public function retryFailed($curlInstances)
    {
        $failed = 0;

        $curlMultiInstance = new CurlMulti();

        /**
         * @var $curlInstance Curl
         */
        foreach ($curlInstances as $curlInstance) {
            if ($curlInstance->isSuccessful()) {
                continue;
            }

            $curlInstance->setRetryCount($curlInstance->getRetryCount() + 1);

            $curlMultiInstance->addInstance($curlInstance);

            $failed++;
        }

        $curlMultiInstance->exec();

        return $failed;
    }