What is Caching?

• Serving static content to a visitor rather than something generated upon request.
• Two major types in WordPress context (there are more):
  • Page - whole page is static and, therefore, could be outdated.
  • Fragment - cache pieces of a page rather than the entire page.

Fragment Caching Benefits

Allow dynamic and static content to coexist

Fragment Caching Benefits

Common elements can be reused throughout a site

Fragment Caching Benefits

Reduce calls to APIs

WordPress' Native Caching APIs

Fragment Caching Basics: Creating

<?php
	function generate_cached_output() {
		if ( false === ( $output = wp_cache_get( $cache_key, $cache_group ) ) ) {
			$output = 'Something to be cached';

			wp_cache_set( $cache_key, $output, $cache_group, 86400 );
		}

		return $output;
	}
?>

• wp_cache_get() returns a boolean false if the requested cache isn't set or has expired.
• Everything that is to be cached must be accessible via a function that returns its results rather than echoing them, otherwise output buffering is needed.
• wp_cache_add() will not overwrite an existing, unexpired cache, whereas wp_cache_set() does.

Fragment Caching Basics: Clearing

<?php
	function clear_cached_output( $new_status, $old_status ) {
		if ( 'publish' == $new_status || 'publish' == $old_status )
			wp_cache_delete( $cache_key, $cache_group );
	}
	add_action( 'transition_post_status', 'clear_cached_output', 10, 2 );
?>

This above example clears a cache when anything is published or something that is published is modified. The "something" could be a post, page, or custom post type object.

If, instead, the cache should be rebuilt only when posts are edited, one additional argument from transition_post_status can be employed.

<?php
	function clear_cached_output( $new_status, $old_status, $post ) {
		if ( ( 'publish' == $new_status || 'publish' == $old_status ) && 'post' == $post->post_type )
			wp_cache_delete( $cache_key, $cache_group );
	}
	add_action( 'transition_post_status', 'clear_cached_output', 10, 3 );
?>

Fragment Caching Basics: Clearing

Same cache generation function from two slides ago, with a minor change

<?php
	function generate_cached_output( $force_refresh = false ) {
		if ( $force_refresh || false === ( $output = wp_cache_get( $cache_key, $cache_group ) ) ) {
			$output = 'Something to be cached';

			wp_cache_set( $cache_key, $output, $cache_group, 86400 );
		}

		return $output;
	}
?>

Clear by rebuilding cache

<?php
	function clear_cached_output( $new_status, $old_status ) {
		if ( 'publish' == $new_status || 'publish' == $old_status )
			generate_cached_output( true );
	}
	add_action( 'transition_post_status', 'clear_cached_output', 10, 2 );
?>

Unpredictable Keys

• Object caching doesn’t provide a way to clear all caches in a given group.
• Therefore, if the cache key is unpredictable, how can we clear it?
• For example, a list of recent posts to be displayed on an individual post, but that excludes the current post.

Unpredictable Keys: Recent Posts

<?php
	function recent_posts( $post_id = false, $qty = 3 ) {
		$post_id = (int) $post_id;
		if ( ! $post_id )
			return false;

		$qty = (int) $qty ? (int) $qty : 3;

		$cache_key = $post_id . '_' . $qty;

		if ( false === ( $output = wp_cache_get( $cache_key, 'recent_posts' ) ) ) {
			$output = 'Something to be cached';

			wp_cache_set( $cache_key, $output, 'recent_posts', 86400 );
		}

		return $output;
	}
?>

Unpredictable Keys: Cached Array

<?php
	function recent_posts( $post_id = false, $qty = 3 ) {
		/* Sanitize function arguments */

		$cache_key = $post_id . '_' . $qty;

		$cache = wp_cache_get( 'single', 'recent_posts' );
		if( ! is_array( $cache ) )
			$cache = array();

		if ( ! array_key_exists( $cache_key, $cache ) ) {
			$output = 'Something to be cached';

			$cache[ $cache_key ] = $output;
			wp_cache_set( 'single', $cache, 'recent_posts', 86400 );
		}

		return $output;
	}
?>

Unpredictable Keys: Cached Array

Unpredictable Keys: Incrementor

<?php
	function get_cache_incrementor() {
		$incrementor = wp_cache_get( 'incrementor', 'recent_posts' );
		if ( ! is_numeric( $incrementor ) ) {
			$incrementor = time();
			wp_cache_set( 'incrementor', $incrementor, 'recent_posts', 86400 );
		}

		return $incrementor;
	}

	function recent_posts( $post_id = false, $qty = 3 ) {
		/* Sanitize function arguments */

		$cache_key = get_cache_incrementor() . '_' . $post_id . '_' . $qty;

		if ( false === ( $output = wp_cache_get( $cache_key, 'recent_posts' ) ) ) {
			$output = 'Something to be cached';

			wp_cache_set( $cache_key, $output, 'recent_posts', 86400 );
		}

		return $output;
	}
?>

Where We Use Fragment Caching

• Custom loops
• Anytime a lot of data must be retrieved from WordPress and parsed.
• Most situations where WP_Query generates a subquery outside of the main query.
• Almost anything that is reused across multiple pages.

category__not_in vs post__not_in

<?php
	new WP_Query( array(
		'category__not_in' => 167
	) );
?>

SELECT … WHERE 1=1  AND wp_posts.ID NOT IN ( SELECT tr.object_id FROM wp_term_relationships AS tr INNER JOIN wp_term_taxonomy AS tt ON tr.term_taxonomy_id = tt.term_taxonomy_id WHERE tt.taxonomy = 'category' AND tt.term_id IN ('167') ) …

category__not_in vs post__not_in

<?php
	function cached_get_objects_in_term( $term_ids, $taxonomies, $args ) {
		/* Sanitize function arguments */

		$cache_key = md5( implode( ',', $term_ids ) . $taxonomies . serialize( $args ) );

		if ( false === ( $ids = wp_cache_get( $cache_key, 'get_objects_in_term' ) ) ) {
			$ids = get_objects_in_term( $term_ids, $taxonomies, $args );
			/* Error check $ids */
			wp_cache_set( $cache_key, $ids, 'get_objects_in_term', 86400 );
		}

		return $ids;
	}
?>

category__not_in vs post__not_in

<?php
	new WP_Query( array(
		'post__not_in' => cached_get_objects_in_term( 167, 'category' )
	) );
?>

Before

SELECT … WHERE 1=1  AND wp_posts.ID NOT IN ( SELECT tr.object_id FROM wp_term_relationships AS tr INNER JOIN wp_term_taxonomy AS tt ON tr.term_taxonomy_id = tt.term_taxonomy_id WHERE tt.taxonomy = 'category' AND tt.term_id IN ('167') ) …

After

SELECT … WHERE 1=1  AND wp_posts.ID NOT IN ( '1','2','3','4','5' ) …

Menu Caching: No Active States

<?php
	function cache_wp_nav_menu( $args = false, $skip_cache = false ) {
		/* Sanitize function arguments */

		$echo = (bool) $args[ 'echo' ];
		$args[ 'echo' ] = false;

		$cache_key = md5( implode( '|', $args ) );

		if( $skip_cache || false === ( $menu = wp_cache_get( $cache_key, $this->cache_group ) ) ) {
			$menu = wp_nav_menu( $args );

			wp_cache_set( $cache_key, $menu, $this->cache_group, 86400 );
		}

		if( $echo )
			echo $menu;
		else
			return $menu;
	}
?>

Menu Caching: Active States

<?php
	function cache_wp_nav_menu( $args = false, $skip_cache = false ) {
		/* Sanitize function arguments */

		$echo = (bool) $args[ 'echo' ];
		$args[ 'echo' ] = false;

		$queried_object = (int) get_queried_object_id();
		if ( ! $queried_object )
			$queried_object = 0;

		$cache_key = get_cache_incrementor();
		$cache_key .= $queried_object . '|';
		$cache_key .= implode( '|', $args );
		$cache_key = md5( $cache_key );

		...
?>

Menu Caching: Keys & Clearing

• get_queried_object_id() returns an integer representing the post ID or term ID.
• Front page and custom post type archives return 0.
• Menu caches must be cleared when four different actions fire to ensure consistency:
  • wp_update_nav_menu
  • wp_update_nav_menu_item
  • wp_delete_nav_menu
  • wp_setup_nav_menu_item

query_posts() vs pre_get_posts

<?php
	//In home.php
	query_posts( array(
		'posts_per_page' => 5
	) );
?>

<?php
	function action_pre_get_posts( $query ) {
		if ( $query->is_home() )
			$query->set( 'posts_per_page', 5 );
	}
	add_action( 'pre_get_posts', 'action_pre_get_posts' );
?>

How Does This Factor Into Our Work On WordPress.com VIP?

• Page caches only last for five minutes.
• No page caching for logged-in users.
• Sites that publish with great frequency trigger regular invalidations of homepage and category pages.
• Web servers outnumber database servers.

Want to know more about WordPress.com infrastructure? Check out http://goo.gl/lYpJH.