mod_qos is a quality of service module for the Apache Web Server. It implements control mechanisms that can provide different priority to different requests and controls server access based on available resources.

mod_qos may be used to determine which requests should be served and which shouldn't in order to avoid resource oversubscription. The module collects different attributes such as the request URL, HTTP request and response headers, the IP source address, the HTTP response code, history data (user session and on per source IP address basis), the number of concurrent requests to the server (total or requests having similar attributes), the number of concurrent TCP connections (total or from a single source IP), and so forth.