public function setRequestsPerTimeSpan($requests)
    {
        if (!is_numeric($requests)) {
            throw new InvalidArgumentException('requests per timespan is not numeric');
        }
        $requests = (int) $requests;
        if ($requests < 1) {
            throw new InvalidArgumentException('requests per timespan cannot be smaller than 1');
        }
        $this->requestsPerTimeSpan = $requests;
    }