Memcached Object Cache

Description

Memcached Object Cache provides a persistent backend for the WordPress object cache. A memcached server and the PECL memcache extension are required.

Installation

1. Install memcached on at least one server. Note the connection info. The default is 127.0.0.1:11211.

2. Install the PECL memcache extension

3. Copy object-cache.php to wp-content

4. Add the WP_CACHE_KEY_SALT constant to the wp-config.php:

   php
define( 'WP_CACHE_KEY_SALT', '...long random string...' );

This helps prevent cache pollution when multiplte WordPress installs are using the same Memcached server. The value must be unique for each WordPress install.

FAQ

How can I manually specify the memcached server(s)?

Add something similar to the following to wp-config.php above /* That's all, stop editing! Happy blogging. */:

$memcached_servers = array(
    'default' => array(
        '10.10.10.20:11211',
        '10.10.10.30:11211'
    )
);

The top level array keys, are cache groups, where ‘default’ corresponds to any cache group that is not explicitly defined. This allows for specifying memcached servers that only handle certain cache groups. The most common use is only specifying ‘default’.

Possible cache groups are:

{$taxonomy}_relationships
{$meta_type}_meta
{$taxonomy}_relationships
blog-details
blog-id-cache
blog-lookup
bookmark
calendar
category
comment
counts
general
global-posts
options
plugins
post_ancestors
post_meta
posts
rss
site-lookup
site-options
site-transient
terms
themes
timeinfo
transient
user_meta
useremail
userlogins
usermeta
users
userslugs
widget

Reviews

Amazing performance improvement!

rambillo October 21, 2023

Install is not like other plugins — and in fact if you do upload it to plugins and then activate it, it’ll bring your site down in many cases. You can upload it (but don’t activate!) and then use the server side to move the plugin file manually into wp-content (and then delete the uploaded plugin folder and readme.txt). But that’s not enough to make the whole thing work! You also need memcached running on your server and properly configured, and the pecl memcache extension compiled on the server side in your php configuration, too. Make sure to use a unique salt as well. Once it’s all there, the speed improvement is significant! Happy to try and help those that respond to this thread if you need — or at least try to get you pointed in the right direction. Thanks for the plugin, devs! Appreciate as you update and keep current as well!

Can t install the plugin

kalagan80 September 4, 2023

Unable to install the plugin : application not recognized.

@terrykernan I get critical error when moving

markleeuw July 8, 2023

@terrykernan I moved like you said but then i get ciritcal error and can enter the Admin section of wordpress, when i remove it from WP-Content everything works again. I installed the plugin only because i have Redis on my host

Cannot redeclare wp_cache_add

terrykernan May 17, 2023 1 reply

Tip for others here, if you are getting this error: “Cannot redeclare wp_cache_add” It probably means you have the object-cache.php file in the wrong directory, it should not be in the plugins/memcached/ folder, it should be moved to the wp-content folder!

FATAL ERROR PHP

legarsfred April 5, 2023 1 reply

Fatal error: Cannot redeclare wp_cache_add() (previously declared in /htdocs/wp-includes/cache.php:41) in /htdocs/wp-content/plugins/memcached/object-cache.php on line 19

Works great!

Miguel February 16, 2023

It works well even with WPRocket Cache plugin. If you get fatal errors don’t forget to install PECL memcache module to PHP.

Contributors & Developers

Changelog

4.0.0

• Add preemptive filter pre_wp_cache_get
• Add flush_number replication to prevent accidental flush due to flush_number eviction, server rotation, etc.

3.2.2

• Remove filter, and base key stripping on presence of key_salt

3.2.1

• Fix bug allowing slow-ops entries to have the same key, so toggling doesn’t work

3.2.0

• Better stats(). Now shows cache group/individual calls with size of the payload and timings.
• PHP 5.6.20 is now required
• Fix get_multi to show per group calls
• Added filter (memcached_strip_keys) to bypass memcached key stripping
• Special group for slow-ops ( > 5ms ) with backtrace

3.1.0

• Add wp_cache_get_multi
• Add support for the $found parameter
• Set a variable for $max_expiration to 30 days
• Code style changes
• Different coloring for unknown stats group
• Store host/port on failure_callback
• Default stats counts

3.0.2

• Better output of HTML

3.0.1

• Fix key generation error in switch_to_blog()

3.0.0

• Flush site cache by rotating keys
• Flush global cache when flushing main site

2.0.6

• Flush the local cache on wp_cache_flush()

2.0.5

• Fix missing global in switch_to_blog

2.0.4

• Remove deprecated constructor

2.0.3

• Support for unix sockets

2.0.2

• Break references by cloning objects
• Keep local cache in sync with memcached when using incr and decr
• Handle limited environments where is_multisite() is not defined
• Fix setting and getting 0
• PHP 5.2.4 is now required
• Use the WP_CACHE_KEY_SALT constant if available to guarantee uniqueness of keys