protected function addServers($cache, $servers)
    {
        $existingServers = [];
        if ($this->persistentId !== null) {
            foreach ($cache->getServerList() as $s) {
                $existingServers[$s['host'] . ':' . $s['port']] = true;
            }
        }
        foreach ($servers as $server) {
            if (empty($existingServers) || !isset($existingServers[$server->getHost() . ':' . $server->getPort()])) {
                $cache->addServer($server->getHost(), $server->getPort(), $server->getWeight());
            }
        }
    }