public function clean($mode = 'all', $tags = array())
    {
        if (!$this->_options['caching']) {
            return true;
        }
        if (!in_array($mode, array(Zend_Cache::CLEANING_MODE_ALL,
                                   Zend_Cache::CLEANING_MODE_OLD,
                                   Zend_Cache::CLEANING_MODE_MATCHING_TAG,
                                   Zend_Cache::CLEANING_MODE_NOT_MATCHING_TAG,
                                   Zend_Cache::CLEANING_MODE_MATCHING_ANY_TAG))) {
            Zend_Cache::throwException('Invalid cleaning mode');
        }
        self::_validateTagsArray($tags);

        return $this->_backend->clean($mode, $tags);
    }