The Many Challenges of Object Caching in WordPress

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The Many Challenges of Object Caching in WordPress Zack Tollman @tollmanz

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I know what you are thinking

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“There are two hard things in computer science: cache invalidation, and naming things”

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Cache Rules Everything Around Me

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WordPress can get slow

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Database queries

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API requests

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Long running routines

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Once is usually enough

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Object caching can improve performance

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http://www.fs.usda.gov/

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Making data easier to access on subsequent requests

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1. Caching primer 2. Caching patterns

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wp-content/object-cache.php

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wp_cache_init();

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function wp_cache_init() { global $wp_object_cache; $wp_object_cache = new WP_Object_Cache(); }

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Persistence

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APC https://wordpress.org/plugins/apc/

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Memcached https://github.com/tollmanz/ wordpress-pecl-memcached-object-cache https://wordpress.org/plugins/memcached/

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“There are two hard things in computer science: cache invalidation, and naming things”

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Redis https://github.com/ericmann/Redis-Object-Cache/

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wp_cache_set($key, $value, $prefix, $ttl)

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wp_cache_get($key, $prefix)

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Grokking the WordPress Object Cache https://www.tollmanz.com/grokking-the-wp-object-cache/

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Caching patterns

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On demand

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$related = wp_cache_get( get_the_ID(), ‘related’); if ( false === $related ) { $related = generate_related( get_the_ID() ); wp_cache_set( get_the_ID(), $value, ‘related’, 86400 ); } foreach ( $related as $related_post ) { // Display the related post }

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Stampeding herd Easy to implement

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On event

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add_action( ‘save_post’, function($post_id) { $value = generate_related( $post_id ); wp_cache_set( $post_id, $value, ‘related’ ); } );

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$related = wp_cache_get( get_the_ID(), ‘related’ ); if ( empty( $related ) ) { $related = get_default_related(); } foreach ( $related as $related_post ) { // Display the related post }

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No stampeding herd Must be able to listen to event

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On interval

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add_action( ‘init’, function() { wp_schedule_event( time(), 'hourly', ‘update_tweets’ ); } ); function update_tweets() { $tweets = generate_tweets(); if ( empty( $tweets ) ) { $tweets = ‘none-available’; } wp_cache_set( ‘tweets’, $tweets ); }

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$tweets = wp_cache_get( ‘tweets’ ); if ( empty( $tweets ) || ‘none- available’ === $tweets) { $tweets = get_default_tweets(); } foreach ( $tweets as $tweets ) { // Get your social on }

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No stampeding herd Doesn’t need an event Relies on WP Cron

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Stale while revalidate https://tools.ietf.org/html/rfc5861

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Use stale data while generating new data

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$related = wp_cache_get( get_the_ID(), ‘related’ ); if ( false === $value ) { queue_related( get_the_ID() ); $related = wp_cache_get( get_the_ID(), ‘related-stale’ ); } foreach ( $related as $related_post ) { // Display the related post }

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TLC Transients https://github.com/markjaquith/WP-TLC-Transients

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$related = tlc_transient( ‘related-’ . $post_id ) ->updates_with( ‘queue_related’, [ $post_id ] ) ->expires_in( 300 ) ->get(); foreach ( $related as $related_post ) { // Display the post } ———————————————— function queue_related( $post_id ) { return generate_related( $post_id ); }