I recently faced an interesting challenge: a site has one primary domain name and several secondary names. The site(s) have the same codebase, the same database, but several domain names. The site used an object cache drop-in. Site administrators used only the primary domain for posting. All the administrative operations triggered cache updates only for the primary domain; the mirrors on the secondary domains could show stale data. Site owners somehow needed to disable the object cache for all domains except the primary one.
Surprisingly, WordPress has a enable_loading_object_cache_dropin filter which controls whether to enable loading of the
object-cache.php drop-in. That filter comes with one caveat, though:
This filter runs before it can be used by plugins. It is designed for non-web run-times. If false is returned, object-cache.php will never be loaded.
The above means that there is no way to write a plugin or a “must-use” plugin to control the loading of the drop-in.
However, there is a workaround (or a hack, if you will) to add a filter in your
wp-config.php.
wp_debug_mode() function gives a hint how to do this:
* This filter runs before it can be used by plugins. It is designed for * non-web run-times. Returning false causes theWP_DEBUGWP_DEBUGand related * constants to not be checked and the default PHP values for errors * will be used unless you take care to update them yourself. * * To use this filter you must define a$wp_filter$wp_filterglobal before * WordPress loads, usually inwp-config.phpwp-config.php. * * Example: * * $GLOBALS['wp_filter'] = array( * 'enable_wp_debug_mode_checks' => array( * 10 => array( * array( * 'accepted_args' => 0, * 'function' => function() { * return false; * }, * ), * ), * ), * );
Of course, we can employ the same technique for the
enable_loading_object_cache_dropin filter. All we need is to place this piece of code into wp-config.php above the require_once(ABSPATH . 'wp-settings.php'); line:
$GLOBALS['wp_filter'] = [
'enable_loading_object_cache_dropin' => [
10 => [
[
'accepted_args' => 1,
'function' => function ( $load ) {
/* Here comes the logic that determines whether WP should load object-cache.php */
return $load;
},
],
],
],
];