityService#processRequest}.
ambry-frontend/src/main/java/com/github/ambry/frontend/HeadBlobHandler.java