protected function getSystemHash()
    {
        if (!isset($GLOBALS['TYPO3_CONF_VARS']['SYS']['sitename'])) {
            throw new \InvalidArgumentException("No sitename set in TYPO3_CONF_VARS|SYS|sitename");
        }

        $siteName = $GLOBALS['TYPO3_CONF_VARS']['SYS']['sitename'];
        $systemKey = 'tx_solr' . $siteName;
        return GeneralUtility::hmac($systemKey);
    }