Smart HttpCache clearing

EZP >=3D 2015.01

Smart Http cache clearing refers to the ability to clea= r cache for locations/content that can be in relation with the content bein= g currently cleared.

When published, any content item usually has at least one location, figu= red by its URL. Therefore Http cache being bound to URLs, if content = A is updated (new version is published), we want Http cache for all = its locations to be cleared, so the content itself can appear up-to-date ev= erywhere it is supposed to be displayed. Sometimes, clearing cache for the = content's locations is not sufficient. You can for instance have an excerpt= of it displayed in a list from the parent location, or from within a relat= ion. In this case, cache for the parent location and/or the relation need t= o be cleared as well (at least if an ESI is not used).

The mechanism

Smart Http cache clearing is an event based mechanism. = Whenever a content item needs its cache to be cleared, the cache purger ser= vice sends an ezpublish.cache_clear.content event (also identi= fied by eZ\Publish\Core\MVC\Symfony\MVCEvents::CACHE_CLEAR_CONTENT constant) and passes a eZ\Publish\Core\MVC\Symfony\Event\Content= CacheClearEvent event object. This object contains the ContentInfo o= bject we need to clear the cache for. Every listener for this event can add= location objects to the cache clear list.


Once the event dispatched, the purger passes collected location objects =
to the purge client, which will effectively send the cache BAN=
 request.



Note: The event is dispatched with a dedicated event dispa=
tcher,=20
ezpublish.http_cache.event_dispatcher.


Default behavior
By default, following locations will be added to the cache clear list:

All locations assigned to content (AssignedLocationsListener)

Parent location of all content's locations (ParentLocationsListen=
er)
Locations for content's relations, including reverse relations (R=
elatedLocationsListener)

Implementing =
a custom listener
By design, smart Http cache clearing is extensible. One can easily imple=
ment an event listener/subscriber to the ezpublish.cache_clear.conten=
t event and add locations to the cache clear list.
Example
Here's a very simple custom listener example, adding an arbitrary locati=
on to the list.



Important: Cache clear listener services=20
must be tagged as=20
ezpublish.http_cache.event_subscriber or=20
ezpublish.http_cache.event_listener.



=20
namespace Acme\AcmeTestBundle\EventListener;

use eZ\Publish\API\Repository\LocationService;
use eZ\Publish\Core\MVC\Symfony\Event\ContentCacheClearEvent;
use eZ\Publish\Core\MVC\Symfony\MVCEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class ArbitraryLocationsListener implements EventSubscriberInterface
{
    /**
     * @var LocationService
     */
    private $locationService;

    public function __construct( LocationService $locationService )
    {
        $this->locationService =3D $locationService;
    }

    public static function getSubscribedEvents()
    {
        return [MVCEvents::CACHE_CLEAR_CONTENT =3D> ['onContentCacheClea=
r', 100]];
    }

    public function onContentCacheClear( ContentCacheClearEvent $event )
    {
        // $contentInfo is the ContentInfo object for the content being cle=
ared.
        // You can extract information from it (e.g. ContentType from its c=
ontentTypeId), using appropriate Repository services.
        $contentInfo =3D $event->getContentInfo();

        // Adding arbitrary locations to the cache clear list.
        $event->addLocationToClear( $this->locationService->loadLo=
cation( 123 ) );
        $event->addLocationToClear( $this->locationService->loadLo=
cation( 456 ) );
    }
}=20



=20
parameters:
    acme.cache_clear.arbitrary_locations_listener.class: Acme\AcmeTestBundl=
e\EventListener\ArbitraryLocationsListener

services:
    acme.cache_clear.arbitrary_locations_listener:
        class: %acme.cache_clear.arbitrary_locations_listener.class%
        arguments: [@ezpublish.api.service.location]
        tags:
            - { name: ezpublish.http_cache.event_subscriber }

=20

Smart HttpCache clearing

The mechanism

Default behavior

Implementing = a custom listener

Example