6.0 Apache Tomcat Connector

Introduction

It is often useful to have Apache front Tomcat.

There are two main methods of doing this,

1. Apache Tomcat Connector - tried and true
2. Reverse Proxy - relatively new (Nov 4, 2009) and part of Apache

In our Enterprise environments we currently only use the tried and true Apache Tomcate Connector.

The Apache Tomcat Connector is often called by its binary file mod_jk which I will use from this point onwards.

Install mod_jk

There are two different methods of installing mod_jk. The simper being using apt-get if you have Ubuntu.

Using apt-get

With Ubuntu you can have mod_jk almost automatically install for you via,

sudo apt-get install libapache2-mod-jk

Running this command results in the following,

• /usr/lib/apache2/modules/mod_jk.so (this will not always be the most current - you can see the current version by looking at the package info)
• /etc/libapache2-mod-jk/workers.properties (create a generic workers.properties file for generic setup of Tomcat 6.x)
• /etc/apache2/mods-available/jk.load (file which loads the mod_jk module)
• /etc/apache2/mods-enabled/jk.load (symbolic link which starts the mod_jk module)

BUT it is not quite working yet. Continue reading.

Create jk.conf

The approach I take here is to make the mod_jk available at the global level to all Apache Virtual hosts. If we wanted things to be more granular you could instead put the contents of jk.conf in a specific virtual host.

mod_jk may be installed but it still needs to be configured. Create the file /etc/apache2/mods-available/jk.conf using sudo with the following contents,

<IfModule mod_jk.c>

JkWorkersFile "/etc/libapache2-mod-jk/workers.properties"
JkLogFile "/var/log/apache2/mod_jk.log"

# Use debug mode if you have trouble
# JkLogLevel debug

JkLogLevel info

</IfModule>

There are much more configuration options. But this should be enough to get you started for a small to medium scale website.

Enable mod_jk

In Ubuntu it is very easy to enable a module,

sudo a2enmod jk

This basically creates symbolic links for you in the /etc/apache2/mods-enabled directory. The two symbolic links will be,

1. jk.conf
2. jk.load

You can verify that the module loaded properly,

sudo apache2ctl -l
Compiled in modules:
  core.c
  mod_log_config.c
  mod_logio.c
  worker.c
  http_core.c
  mod_so.c

worker.c in the resulting output shows that the module loaded properly.

Configure Mod_jk

workers.properties Configuration File

Edit or create (in the case of a manual setup) /etc/libapache2-mod-jk/workers.properties which defines how Apache will pass the traffic to Tomcat.

# Define workers using ajp13
worker.list=tomcat_a_worker

# Set properties for worker
worker.tomcat_a_worker.type=ajp13
worker.tomcat_a_worker.host=localhost
worker.tomcat_a_worker.port=8009
worker.tomcat_a_worker.lbfactor=1

In the above case are creating a worker for a web application to connect to TomcatA which will run in the www.krypton.com virtualhost. Notice the host=localhost line in yellow. This should point to the server that Tomcat is running on. In this case, we are have Apache and Tomcat on the same server so we use localhost.

To keep the example simple I have not added load balancer support yet. If you want load do load balancing a simple configuration would look like this,

# Define workers using ajp13
worker.list=loadbalancer

# Set properties for worker
worker.tomcat_a_worker.type=ajp13
worker.tomcat_a_worker.host=localhost
worker.tomcat_a_worker.port=9009
worker.tomcat_a_worker.lbfactor=1

# Set properties for worker
worker.tomcat_b_worker.type=ajp13
worker.tomcat_b_worker.host=localhost
worker.tomcat_b_worker.port=8109
worker.tomcat_b_worker.lbfactor=1

# Set up load balancer using ajp13 workers
work.loadbalancer.type=lb
work.loadbalancer.balance_workers=tomcat_a_worker,tomcat_b_worker

The load balancing uses weighted round-robin with sticky sessions. The lower the lbfactor number the less weight and as such the less work done by the worker. In our example, since both tomcat a and b have a factor of 1 the load balance is split about 50/50.

Tin's Adjustments

What you have so far should work. However, if you want to match what I use in production I also made the following adjustments.

Look for this block and comment out.

# Tin: This directory does not exist.
#
# workers.tomcat_home=/usr/share/tomcat5

Look for this block and comment out.

# Tin: This directory does not exist.
#
# workers.java_home=/usr/lib/jvm/java-gcj

Configure Map Points in Apache

This last step will be different depending on if you are using virtual hosts or not.
 
First verify that you can hit the examples application without mod_jk by directly hitting the tomcat a server running on port 8080.

Launch a browser and to to, http://www.krypton.com:8080/examples/. You should see the following page show up.

In our examples we will be using virtual hosts. Once you get virtual hosts working, add this entry to your virtual host file.

JkMount /examples tomcat_a_worker
JkMount /examples/* tomcat_a_worker

If you are not using virtual hosts you can add the following to /etc/apache2/sites-enabled/000-default.

If you wanted to use the load balancer you would change tomcatA to loadbalancer.

Restart Apache

Finally you must restart Apache for the changes to take effect.

sudo /etc/init.d/apache2 restart

Now you should be able to view the examples site without specifying port 8080, http://www.krypton.com/examples/

Manually Installing mod_jk

I often have to do this on Solaris or alternative operating systems so the manual process of installing mod_jk is good to have handy

Resources

http://www.gustavomejia.com/blog/2008/03/02/1204455261015.html - seems to have decent instructions using ubuntu apt-get to set up mod_jk. Don't understand the java_home thing though.

http://wiki.zimbra.com/index.php?title=Zimbra_with_Apache_using_mod_jk - article from Zimbra