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.

Note

Symfony HttpCache support is currently limited to following features:

  • Purge
  • Refresh
  • Cache Tags
  • User Context

Ban operations are not supported.

Event Dispatching HttpCache

You need to adjust your AppCache to support event handling and register the cache event listeners for the functionality you want to use.

To adjust your cache kernel, follow the instructions in the FOSHttpCache Symfony Cache documentation section.

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.

Optimization for Single Server Installations

Normally, cache invalidation is done with a HTTP request to each cache server. If your application runs on one single server, you can use the kernel dispatcher to have PHP code call the HttpCache in the same PHP process, rather than sending an actual web request. This is more efficient, and you don’t need to configure the server IP address.

For this to work, your kernel needs to implement the HttpCacheProvider interface and know about the cache kernel. The cache is implemented with the decorator pattern and thus the application kernel does not normally know about the cache. FOSHttpCacheBundle provides the HttpCacheAware trait to simplify making your kernel capable of providing the cache.

The recommended way to wire things up is to instantiate the cache kernel in the kernel constructor to guarantee consistent setup over all entry points. Adjust your kernel like this:

// src/AppKernel.php

namespace App;

use FOS\HttpCache\SymfonyCache\HttpCacheAware;
use FOS\HttpCache\SymfonyCache\HttpCacheProvider;
use Symfony\Component\HttpKernel\Kernel;

class AppKernel extends Kernel implements HttpCacheProvider
{
    use HttpCacheAware;
    // ...

    public function __construct(...)
    {
        // ...
        $this->setHttpCache(new AppCache($this));
    }
}

Now you need to adjust your front controller to use that cache instance rather than creating one:

// public/index.php

use App\AppKernel;

// ...

$kernel = new AppKernel($env, $debug);
if ('prod' === $env) {
    $kernel = $kernel->getHttpCache();
}

Warning

If you do not want to instantiate the cache kernel in your kernel constructor, you need to make sure it is always available and consistently configured. Notably, the bin/console must also have access to the kernel to support invalidation on the command line.

Once your bootstrapping is adjusted, set the configuration option fos_http_cache.proxy_client.symfony.use_kernel_dispatcher: true.