2nd Wordpress Conference in Hluboká nad Vltavou, CZ

1. Caching in
WordPress
David Biňovec
david_binovec@parade.com

2. Presentation outline
● What cache is, what to cache
● Caching options in PHP and MySQL
o Memcached, Alternative PHP Cache (APC)
o MySQL Query Cache
● Page level a Object Cache
● Is WordPress cache-ready?
o Transients API
o WP_Object_Cache a wp_cache funkce
o oEmbed cache

3. What is a cache and what to cache?
● transparently stored data so that future requests for that
data can be served faster
● You can cache almost everything
o whole pages, objects
o external API requests, resource demanding
operations ...
● … and even more
o MySQL Query Cache
o OpCode

4. MySQL Query Cache
● only identical and deterministic SELECT
requests
● inter session persistent
● cache flushing after the change of relevant
data
● non-distributed

5. Cache engines
● Memcached
o distributed (server accessible via telnet)
o key-value pairs saved in a memory
o PECL extension for PHP (memcached / memcache)
● Alternative PHP Cache (APC)
o Opcode and user cache (PHP <= 5.4), APCu (PHP 5.5)
o non-distributed
● Flat file
o eg. mod_rewrite (WP Super Cache), CDN

6. Page Level vs. Object Cache
● Page level cache (e.g. WP Super Cache)
o saves only whole pages
o every single page has to be generated on it’s own
with all database requests, API call etc..
o usually is not available for non logged in users
● Object cache (APC, Memcached, W3TC)
o reusable for a generation of multiple pages
o works also for logged in users
o has affect even on WordPress administration

7. Page Level Cache
● HTML files saved on hard drive
o loading via PHP or mod_rewrite (WP Super Cache)
● memcached (Batcache)
o can be used even without PHP (NginX, Apache ...)
● CDN
o cloudflare (W3TC)
o anything else (WP Super Cache)

8. How to cache a page in WordPress

9. WP Super Cache - mod_rewrite

10. Object Cache
● reusable objects
o either in terms of a single requests
o or, and with better performance, in multiple requests
● object is created via one or multiple
SELECTs
o is an output from multiple functions, methods ...
● For example:
o WP_User (capabilities, user_meta)
o WP_Post ...

11. Is WordPress cache-ready? It is!
● Transients API
o set_transient, get_transient, delete_transient
o set_site_transient, get_site_transient …
● WP_Object_Cache and wp_cache functions
o wp-includes/cache.php
 wp_cache_add, wp_cache_set,
wp_cache_replace
 wp_cache_get
 wp_cache_delete, wp_cache_flush

12. Transients API
● temporary saving of cached data
● uses wp_options table by default
o add_option with param autoload = “no”
 if no expiration is set, then autoload = “yes”
o adds two entries to the database
 transient value
 maximal expiration time (vs. guaranted time)
● it’s guaranted by default, but saving into database can be
overriden by memcache server, for instance

13. ● core implementation of this class uses global
variable for saving data
o thus, from nature, is cached only per requests
● has it’s meaning for repeated database
requests
o you can call get_post_meta multiple time for one
request, but you reach database once only
WP_Object_Cache

14. Where is WP_Object_Cache used
● get_metadata (post, user, comment ...)
● get_option (autoload options)
● get_post, get_user, get_comment, get_term
● transients
● ...

15. wp_cache functions
● wp_cache_set($key, $data, $group, $expire)
o no expiration by default ($expire = 0)
● wp_cache_get( $key, $group )
o group allows to use same keys across multiple
groups

16. WP_Object_Cache dropins
● dropins
o wp-content/advanced-cache.php
o wp-content/object-cache.php
● object-cache.php
o prevents loading of wp-includes/wp_cache.php
o redefines WP_Object_Cache and wp_cache
functions
o uses any of external caching engines
 memcache, APC, filesystem ...

17. oEmbed cache using post_meta

18. WordPress Cache Plugins
● WP Super Cache http://wordpress.org/plugins/wp-super-
cache/
o page level cache using mod_rewrite
● W3 Total Cache http://wordpress.org/plugins/w3-total-cache/
o multiple object-cache engines
● Batcache http://wordpress.org/plugins/batcache/
● object-cache.php dropins
http://wordpress.org/plugins/memcached/
http://wordpress.org/plugins/apc/

19. Understanding Cache in WP - Summary
● Memcached, APC, MySQL Query Cache
● Page level vs. Object Cache
o go for a mix of page level and object
cache
● Transients API
● WP_Object_Cache a wp_cache funkce
o dropins: object-cache.php, advanced-cache.php
● oEmbed cache using post_meta table