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.
Within httpd, mod_cluster is implemented as a set of modules for httpd with mod_proxy enabled. Much of the logic comes from mod_proxy, e.g. mod_proxy_ajp provides all the AJP logic needed by mod_cluster.