Apache module mod_athena manual

• Summary
• Install / Compile / Tune
• Architectural suggestions
• Securing the system
• Configuration suggestions
• Directives
• Sending statistics
• Runtime management
• Querying the balancer

Description:	Full-featured reverse-proxying and/or stand-alone load balancer
Status:	Extension
Module Identifier:	athena_module
Source File:	mod_athena.c (plus others)
Compatibility:	v 1.x Requires httpd-2.0.43 or newer
Compatibility:	v 2.x Requires httpd-2.2.0 or newer

Summary

This module is designed to allow httpd to act as a load balancer, either internally to apache's own mod_proxy (for reverse proxying), or externally to machines querying it. Arbitrary statistics are sent to the engine via a simple GET plus query-string interface, from which it will then make decisions based on chosen algorithms. You are able to manage farms of servers, mark them in different states, and forward disabled or down systems or farms to new targets, among other administrative features. In version 2.x (requires httpd-2.2.x), you can manipulate loadbalancing decisions with your back-end application by setting a secret key secured cookie that the load balancer will intercept and use to modify the algorithm. This feature allows you to maintain stick sessions to a specific server, or prioritize farms using business rules (QoS).

If you are using commercial hardware or software to load balance protocols that can be handled by apache's mod_proxy, then you should be able to use this as a drop in replacement.

In the reverse http(s) proxy configuration, a request from a client is answered by apache. The request is handled by mod_proxy, which would then rewrite the url to substitute a mod_athena "AthFarm" where the target hostname usually resides.

httpd.conf: ProxyPass     /reports/     http://farm_one/reports/

Then, mod_athena takes over the request. It will look up the farm, run the algorithm and analyze health status for that farm, and then substitute a real address for for the farm name. If the farm or servers are disabled or all are sick, a complete alternative URL will be substitued. The result is handed back to mod_proxy to finish the request.

pseudo-code: http://farm_one/reports/ --> http://chosen_server/reports/

pseudo-code: http://farm_one/reports/ --> http://www.yoursite.com/outage/

In the query configuration, a client (in this case probably another application) sends a GET request with a query string containing the desired farm(s) to httpd.

> fetch -q -o - http://my_athena/ath/balance/?farm_one
chosen_server

The module engine will catch this request, run the appropriate algorithm(s), and return the appropriate result(s) as text content to the client.

You can easily use mod_athena in both modes in the same instance of the engine.

The package comes with scripts that can be used to mirror and monitor the system, along with a perl package that provides an HTML based front end for runtime management.