protected function get_cache_identifier() {
        $identifiers = array();
        if ($this->selectedsharingoption & self::SHARING_ALL) {
            // Nothing to do here.
        } else {
            if ($this->selectedsharingoption & self::SHARING_SITEID) {
                $identifiers[] = cache_helper::get_site_identifier();
            }
            if ($this->selectedsharingoption & self::SHARING_VERSION) {
                $identifiers[] = cache_helper::get_site_version();
            }
            if ($this->selectedsharingoption & self::SHARING_INPUT && !empty($this->userinputsharingkey)) {
                $identifiers[] = $this->userinputsharingkey;
            }
        }
        return join('/', $identifiers);
    }