Apache Module mod_proxy_balancer

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Description:	`mod_proxy` extension for load balancing
Status:	Extension
Module Identifier:	proxy_balancer_module
Source File:	mod_proxy_balancer.c
Compatibility:	Available in version 2.1 and later

Summary

This module requires the service of mod_proxy and it provides load balancing for all the supported protocols. The most important ones are:

HTTP, using mod_proxy_http
FTP, using mod_proxy_ftp
AJP13, using mod_proxy_ajp
WebSocket, using mod_proxy_wstunnel

The Load balancing scheduler algorithm is not provided by this module but from other ones such as:

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.

Directives

This module provides no directives.

Bugfix checklist

Load balancer scheduler algorithm

At present, there are 4 load balancer scheduler algorithms available for use: Request Counting (mod_lbmethod_byrequests), Weighted Traffic Counting (mod_lbmethod_bytraffic), Pending Request Counting (mod_lbmethod_bybusyness) and Heartbeat Traffic Counting (mod_lbmethod_heartbeat). These are controlled via the lbmethod value of the Balancer definition. See the ProxyPass directive for more information, especially regarding how to configure the Balancer and BalancerMembers.

Load balancer stickyness

The balancer supports stickyness. When a request is proxied to some back-end, then all following requests from the same user should be proxied to the same back-end. Many load balancers implement this feature via a table that maps client IP addresses to back-ends. This approach is transparent to clients and back-ends, but suffers from some problems: unequal load distribution if clients are themselves hidden behind proxies, stickyness errors when a client uses a dynamic IP address that changes during a session and loss of stickyness, if the mapping table overflows.

The module mod_proxy_balancer implements stickyness on top of two alternative means: cookies and URL encoding. Providing the cookie can be either done by the back-end or by the Apache web server itself. The URL encoding is usually done on the back-end.

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"
ProxyPassReverse "/test" "balancer://mycluster"

Another example of how to provide load balancing with stickyness using mod_headers, even if the back-end 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"
ProxyPassReverse "/test" "balancer://mycluster"

Exported Environment Variables

At present there are 6 environment variables exported:

BALANCER_SESSION_STICKY: This is assigned the stickysession value used for the current request. It is the name of the cookie or request parameter 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 of 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
    Require host 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. Please note that only Balancers defined outside of <Location ...> containers can be dynamically controlled by the Manager.

Details on load balancer stickyness

When using cookie based stickyness, you need to configure the name of the cookie that contains the information about which back-end to use. This is done via the stickysession attribute added to either ProxyPass or ProxySet. The name of the cookie is case-sensitive. The balancer extracts the value of the cookie and looks for a member worker with route equal to that value. The route must also be set in either ProxyPass or