Symfony HttpCache

Symfony comes with a built-in reverse proxy written in PHP, known as HttpCache. While it is certainly less efficient than using Varnish or Nginx, it can still provide considerable performance gains over an installation that is not cached at all. It can be useful for running an application on shared hosting for instance (see the Symfony HttpCache documentation).

You can use features of this library with the Symfony HttpCache. The basic concept is to use event listeners on the HttpCache class.

Warning

Symfony HttpCache support is currently limited to following features:

  • Purge
  • Refresh
  • Cache Tags
  • User Context

Extending the correct HttpCache

Instead of extending Symfony\Bundle\FrameworkBundle\HttpCache\HttpCache, your AppCache should extend FOS\HttpCacheBundle\SymfonyCache\EventDispatchingHttpCache:

require_once __DIR__.'/AppKernel.php';

use FOS\HttpCacheBundle\SymfonyCache\EventDispatchingHttpCache;

class AppCache extends EventDispatchingHttpCache
{
}

Tip

If your class already needs to extend a different class, simply copy the event handling code from the EventDispatchingHttpCache into your AppCache class. The drawback is that you need to manually check whether you need to adjust your AppCache each time you update the FOSHttpCache library.

By default, the event dispatching cache kernel registers all event listeners it knows about. You can disable listeners, or customize how they are instantiated.

If you do not need all listeners, or need to register some yourself to customize their behavior, overwrite getOptions and return the right bitmap in fos_default_subscribers. Use the constants provided by the EventDispatchingHttpCache:

public function getOptions()
{
    return array(
        'fos_default_subscribers' => self::SUBSCRIBER_NONE,
    );
}

To register event listeners that you want to instantiate yourself in the cache kernel, overwrite getDefaultSubscribers:

use FOS\HttpCache\SymfonyCache\UserContextSubscriber;

// ...

public function getDefaultSubscribers()
{
    // get enabled listeners with default settings
    $subscribers = parent::getDefaultSubscribers();

    $subscribers[] = new UserContextListener(array(
        'session_name_prefix' => 'eZSESSID',
    ));

    $subscribers[] = new CustomListener();

    return $subscribers;
}

You can also register event listeners from outside the kernel, e.g. in your app.php with the addListener and addSubscriber methods.

Warning

Since Symfony 2.8, the class cache (classes.php) is compiled even in console mode by an optional warmer (ClassCacheCacheWarmer). This can produce conflicting results with the regular web entry points, because the class cache may contain definitions (such as the subscribers above) that are loaded before the class cache itself; leading to redeclaration fatal errors.

There are two workarounds:

  • Disable class cache warming in console mode with e.g. a compiler pass:

    $container->getDefinition('kernel.class_cache.cache_warmer')->clearTag('kernel.cache_warmer');
    
  • Force loading of all classes and interfaced used by the HttpCache in app/console to make the class cache omit those classes. The simplest way to achieve this is to call class_exists resp. interface_exists with each of them.

Event Listeners

Each cache feature has its own event listener. The listeners are provided by the FOSHttpCache library. You can find the documentation for those listeners in the FOSHttpCache Symfony Cache documentation section.