public function write($key, $data) {
        $cache = Zend_Registry::get('CacheFrontend');
        // include the current version in the cached results
        $version = (int)$cache->load($this->_versionKey);
        // save the data to cache but include the version number
        return $cache->save(
            array(
                'data' => $data,
                'version' => $version
            ),
            $key
        );
    }