Pantheon Advanced Page Cache

For sites wanting fine-grained control over how their responses are represented in their edge cache, Pantheon Advanced Page Cache is the golden ticket. Here’s a high-level overview of how the plugin works:

1. When a response is generated, the plugin uses surrogate keys based on WordPress’ main WP_Query object to “tag” the response with identifers for the data used in the response. See the “Adding Custom Keys” section for including your own surrogate keys.
2. When WordPress data is modified, the plugin triggers a purge request for the data’s corresponding surrogate keys.

Because of its surrogate key technology, Pantheon Advanced Page Cache empowers WordPress sites with a significantly more accurate cache purge mechanism, and generally higher cache hit rate. It even works with the WordPress REST API.

Go forth and make awesome! And, once you’ve built something great, send us feature requests (or bug reports).

How It Works

Pantheon Advanced Page Cache makes heavy use of surrogate keys, which enable responses to be “tagged” with identifiers that can then later be used in purge requests. For instance, a home page response might include the Surrogate-Key header with these keys:

Surrogate-Key: front home post-43 user-4 post-41 post-9 post-7 post-1 user-1

Similarly, a GET requests to /wp-json/wp/v2/posts might include the Surrogate-Key header with these keys:

Surrogate-Key: rest-post-collection rest-post-43 rest-post-43 rest-post-9 rest-post-7 rest-post-1

Because cached responses include metadata describing the data therein, surrogate keys enable more flexible purging behavior like:

• When a post is updated, clear the cache for the post’s URL, the homepage, any index view the post appears on, and any REST API endpoints the post is present in.
• When an author changes their name, clear the cache for the author’s archive and any post they’ve authored.

There is a limit to the number of surrogate keys in a response, so we’ve optimized them based on a user’s expectation of a normal WordPress site. See the “Emitted Keys” section for full details on which keys are included, and the “Adding Custom Keys” section following for information on how to add your own.

Adding Custom Keys

By default, Pantheon Advanced Page Cache generates surrogate keys based on an interpretation of the main WP_Query query object. Because WordPress sends headers before the page is rendered, you need to use the pantheon_wp_main_query_surrogate_keys filter to include additional surrogate keys for any data present on the page.

For example, to include surrogate keys for a sidebar rendered on the homepage, you can filter the keys using the is_home() template tag:

/**
 * Add surrogate key for the featured content sidebar rendered on the homepage.
 */
add_filter( 'pantheon_wp_main_query_surrogate_keys', function( $keys ){
    if ( is_home() ) {
        $keys[] = 'sidebar-home-featured';
    }
    return $keys;
});

Then, when sidebars are updated, you can use the pantheon_wp_clear_edge_keys() helper function to emit a purge event specific to the surrogate key:

/**
 * Trigger a purge event for the featured content sidebar when widgets are updated.
 */
add_action( 'update_option_sidebars_widgets', function() {
    pantheon_wp_clear_edge_keys( array( 'sidebar-home-featured' ) );
});

Similarly, to include surrogate keys for posts queried on the homepage, you can pre-fetch the posts before the page is rendered:

/**
 * An example of pre-fetching a WP_Query to tag the
 * response with queried data. You'd use `papcx_wp_query()`
 * a second time within your template to use the data.
 */
add_filter( 'pantheon_wp_main_query_surrogate_keys', function( $keys ) {
    if ( is_home() ) {
        $query = papcx_wp_query( array(
            'post_type' => 'page',
        ) );
        foreach( $query->posts as $post ) {
            $keys[] = 'post-' . $post->ID;
        }
    }
    return $keys;
});

/**
 * Register a 'papc-non-persistent' cache group to cache data
 * in a non-persistent manner. We only want data in this group
 * to be cached within the page request.
 */
add_action( 'init', function(){
    wp_cache_add_non_persistent_groups( array( 'papc-non-persistent' ) );
});

/**
 * Helper function to instantiate a WP_Query object only
 * once per page request.
 *
 * @param array $args Arguments to pass to WP_Query.
 * @return WP_Query
 */
function papcx_wp_query( $args = array() ) {
    $cache_key = md5( serialize( $args ) );
    // WP_Query object will be in cache the second time we use the function.
    $cache_value = wp_cache_get( $cache_key, 'papc-non-persistent' );
    if ( false !== $cache_value ) {
        return $cache_value;
    }
    $query = new WP_Query( $args );
    wp_cache_set( $cache_key, $query, 'papc-non-persistent' );
    return $query;
}

Because Pantheon Advanced Page Cache already handles WordPress post purge events, there’s no additional call to pantheon_wp_clear_edge_keys().

Lastly, the pantheon_wp_rest_api_surrogate_keys filter lets you filter surrogate keys present in a REST API response.

Need a bit more power? In addition to pantheon_wp_clear_edge_keys(), there are two additional helper functions you can use:

• pantheon_wp_clear_edge_paths( $paths = array() ) – Purge cache for one or more paths.
• pantheon_wp_clear_edge_all() – Warning! With great power comes great responsibility. Purge the entire cache, but do so wisely.

Ignoring Specific Post Types

By default, Pantheon Advanced Page Cache is pretty aggressive in how it clears its surrogate keys. Specifically, any time wp_insert_post is called (which can include any time a post of any type is added or updated, even private post types), it will purge a variety of keys including home, front, 404 and feed. To bypass or override this behavior, since 1.5.0-dev we have a filter allowing an array of post types to ignore to be passed before those caches are purged. By default, the revision post type is ignored, but others can be added:

/**
* Add a custom post type to the ignored post types.
*
* @param array $ignored_post_types The array of ignored post types.
* @return array
*/
function filter_ignored_posts( $ignored_post_types ) {
    $ignored_post_types[] = 'my-post-type'; // Ignore my-post-type from cache purges.
    return $ignored_post_types;
}

add_filter( 'pantheon_purge_post_type_ignored', 'filter_ignored_posts' );

This will prevent the cache from being purged if the given post type is updated.

WP-CLI Commands

This plugin implements a variety of WP-CLI commands. All commands are grouped into the wp pantheon cache namespace.

$ wp help pantheon cache

NAME

  wp pantheon cache

DESCRIPTION

  Manage the Pantheon Advanced Page Cache.

SYNOPSIS

  wp pantheon cache <command>

SUBCOMMANDS

  purge-all       Purge the entire page cache.
  purge-key       Purge one or more surrogate keys from cache.
  purge-path      Purge one or more paths from cache.

Use wp help pantheon cache <command> to learn more about each command.

Debugging

By default, Pantheon’s infrastructure strips out the Surrogate-Key response header before responses are served to clients. The contents of this header can be viewed as Surrogate-Key-Raw by adding on a debugging header to the request.

A direct way of inspecting headers is with curl -I. This command will make a request and show just the response headers. Adding -H "Pantheon-Debug:1" will result in Surrogate-Key-Raw being included in the response headers. The complete command looks like this:

curl -IH "Pantheon-Debug:1" https://scalewp.io/

Piping to grep will filter the output down to just the Surrogate-Key-Raw header:

curl -IH "Pantheon-Debug:1" https://scalewp.io/ | grep -i Surrogate-Key-Raw

Tada!

Emitted Keys and Purge Events

Emitted Keys on Traditional Views

Home /

• Emits surrogate keys: home, front, post-<id> (all posts in main query)

Single post /2016/10/14/surrogate-keys/

• Emits surrogate keys: single, post-<id>, post-user-<id>, post-term-<id> (all terms assigned to post)

Author archive /author/pantheon/

• Emits surrogate keys: archive, user-<id>, post-<id> (all posts in main query)

Term archive /tag/cdn/

• Emits surrogate keys: archive, term-<id>, post-<id> (all posts in main query)

Day archive /2016/10/14/

• Emits surrogate keys: archive, date, post-<id> (all posts in main query)

Month archive /2016/10/

• Emits surrogate keys: archive, date, post-<id> (all posts in main query)

Year archive /2016/

• Emits surrogate keys: archive, date, post-<id> (all posts in main query)

Search /?s=<search>

• Emits surrogate keys: search, either search-results or search-no-results, post-<id> (all posts in main query)

Not found (404)

• Emits surrogate keys: 404

Emitted Keys on REST API Endpoints

Posts

• /wp-json/wp/v2/posts emits surrogate keys: rest-post-collection, rest-post-<id>
• /wp-json/wp/v2/posts/<id> emits surrogate keys: rest-post-<id>

Pages

• /wp-json/wp/v2/pages emits surrogate keys: rest-page-collection, rest-post-<id>
• /wp-json/wp/v2/pages/<id> emits surrogate keys: rest-post-<id>

Categories

• /wp-json/wp/v2/categories emits surrogate keys: rest-category-collection, rest-term-<id>
• /wp-json/wp/v2/categories/<id> emits surrogate keys: rest-term-<id>

Tags

• /wp-json/wp/v2/tags emits surrogate keys: rest-post_tag-collection, rest-term-<id>
• /wp-json/wp/v2/tags/<id> emits surrogate keys: rest-term-<id>

Comments

• /wp-json/wp/v2/comments emits surrogate keys: rest-comment-collection, rest-comment-post-<post-id>, rest-comment-<id>
• /wp-json/wp/v2/comments/<id> emits surrogate keys: rest-comment-post-<post-id>, rest-comment-<id>

Users

• /wp-json/wp/v2/users emits surrogate keys: rest-user-collection, rest-user-<id>
• /wp-json/wp/v2/users/<id> emits surrogate keys: rest-user-<id>

Settings

• /wp-json/wp/v2/settings emits surrogate keys: rest-setting-<name>

Purge Events

Different WordPress actions cause different surrogate keys to be purged, documented here.

wp_insert_post / transition_post_status / before_delete_post / delete_attachment

• Purges surrogate keys: home, front, 404, post-<id>, user-<id>, term-<id>, rest-<type>-collection, rest-comment-post-<id>
• Affected views: homepage, single post, any page with 404 header, any archive where post displays, author archive, term archive, REST API collection and resource endpoints

clean_post_cache

• Purges surrogate keys: post-<id>, rest-post-<id>
• Affected views: single post, REST API resource endpoint

created_term / edited_term / delete_term

• Purges surrogate keys: term-<id>, post-term-<id>, rest-<taxonomy>-collection
• Affected views: term archive, any post where the term is assigned, REST API collection and resource endpoints

clean_term_cache

• Purges surrogate keys: term-<id>, rest-term-<id>
• Affected views: term archive, REST API resource endpoint

wp_insert_comment / transition_comment_status

• Purges surrogate keys: rest-comment-collection, rest-comment-<id>
• Affected views: REST API collection and resource endpoints

clean_comment_cache

• Purges surrogate keys: rest-comment-<id>
• Affected views: REST API resource endpoint

clean_user_cache

• Purges surrogate keys: user-<id>, rest-user-<id>
• Affected views: author archive, any post where the user is the author

updated_option

• Purges surrogate keys: rest-setting-<name>
• Affected views: REST API resource endpoint

Surrogate Keys for taxonomy terms

Setting surrogate keys for posts with large numbers of taxonomies (such as WooCommerce products with a large number of global attributes) can suffer from slower queries. Surrogate keys can be skipped for ‘product’ post types’ taxonomy terms (or any other criteria you see fit) with the following filter:

function custom_should_add_terms($should_add_terms, $wp_query) {
    if ( $wp_query->is_singular( 'product' ) ) {
        return false;
    }
    return $should_add_terms;
}
add_filter('pantheon_should_add_terms', 'custom_should_add_terms', 10, 2);<h3>Plugin Integrations</h3>

Pantheon Advanced Page Cache integrates with WordPress plugins, including:

• WPGraphQL

Contributing

See CONTRIBUTING.md for information on contributing.