Apache Web Server to Tomcat Communication

mod_jk

mod_jk is an Apache httpd to Tomcat connector. It  handles the communication between Tomcat and Apache.

Basic Terms

• Worker

• A Tomcat instance. In most cases there is only a single worker (single Tomcat process) but sometimes you will run multiple workers to achieve load balancing or site partitioning.

• Each worker is identified to the web server by the host were it is located, the port where it listens and the communication protocol used to exchange messages.

• In-Process Worker - A special Tomcat worker that is run and executed on a JVM that the web server (Apache) opens inside the web server process address space.

• Web Server Plug-in/Tomcat Redirector - A Tomcat agent that sits on the web server (Apache) and redirects to Tomcat servlet requests. In our case it is mod_jk.

• Plug-in Configuration - We need to configure the web server plug-in so that it knows where the different Tomcat workers are and to which of them it should forward requests. This information, accompanied with some internal parameter, such as the log level, comprises the plug-in configuration. 

• Web Server Configuration - Each web server has some configuration that defines its behavior, e.g. on which port to listen, what files to serve, what web server plug-ins to load, etc. You will need to modify  your web server configuration to instruct it to load the Tomcat redirector mod_jk.