public function set($userId, $callId, $data)
    {
        $key = $this->getCacheKey($userId, $callId);
        $cache = $this->getFromCache($key);
        if (is_null($cache) || $cache != $data) {
            $this->putInCache($key, $userId, $callId, $data, self::STATE_PENDING_WRITE);
        }
        return true;
    }