mod_proxy_balancer mod_proxy extension for load balancing Extension mod_proxy_balancer.c proxy_balancer_module Available in version 2.1 and later

This module requires the service of mod_proxy. It provides load balancing support for HTTP, FTP and AJP13 protocols

Load balancing scheduler algorithm is provided by not this module but other modules such as: mod_lbmethod_byrequests, mod_lbmethod_bytraffic and mod_lbmethod_bybusyness.

Thus, in order to get the ability of load balancing, mod_proxy, mod_proxy_balancer and at least one of load balancing scheduler algorithm modules have to be present in the server.

Warning

Do not enable proxying until you have secured your server. Open proxy servers are dangerous both to your network and to the Internet at large.

 mod_proxy
Load balancer scheduler algorithm

At present, there are 3 load balancer scheduler algorithms available for use: Request Counting, Weighted Traffic Counting and Pending Request Counting. These are controlled via the lbmethod value of the Balancer definition. See the ProxyPass directive for more information.

Examples of a balancer configuration

Before we dive into the technical details, here's an example of how you might use mod_proxy_balancer to provide load balancing between two back-end servers:

<Proxy balancer://mycluster>
BalancerMember http://192.168.1.50:80
BalancerMember http://192.168.1.51:80
</Proxy>
ProxyPass /test balancer://mycluster

Another example of how to provide load balancing with stickyness using mod_headers, even if the backend server does not set a suitable session cookie:

Header add Set-Cookie: "ROUTEID=.%{BALANCER_WORKER_ROUTE}e; path=/" env=BALANCER_ROUTE_CHANGED
<Proxy balancer://mycluster>
BalancerMember http://192.168.1.50:80 route=1
BalancerMember http://192.168.1.51:80 route=2
ProxySet stickysession=ROUTEID
</Proxy>
ProxyPass /test balancer://mycluster
Exported Environment Variables

At present there are 6 environment variables exported:

BALANCER_SESSION_STICKY

This is assigned the stickysession value used in the current request. It is the cookie or parameter name used for sticky sessions

BALANCER_SESSION_ROUTE

This is assigned the route parsed from the current request.

BALANCER_NAME

This is assigned the name of the balancer used for the current request. The value is something like balancer://foo.

BALANCER_WORKER_NAME

This is assigned the name of the worker used for the current request. The value is something like http://hostA:1234.

BALANCER_WORKER_ROUTE

This is assigned the route of the worker that will be used for the current request.

BALANCER_ROUTE_CHANGED

This is set to 1 if the session route does not match the worker route (BALANCER_SESSION_ROUTE != BALANCER_WORKER_ROUTE) or the session does not yet have an established route. This can be used to determine when/if the client needs to be sent an updated route when sticky sessions are used.

Enabling Balancer Manager Support

This module requires the service of mod_status. Balancer manager enables dynamic update of balancer members. You can use balancer manager to change the balance factor or a particular member, or put it in the off line mode.

Thus, in order to get the ability of load balancer management, mod_status and mod_proxy_balancer have to be present in the server.

To enable load balancer management for browsers from the example.com domain add this code to your httpd.conf configuration file

<Location /balancer-manager>
SetHandler balancer-manager

Order Deny,Allow
Deny from all
Allow from .example.com
</Location>

You can now access load balancer manager by using a Web browser to access the page http://your.server.name/balancer-manager