modproxybalancer

mod_proxy_balancer, sometimes referred to as modproxybalancer, is a module for the Apache HTTP Server that provides load balancing for requests proxied to multiple backend servers. It is part of the mod_proxy family and enables distribution of traffic across a pool of backends behind a single virtual URL. The module defines one or more balancer clusters, each identified by a BalancerName, and uses BalancerMember directives to register backend servers. Clients connect to Apache as the front end, while requests are routed to a suitable backend according to the configured balancing strategy.

Key features include configurable load balancing algorithms, session affinity, and retry behavior. The load balancing method is selected with ProxySet lbmethod, with common options such as byrequests, bytraffic, and bybusyness. Each BalancerMember can carry per-member options, for example retry to control how long to wait before retrying a failed member. Sticky sessions are supported by enabling a sticky cookie name via ProxySet stickysession or by configuring a member route, which helps maintain session affinity for applications that rely on server-side state.

Management and observability are provided by the BalancerManager interface, exposed via a URL such as /balancer-manager, which allows viewing status and enabling or disabling members. ProxyPass and ProxyPassReverse route requests to the balancer, typically in the form of ProxyPass / balancer://mycluster/ and ProxyPassReverse / balancer://mycluster/.

Example configuration (plain text):

<Proxy balancer://mycluster>

BalancerMember http://backend1.example.com:80

BalancerMember http://backend2.example.com:80 retry=5

ProxySet lbmethod=byrequests

ProxySet stickysession=JSESSIONID

</Proxy>

ProxyPass / balancer://mycluster/

ProxyPassReverse / balancer://mycluster/