wp_start_object_cache()

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' ) );
	}
}