class JsonapiMaintenanceModeSubscriber

class JsonapiMaintenanceModeSubscriber implements EventSubscriberInterface {
    
    /**
     * The maintenance mode.
     *
     * @var \Drupal\Core\Site\MaintenanceMode
     */
    protected $maintenanceMode;
    
    /**
     * The configuration factory.
     *
     * @var \Drupal\Core\Config\ConfigFactoryInterface
     */
    protected $config;
    
    /**
     * Constructs a new JsonapiMaintenanceModeSubscriber.
     *
     * @param \Drupal\Core\Site\MaintenanceModeInterface $maintenance_mode
     *   The maintenance mode.
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The config factory.
     */
    public function __construct(MaintenanceModeInterface $maintenance_mode, ConfigFactoryInterface $config_factory) {
        $this->maintenanceMode = $maintenance_mode;
        $this->config = $config_factory;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        $events = [];
        $events[MaintenanceModeEvents::MAINTENANCE_MODE_REQUEST][] = [
            'onMaintenanceModeRequest',
            -800,
        ];
        return $events;
    }
    
    /**
     * 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() !== '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);
    }

}