private function canPurge()
    {
        // check if purge header exists
        if (!$this->request->matchHeader(self::H_PURGE)) {
            return false;
        }

        if ($this->controlKey == '') {
            return true; // everybody can purge
        }

        // only users with valid control key can purge
        if ($this->request->matchHeader(self::H_CKEY, $this->controlKey)) {
            return true;
        }

        return false;
    }