public function fetch($url, $key)
    {
        $bucketMgr = $this->getBucketManager();
        list($ret, $err) = $bucketMgr->fetch($url, $this->bucket, $key);
        if ($err !== null) {
            $this->logQiniuError($err);
            return false;
        } else {
            return $ret['key'];
        }
    }