Mod_cluster is an httpd-based load balancer. Like mod_jk and mod_proxy,
mod_cluster uses a communication channel to forward requests from httpd to one
of a set of application server nodes. Unlike mod_jk and mod_proxy, mod_cluster
leverages an additional connection between the application server nodes and
httpd. The application server nodes use this connection to transmit server-side
load balance factors and lifecycle events back to httpd via a custom set of
HTTP methods, affectionately called the Mod-Cluster Management Protocol (MCMP).
This additional feedback channel allows mod_cluster to offer a level of
intelligence and granularity not found in other load balancing solutions.