public function exists($storage_name, $key = null)
    {
        // No app found = false
        if (!isset($this->storage[$storage_name])) {
            return false;
        }

        // app found and no key requested? true
        if (!isset($key)) {
            return true;
        }

        // key requested and found? true
        return isset($this->storage[$storage_name]->{$key}) && !empty($this->storage[$storage_name]->{$key});
    }