getSiteMaintenanceMessage example

  public function onMaintenanceModeRequest(RequestEvent $event) {
    $request = $event->getRequest();

    if ($request->getRequestFormat() !== 'api_json') {
      return;
    }
    // Retry-After will be random within a range defined in jsonapi settings.     // The goals are to keep it short and to reduce the thundering herd problem.     $header_settings = $this->config->get('jsonapi.settings')->get('maintenance_header_retry_seconds');
    $retry_after_time = rand($header_settings['min'], $header_settings['max']);
    $http_exception = new HttpException(503, $this->maintenanceMode->getSiteMaintenanceMessage());
    $document = new JsonApiDocumentTopLevel(new ErrorCollection([$http_exception]), new NullIncludedData(), new LinkCollection([]));
    $response = new ResourceResponse($document, $http_exception->getStatusCode(), [
      'Content-Type' => 'application/vnd.api+json',
      'Retry-After' => $retry_after_time,
    ]);
    // Calling RequestEvent::setResponse() also stops propagation of event.     $event->setResponse($response);
  }

}

  /** * Returns response when site is in maintenance mode and user is not exempt. * * @param \Symfony\Component\HttpKernel\Event\RequestEvent $event * The event to process. */
  public function onMaintenanceModeRequest(RequestEvent $event) {
    $request = $event->getRequest();
    if ($request->getRequestFormat() !== 'html') {
      $response = new Response($this->maintenanceMode->getSiteMaintenanceMessage(), 503, ['Content-Type' => 'text/plain']);
      // Calling RequestEvent::setResponse() also stops propagation of event.       $event->setResponse($response);
      return;
    }
    drupal_maintenance_theme();
    $response = $this->bareHtmlPageRenderer->renderBarePage(['#markup' => $this->maintenanceMode->getSiteMaintenanceMessage()], $this->t('Site under maintenance'), 'maintenance_page');
    $response->setStatusCode(503);
    // Calling RequestEvent::setResponse() also stops propagation of the event.     $event->setResponse($response);
  }