mod_cluster 是一个基于HTTP协议的负载均衡和集群的 Apache 模块，支持使用Tomcat的应用服务器，例如 Tomcat 和 JBoss AS
Where mod_cluster differs from mod_jk and mod_proxy is that it provides a back channel from the webservers back to the httpd servers. The webservers use this back channel to provide information to the httpd-side about their current state. The use of this back channel provides a number of advantages, including:
Dynamic configuration of httpd workers.
Server-side load balance factor calculation.
Fine grained web-app lifecycle control.
An added benefit of mod_cluster is that, unlike mod_jk, use of the AJP protocol between httpd and the back-end servers will be optional. The httpd connections to application server nodes can use HTTP, HTTPS, or AJP. (Note that in this first beta release, testing has been focused on AJP.)
