public function addTransaction(TransactionInterface $transaction, $handle)
    {
        if (isset($this->handles[$transaction])) {
            throw new AdapterException('Transaction already registered');
        }

        $code = curl_multi_add_handle($this->multi, $handle);
        if ($code != CURLM_OK) {
            MultiAdapter::throwMultiError($code);
        }

        $this->handles[$transaction] = $handle;
    }