The Many Challenges of Object Caching in WordPress

It's no secret that WordPress can be slow and tough to scale. A key ingredient to scaling WordPress is setting up object caching. Object caching reduces the number of heavy database calls that WordPress needs to satisfy web requests. My talk will discuss the many details of how WordPress loads a third-party object cache, the API that developers can use, and design patterns for effective object caching. Finally, I will share my personal experiences in using object caching to help scale enterprise-level WordPress installations.

Transcript

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

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2. I know what you are
   thinking
   

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

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4. Cache
   
   Rules
   
   Everything
   
   Around
   
   Me
   

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5. WordPress can
   
   get slow
   

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6. Database queries
   

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7. API requests
   

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8. Long running routines
   

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9. Once is usually
   enough
   

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10. Object caching can
    improve performance
    

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

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12. Making data easier to
    access on subsequent
    
    requests
    

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13. 1. Caching primer
    
    2. Caching patterns
    

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14. wp-content/object-cache.php
    

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15. wp_cache_init();
    

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

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17. Persistence
    

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

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

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

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

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

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23. wp_cache_get($key,
    $prefix)
    

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

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26. Caching patterns
    

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27. On demand
    

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29. $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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30. Stampeding herd
    
    Easy to implement
    

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31. On event
    

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

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34. $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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35. No stampeding herd
    
    Must be able to listen to
    event
    

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36. On interval
    

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38. 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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39. $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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40. No stampeding herd
    
    Doesn’t need an event
    
    Relies on WP Cron
    

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

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42. Use stale data while
    generating new data
    

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44. $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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45. function queue_related( $post_id ) {
    $lock = wp_cache_get( $post_id, ‘related-
    lock’ );
    if ( $lock ) {
    return;
    } else {
    wp_cache_set( $post_id, 1, ‘related-lock’ );
    }
    $related = generate_related( $post_id );
    wp_cache_set($post_id, $related, ‘related’,
    300);
    wp_cache_set($post_id, $related, ‘related-
    stale’);
    wp_cache_delete( $post_id, ‘related-lock’ );
    }
    

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

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47. $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 );
    }
    

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48. No stampeding herd
    
    Easy to implement with
    library
    

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49. Tips for the Cacher
    

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50. Avoid front-end
    caching
    

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51. Always set TTL
    

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52. Group level purges
    don’t exist
    https://www.tollmanz.com/invalidation-schemes/
    

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53. Invalidation first
    design
    

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54. #31245 sucks,
    
    a lot
    https://core.trac.wordpress.org/ticket/31245
    

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56. https://speakerdeck.com/tollmanz
    
    @tollmanz
    

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