public function persist($userId = null, $callId = null)
    {
        if ($userId && $callId) {
            $keys = [$this->getCacheKey($userId, $callId)];
        } else {
            $keys = array_keys($this->cache);
        }

        $success = true;
        foreach ($keys as $key) {
            if (!$this->persistCacheEntry($key)) {
                $success = false;
            }
        }
        return $success;
    }