public function get_cache_shared($name, $packed=true)
    {
        $shared_name = "shared_$name";

        if (!array_key_exists($shared_name, $this->caches)) {
            $opt  = strtolower($name) . '_cache';
            $type = $this->config->get($opt);
            $ttl  = $this->config->get($opt . '_ttl');

            if (!$type) {
                // cache is disabled
                return $this->caches[$shared_name] = null;
            }

            if ($ttl === null) {
                $ttl = $this->config->get('shared_cache_ttl', '10d');
            }

            $this->caches[$shared_name] = new rcube_cache_shared($type, $name, $ttl, $packed);
        }

        return $this->caches[$shared_name];
    }