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
2. I know what you are thinking
3. “There are two hard things in computer science: cache invalidation, and naming things”
4. Cache Rules Everything Around Me
5. WordPress can get slow
6. Database queries
7. API requests
8. Long running routines
9. Once is usually enough
10. Object caching can improve performance
11. http://www.fs.usda.gov/
12. Making data easier to access on subsequent requests
13. 1. Caching primer 2. Caching patterns
14. wp-content/object-cache.php
15. wp_cache_init();
16. function wp_cache_init() { global $wp_object_cache; $wp_object_cache = new WP_Object_Cache(); }
17. Persistence
18. APC https://wordpress.org/plugins/apc/
19. Memcached https://github.com/tollmanz/ wordpress-pecl-memcached-object-cache https://wordpress.org/plugins/memcached/
20. “There are two hard things in computer science: cache invalidation, and naming things”
21. Redis https://github.com/ericmann/Redis-Object-Cache/
22. wp_cache_set($key, $value, $prefix, $ttl)
23. wp_cache_get($key, $prefix)
24. Grokking the WordPress Object Cache https://www.tollmanz.com/grokking-the-wp-object-cache/
26. Caching patterns
27. On demand
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 $related_post ) { // Display the related post }
30. Stampeding herd Easy to implement
31. On event
33. add_action( ‘save_post’, function($post_id) { $value = generate_related( $post_id ); wp_cache_set( $post_id, $value, ‘related’ ); } );
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 }
35. No stampeding herd Must be able to listen to event
36. On interval
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 ); }
39. $tweets = wp_cache_get( ‘tweets’ ); if ( empty( $tweets ) || ‘none- available’ === $tweets) { $tweets = get_default_tweets(); } foreach ( $tweets as $tweets ) { // Get your social on }
40. No stampeding herd Doesn’t need an event Relies on WP Cron
41. Stale while revalidate https://tools.ietf.org/html/rfc5861
42. Use stale data while generating new data
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 }
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’ ); }
46. TLC Transients https://github.com/markjaquith/WP-TLC-Transients
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 ); }
48. No stampeding herd Easy to implement with library
49. Tips for the Cacher
50. Avoid front-end caching
51. Always set TTL
52. Group level purges don’t exist https://www.tollmanz.com/invalidation-schemes/
53. Invalidation first design
54. #31245 sucks, a lot https://core.trac.wordpress.org/ticket/31245
56. https://speakerdeck.com/tollmanz @tollmanz