Maintenance Mode for Symfony

config.yml

# saves sessions by (default) in /app/sessions/[environment], e.g. /app/sessions/dev or /app/sessions/prod
# be sure that the write permissions are set correctly for the folder. Should be the same as /cache/ or /logs/
framework:
    session:
        save_path: %kernel.root_dir%/sessions/%kernel.environment% 

MaintenanceListener.php

<?php
/**
 * Based on http://www.wenigersh.com/blog/post/maintenance-mode-for-symfony-2-applications
 * 
 * Includes support of user permisson check (here: check if user has role ADMIN), so admins can still surf the site while in maintenance mode
 * Be aware that if you clear the cache (to start maintenance in prod environment) your sessions might be cleared as well, so people get logged out
 * The config.yml will move the sessions to /app/sessions/[environment]
 * 
 * Also be aware that you need at least Symfony 2.6 for this permission check. For older versions the call is different
 * @see http://symfony.com/blog/new-in-symfony-2-6-security-component-improvements
 * 
 * HTTP Response is a 503 Service Unavailable
 * 
 * Don't forget to create the template that is called if you're in maintenance mode. See Comment in this file below
 */
namespace Acme\DemoBundle\Listener;

use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\DependencyInjection\ContainerInterface;

class MaintenanceListener
{
    private $container;

    public function __construct(ContainerInterface $container)
    {
        $this->container = $container;
    }

    public function onKernelRequest(GetResponseEvent $event)
    {
        // get maintenance parameters
        $underMaintenanceUntil = $this->container->hasParameter('underMaintenanceUntil') ? $this->container->getParameter('underMaintenanceUntil') : false;
        $maintenance = $this->container->hasParameter('maintenance') ? $this->container->getParameter('maintenance') : null;

        $debug = in_array($this->container->get('kernel')->getEnvironment(), array('test'));

        if ($maintenance && !$debug && !$this->container->get('security.authorization_checker')->isGranted('ROLE_ADMIN')) {
            $engine = $this->container->get('templating');
            // be sure to create the template and link to it. Location doesn't matter (for me), /app/Resources/views/ might be your best bet
            $content = $engine->render('::maintenance.html.twig', array('underMaintenanceUntil' => $underMaintenanceUntil));
            $event->setResponse(new Response($content, 503));
            $event->stopPropagation();
        }

    }
}

parameters.yml

parameters:
    maintenance: false  #turn it to true to enable maintenance, false if disable
    underMaintenanceUntil: tommorow 8 AM # message you can display on your view, will be null if not set

services.yml

services:
    acme.listener.maintenance:
        class: Acme\DemoBundle\Listener\MaintenanceListener
        arguments:
            container: "@service_container"
        tags:
            - { name: kernel.event_listener, event: kernel.request, method: onKernelRequest }

use Symfony\Component\HttpKernel\Event\GetResponseEvent;
is not found. Have an explanation ?

use Symfony\Component\HttpKernel\Event\GetResponseEvent;
is not found. Have an explanation ?

Use RequestEvent instead. Here is the list of kernel event and its argument. https://symfony.com/doc/current/components/http_kernel.html#component-http-kernel-event-table