wp_start_object_cache

/**
 * Start the WordPress object cache.
 *
 * If an object-cache.php file exists in the wp-content directory,
 * it uses that drop-in as an external object cache.
 *
 * @since 3.0.0
 * @access private
 *
 * @global array $wp_filter Stores all of the filters.
 */
function wp_start_object_cache() {
    global $wp_filter;

    $first_init = false;
    if ( ! function_exists( 'wp_cache_init' ) ) {
        if ( file_exists( WP_CONTENT_DIR . '/object-cache.php' ) ) {
            require_once ( WP_CONTENT_DIR . '/object-cache.php' );
            if ( function_exists( 'wp_cache_init' ) ) {
                wp_using_ext_object_cache( true );
            }

            // Re-initialize any hooks added manually by object-cache.php
            if ( $wp_filter ) {
                $wp_filter = WP_Hook::build_preinitialized_hooks( $wp_filter );
            }
        }

        $first_init = true;
    } elseif ( ! wp_using_ext_object_cache() && file_exists( WP_CONTENT_DIR . '/object-cache.php' ) ) {
        /*
         * Sometimes advanced-cache.php can load object-cache.php before
         * it is loaded here. This breaks the function_exists check above
         * and can result in `$_wp_using_ext_object_cache` being set
         * incorrectly. Double check if an external cache exists.
         */
        wp_using_ext_object_cache( true );
    }

    if ( ! wp_using_ext_object_cache() ) {
        require_once ( ABSPATH . WPINC . '/cache.php' );
    }

    /*
     * If cache supports reset, reset instead of init if already
     * initialized. Reset signals to the cache that global IDs
     * have changed and it may need to update keys and cleanup caches.
     */
    if ( ! $first_init && function_exists( 'wp_cache_switch_to_blog' ) ) {
        wp_cache_switch_to_blog( get_current_blog_id() );
    } elseif ( function_exists( 'wp_cache_init' ) ) {
        wp_cache_init();
    }

    if ( function_exists( 'wp_cache_add_global_groups' ) ) {
        wp_cache_add_global_groups( array( 'users', 'userlogins', 'usermeta', 'user_meta', 'useremail', 'userslugs', 'site-transient', 'site-options', 'blog-lookup', 'blog-details', 'site-details', 'rss', 'global-posts', 'blog-id-cache', 'networks', 'sites' ) );
        wp_cache_add_non_persistent_groups( array( 'counts', 'plugins' ) );
    }
}