Caching for Coders:
How, What, and When to Cache in WordPress

Metaphor

Source: http://toblender.com/memcached-story-part-1/

Metaphor

What We Are Talking About

• Improving site performance through object (fragment) caching
  • How to cache
  • What to cache
  • When to cache
• No more cookie cutter approaches to caching
• Not talking about caching plugins or server setups
  • Chris Scott and Sean O’Shaughnessy: Caching in WordPress

How to Cache

Transients

• set_transient, get_transient, delete_transient
• Uses the database if an object cache is not present

function zt_display_leaders() {
    $leaders = get_transient( 'zt-leaders' );
    if ( false === $leaders ) {
        $leaders = zt_get_leaders();
        set_transient( 'zt-leaders', $leaders, 300 );
    }

    return $leaders;
}

How to Cache

WP_Object_Cache

• wp_cache_add, wp_cache_get, wp_cache_delete (amongst others)
• Success depends on server environment

function zt_display_leaders() {
    $leaders = wp_cache_get( 'zt-leaders', 'zt-stats' );
    if ( false === $leaders ) {
        $leaders = zt_get_leaders();
        wp_cache_set( 'zt-leaders', $leaders, 'zt-stats', 300 );
    }

    return $leaders;
}

Discussion of some differences by Otto

What to Cache

• Requests to external services
  • wp_remote_*
• Expensive queries
  • 'post__not_in', 'meta_query', 'tax_query'
  • Identify slow queries with Debug Bar/Extender
• Repeated routines/queries
  • First image in post, dealing with child taxonomy terms
• Infrequently updated data
  • menus , related posts, lists of data

When to Cache

• The goal is to reduce the server's workload
• How can you remove queries or expensive processes?

Gameplan

• Ideally, refresh cache ONLY when data is added/changed
• Minimize running intensive processes
• Avoid caching on front end page requests
• Design to fail gracefully

When to Cache — A Common Solution

Cache when the data is not cached

// functions.php
function zt_get_tweet() {
    if ( false === ( $tweet = get_transient( 'zt-tweet' ) ) ) {
        $tweet = zt_request_tweet(); // Talk to Twitter
        set_transient( 'zt-tweet', $tweet, 300 );
    }
    return $tweet;
}

// sidebar.php
<h3>Latest Tweet:</h3>
<p><?php echo zt_get_tweet(); ?></p>

When to Cache — When the Data Changes

• Generate the data on an admin event
  • Generates data only when changed
  • Reduces the chance of front end generation
• Helpful Events/Hooks
  • Publishing/Updating posts: publish_post
  • Changing post status: transition_post_status
  • Creating terms: created_{$taxonomy}
  • Updating terms: edited_{$taxonomy}

When to Cache — When the Data Changes

function zt_set_best_games( $post_id, $post ) {
    if ( 'zt-game' != $post->post_type )
        return false;

    zt_determine_best_games();
}

add_action( 'publish_post', 'zt_set_best_games', 99, 2 );

function zt_determine_best_games() {
    /* Get best reviewed games; set to $top_games */
    set_transient( 'zt-top-games', $top_games );
    return $top_games;
}

In 2 years

1 post per day: 730 runs

Common solution: 210,240 runs

When to Cache — On an Interval

• Use scheduled events to cache unpredictable data
  • e.g., external data
• Avoids unnecessary requests
• Reduces the chance of front end caching

When to Cache — On an Interval

function zt_schedule_tweet_retrieval() {
    if ( ! wp_next_scheduled( 'zt_tweet_sched' ) )
        wp_schedule_event( time(), 'hourly', 'zt_tweet_sched' );
    add_action( 'zt_tweet_sched', 'zt_request_tweet' );
}
add_action( 'init', 'zt_schedule_tweet_retrieval' );

function zt_request_tweet() {
    // Talk to Twitter
    set_transient( 'zt-tweet', $tweet );
    return $tweet;
}

function zt_get_tweet() {
    if ( false === ( $tweet = get_transient( 'zt-tweet' ) ) ) {
        $tweet = zt_request_tweet(); // Talk to Twitter
        set_transient( 'zt-tweet', $tweet );
    }
    return $tweet;
}

When to Cache — Failing Gracefully

• Never trust that data is in the cache
• Have a backup plan
  • Display placeholder text
  • Regenerate data
  • Backup storage (Options API)

When to Cache — Failing Gracefully

Placeholder text

function zt_display_tweet() {
    if ( $tweet = get_transient( 'zt-tweet' ) ) {
        return $tweet;
    else
        return 'Latest Tweet Unavailable';
}

Regenerate data

function zt_display_tweet() {
    if ( $tweet = get_transient( 'zt-tweet' ) ) {
        return $tweet;
    else
        return zt_get_tweet(); // Talk to Twitter
}

When to Cache — Failing Gracefully

Storing a "backup"

function zt_schedule_tweet_retrieval() {
    if ( ! wp_next_scheduled( 'zt_tweet_sched' ) )
        wp_schedule_event( time(), 'hourly', 'zt_tweet_sched' );
    add_action( 'zt_tweet_sched', 'zt_generate_tweet' );
}

add_action( 'init', 'zt_schedule_tweet_retrieval' );

function zt_generate_tweet() {
    $tweet = zt_request_tweet(); // Talk to Twitter
    set_transient( 'zt-tweet', $tweet );

    if ( ! get_option( 'zt-tweet' ) )
        add_option( 'zt-tweet', $tweet, '', 'no' );
    else
        update_option( 'zt-tweet', $tweet );

    return $tweet;
}

When to Cache — Failing Gracefully

Using the "backup"

function zt_display_tweet() {
    if ( $tweet = get_transient( 'zt-tweet' ) ) {
        return $tweet;
    } elseif ( $tweet = get_option( 'zt-tweet' ) {
        set_transient( 'zt-tweet', $tweet );
        return $tweet;
    } else {
        return zt_generate_tweet();
    }
}

When to Cache — Avoid Front End Caching

• Flooding
  • Many concurrent users cause overload
• Consider "locking" techniques if generating on front end
• Mark Jaquith's WP-TLC-Transients

Beer List

function zt_schedule_generate_beer() {
    if ( ! wp_next_scheduled( 'zt_beer_sched' ) )
        wp_schedule_event( time(), 'daily', 'zt_beer_sched' );
    add_action( 'zt_beer_sched', 'zt_generate_beer_list' );
}
add_action( 'init', 'zt_schedule_generate_beer' );

function zt_generate_beer_list() {
    // Query API and generate HTML
    $beer_list = zt_get_beer_list();
    set_transient( 'zt-beers', $beer_list );
    delete_transient( 'zt-beer-sched-single' );
    return $beer_list;
}

function zt_generate_beer_on_publish( $post_id, $post ) {
    zt_generate_beer_list();
}
add_action( 'publish_post', 'zt_generate_beer_on_publish', 10, 2 );

Beer List

// Same function from previous slide
function zt_generate_beer_list() {
    // Query API and generate HTML
    $beer_list = zt_get_beer_list();
    set_transient( 'zt-beers', $beer_list );
    delete_transient( 'zt-beer-sched-single' );
    return $beer_list;
}

function zt_display_all_the_beers() {
    if ( $beers = get_transient( 'zt-beers' ) ) {
        return $beers;
    } else {
        if ( ! get_transient( 'zt-beer-sched-single' ) ) {
            set_transient( 'zt-beer-sched-single', 'yes' );
            wp_schedule_single_event( time(), 'zt_beer_sched' );
        }
        return 'Beer list is being generated.';
    }
}

Thank You

Andrew Nacin (@nacin)

Daniel Bachhuber (@danielbachhuber)

Helen Hou-Sandi (@helenhousandi)

Laura Milewski (@l

Steve Locker (@slocker3)