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It may be necessary to use the HTTP server as a front-end for Tomcat. For example, you may want to keep more than one application server on the same host, and/or you want to access these app servers with the separate DNS names, without adding a port to the URL.
There are two methods that allow you to "glue" Apache HTTP Daemon and Tomcat application server:
via the HTTP protocol using proxy module.
via the Apache JServ Protocol using Tomcat connector or HTTPD AJP proxy module.