Running Multiple Tomcat Instances on One Server

Here’s a brief step by step guide to running more than one instance of Tomcat on a single machine.

Step 1: Install the Tomcat files

Download Tomcat 5.5, 6.x, or 7.x and unzip it into an appropriate directory. I usually put it in /usr/local, so it ends up in a directory called /usr/local/apache-tomcat-5.5.17, and make a symlink named /usr/local/tomcat to that directory. When later versions come out, I can unzip them and relink, leaving the older version in case things don’t work out (which rarely if ever happens, but I’m paranoid).

Step 2: Make directories for each instance

For each instance of Tomcat you’re going to run, you’ll need a directory that will be CATALINA_BASE. For example, you might make them /var/tomcat/serverA and /var/tomcat/serverB.

In each of these directories you need the following subdirectories: conf, logs, temp, webapps, and work.

Put a server.xml and web.xml file in the conf directory. You can get these from the conf directory of the directory where you put the tomcat installation files, although of course you should tighten up your server.xml a bit.

The webapps directory is where you’ll put the web applications you want to run on the particular instance of Tomcat.

I like to have the Tomcat manager webapp installed on each instance, so I can play with the webapps, and see how many active sessions there are. See my instructions for configuring the Tomcat manager webapp.

Step 3: Configure the ports and/or addresses for each instance

Tomcat listens to at least two network ports, one for the shutdown command, and one or more for accepting requests. Two instances of Tomcat can’t listen to the same port number on the same IP address, so you will need to edit your server.xml files to change the ports they listen to.

The first port to look at is the shutdown port. This is used by the command line shutdown script (actually, but the Java code it runs) to tell the Tomcat instance to shut itself down. This port is defined at the top of the server.xml file for the instance.

<Server port="8001" shutdown="_SHUTDOWN_COMMAND_" debug="0">

Make sure each instance uses a different port value. The port value will normally need to be higher than 1024, and shouldn’t conflict with any other network service running on the same system. The shutdown string is the value that is sent to shut the server down. Note that Tomcat won’t accept shutdown commands that come from other machines.

Unlike the other ports Tomcat listens to, the shutdown port can’t be configured to listen to its port on a different IP address. It always listens on 127.0.0.1.

The other ports Tomcat listens to are configured with the <Connector> elements, for instance the HTTP or JK listeners. The port attribute configures which port to listen to. Setting this to a different value on the different Tomcat instances on a machine will avoid conflict.

Of course, you’ll need to configure whatever connects to that Connector to use the different port. If a web server is used as the front end using mod_jk, mod_proxy, or the like, then this is simple enough - change your web server’s configuration.

In some cases you may not want to do this, for instance you may not want to use a port other than 8080 for HTTP connectors. If you want all of your Tomcat intances to use the same port number, you’ll need to use different IP addresses. The server system must be configured with multiple IP addresses, and the address attribute of the <Connector> element for each Tomcat instance will be set to the appropriate IP address.

Step 4: Startup

Startup scripts are a whole other topic, but here’s the brief rundown. The main different from running a single Tomcat instance is you need to set CATALINA_BASE to the directory you set up for the particular instance you want to start (or stop). Here’s a typical startup routine:

JAVA_HOME=/usr/java
JAVA_OPTS="-Xmx800m -Xms800m"
CATALINA_HOME=/usr/local/tomcat
CATALINA_BASE=/var/tomcat/serverA
export JAVA_HOME JAVA_OPTS CATALINA_HOME CATALINA_BASE

$CATALINA_HOME/bin/catalina.sh start