Smart HTTP cache clearing

Smart HTTP cache clearin= g refers to the ability to clear cache for Locations/content that = is in relation with the content being currently cleared.

When published, any Content item usually has at least one Location, iden= tified by its URL. Therefore, HTTP cache being bound to URLs, if a Content = item is updated (a new version is published), we want HTTP cache for all it= s Locations to be cleared, so the content itself can be updated everywhere = 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 relation. In= this case, cache for the parent Location and/or the relation need to be cl= eared 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 cleared, the cache purger service s= ends an ezpublish.cache_clear.content event (also identified b= y eZ\Publish\Core\MVC\Symfony\MVCEvents::CACHE_CLEAR_CONTENT c= onstant) and passes an eZ\Publish\Core\MVC\Symfony\Event\ContentCache= ClearEvent event object. This object contains the ContentInfo object= we need to clear the cache for. Every listener for this event can add Loca= tion objects to the cache clear list.

Once the event is dispatched, the purger passes collected Location objec= ts to the purge client, which will effectively send the cache BAN request.



Note


The event is dispatched with a dedicated event dispatcher,=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 item's Locations (ParentLocationsL=
istener)
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 HTTP cache clearing

The mechanism

Default behavior

Implementing = a custom listener

Example