Smart HTTP cache clearing 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, identified 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 its 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 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 cleared, the cache purger service sends an ezpublish.cache_clear.content event (also identified by eZ\Publish\Core\MVC\Symfony\MVCEvents::CACHE_CLEAR_CONTENT constant) and passes an eZ\Publish\Core\MVC\Symfony\Event\ContentCacheClearEvent event object. This object contains the ContentInfo object we need to clear the cache for. Every listener for this event can add Location objects to the cache clear list.

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

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 (ParentLocationsListener)
• Locations for content's relations, including reverse relations (RelatedLocationsListener)

Implementing a custom listener

By design, smart HTTP cache clearing is extensible. One can easily implement an event listener/subscriber to the ezpublish.cache_clear.content event and add Locations to the cache clear list.

Example

Here's a very simple custom listener example, adding an arbitrary Location to the list.