Symfony phased migration strategy listener

NoRoutingListener.php

<?php

declare(strict_types=1);

namespace App\Listener;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\Event\ExceptionEvent;
use Symfony\Component\HttpKernel\Event\ResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\Routing\Exception\MethodNotAllowedException;
use Symfony\Component\Routing\Exception\ResourceNotFoundException;

/**
 * This listener is a key component in a phased migration strategy using an AB-proxy.
 *
 * When migrating from one (legacy) application to another (newer) application
 * (placing this listener in the newer application), the proxy will initially
 * direct traffic to this new application. If this application responds with 404
 * and the header `X-Not-Found-Reason: routing` (different to the route being
 * implemented and the controller returning 404 without the header), the proxy
 * will assume the request is asking for a page that has not been implemented
 * yet in the new application and will forward the request to the legacy application.
 */
final class NoRoutingListener implements EventSubscriberInterface
{
    public const array ROUTING_EXCEPTIONS = [ResourceNotFoundException::class, MethodNotAllowedException::class];
    public const array ROUTING_STATUS_CODES = [Response::HTTP_NOT_FOUND, Response::HTTP_METHOD_NOT_ALLOWED];
    public const string HEADER_NAME = 'X-Not-Found-Reason';
    public const string EXCEPTION_REASON_ROUTING = 'routing';
    public const string EXCEPTION_REASON_APPLICATION = 'application';

    private bool $isRoutingException = false;

    public static function getSubscribedEvents(): array
    {
        return [
            KernelEvents::EXCEPTION => ['onKernelException', 1024],
            KernelEvents::RESPONSE => ['onKernelResponse', -1024],
        ];
    }

    public function onKernelException(ExceptionEvent $event): void
    {
        $this->isRoutingException = $this->isRoutingException($event->getThrowable());
    }

    public function onKernelResponse(ResponseEvent $event): void
    {
        $response = $event->getResponse();
        if (in_array($response->getStatusCode(), self::ROUTING_STATUS_CODES, true)) {
            $reason = $this->isRoutingException ? self::EXCEPTION_REASON_ROUTING : self::EXCEPTION_REASON_APPLICATION;
            $response->headers->set(self::HEADER_NAME, $reason);
        }
    }

    private function isRoutingException(\Throwable $exception): bool
    {
        do {
            if (in_array($exception::class, self::ROUTING_EXCEPTIONS, true)) {
                return true;
            }
        } while (null !== $exception = $exception->getPrevious());
        return false;
    }
}