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The Otto & Nacin ShowWordCamp San Francisco 2011August 12, 2011

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Nacin OttoContributing Developers to WordPress Tech Ninjas at Audrey Capitalnacin@wordpress.org otto@wordpress.org@nacin @otto42

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Transients, Caching, andthe Complexities ofMultisite(A brief overview.)

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Basic Caching withTransients

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Transients are ideal for:Temporary data that canexpire at will.

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$value = get_transient( big_data );if ( false === $value ) { // do something that takes a fair amount of time $response = wp_remote_get( $url ); $value = wp_remote_retrieve_body( $response ); set_transient( big_data, $value, 60 * 60 * 24 );}echo $value;

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$value = get_transient( big_data );if ( false === $value ) { // do something that takes a fair amount of time $response = wp_remote_get( $url ); $value = wp_remote_retrieve_body( $response ); set_transient( big_data, $value ); // this one doesnt expire, but it might anyway}echo $value;

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Always persistent:When theres no externalobject cache, WordPressuses the options table.

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Object Caching

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Object caching is ideal for:Caching an object (often aquery result) to reduceoverhead.

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$activity_object = wp_cache_get( $id, activity );if ( false === $activity_object ) { // begrudgingly fetch our object $activity_object = $wpdb->get_row( $wpdb->prepare( "SELECT * FROM $wpdb->activity WHERE ID = %d", $id ) ); wp_cache_set( $id, $activity_object , activity );}// do something with $activity_object

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Ideally persistent:Use an object cachingbackend like APC,Memcached, or WinCache.

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Object caching is not ideal ifyou need both datapersistence and codeportability.No knowledge of the serversetup? Use transients.

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// Cache functions can take an expiration toowp_cache_set( $id, $activity_object , activity, $expires = 0 );

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Enter Multisite(Run.)

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Site Transients!Well, network-wide transients.Pardon the terminology.

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$value = get_site_transient( big_data );if ( false === $value ) { // do something that takes a fair amount of time $response = wp_remote_get( $url ); $value = wp_remote_retrieve_body( $response ); set_site_transient( big_data, $value, 86400 );}echo $value;

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Always persistent:When theres no externalobject cache, WordPressuses the sitemeta table.

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It works in single-site!When not running multisite,it wraps *_transient().

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Global Cache GroupsCache groups (activity) are site-specific. Tell WordPress what isnt.

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// WordPress core (simplified)wp_cache_add_global_groups( array( users, user_meta, site-transient, site-options, site-lookup, blog-lookup, blog-details, ));// You (not simplified)wp_cache_add_global_groups( activity );

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As you might expect, it worksjust fine in single-site.

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And then theresswitch_to_blog()With great power comesgreat responsibility.

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Rule 1Dont use it.If you do, cache around it.

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// Example is a widget with posts from another site:$posts = wp_cache_get( recent_posts, $blog_id );if ( false === $posts ) { switch_to_blog( $blog_id ); $posts = new WP_Query( . . . )->posts; wp_cache_set( $blog_id, $posts, recent_posts ); restore_current_blog(); // OMG BBQ}// do something with $posts

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// And for some extra credit:add_action( init, function() { wp_cache_add_global_groups( recent_posts );} );add_action( save_post, function() { global $blog_id; if ( ! in_array( $blog_id, array( 1, 2 3, 4 ) ) return; wp_cache_delete( $blog_id, recent_posts ); // And for even more extra credit: $posts = new WP_Query( . . . )->posts; wp_cache_add( $blog_id, $posts, recent_posts );} );

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Thanks! Questions?@otto42 @nacin