Mod qos
mod_qos is a Quality of Service (QoS) module for the Apache HTTP server implementing control mechanisms that can provide different priority to different requests.
A web server can only serve a limited number of concurrent requests. QoS is used to ensure that important resources stay available under high server load. mod_qos is used to reject requests to unimportant resources while granting access to more important applications. It is also possible to disable access restrictions, e.g. for requests to very important resources or for very important users.
Control mechanisms are available on two levels:
- Request level control: mod_qos controls the number of concurrent requests to a name space (URL). It is used to define different priorities to different pages or applications within a web server.
- Connection level control: mod_qos controls the number of TCP connections to the web server. This helps to limit the connections coming from a single client, to reduce the maximum of concurrent connections to a virtual server or to implement dynamic HTTP keep-alive settings.
The module can be useful when used in a reverse proxy in order to divide up resources to different webserver but also to prevent denial-of-service attack.