Tomcat Load Balancer using Apache 'mod_proxy_balancer'

1. Setup Overview:

A simple load balancing setup between two Tomcat web servers using ‘mod_proxy’ and ‘mod_proxy_balancer’ Apache modules.

Both the web servers are hosted on SLES 11.1.

2. Tomcat Setup

Apache Tomcat doesn’t come pre-installed with SLES-11.1 OS. Follow the steps below to install Tomcat on both the servers under ‘/webserver’ folder.

a. Download Apache Tomcat tarball ‘apache-tomcat-6.0.32.tar.gz’, from the Tomcat site.

b. Extract Tomcat tarball under ‘/webserver’ folder. The extracted folder will be ‘apache-tomcat-6.0.32’.

d. Create a soft link to the extracted tomcat folder in the same location, by executing the following command,

ln -s /webserver/apache-tomcat-6.0.32 /webserver/active-tomcat

e. Go into the bin directory of tomcat and start tomcat by executing the following script,

./startup.sh

f. Tomcat will now be up and running. You can verify this by accessing the following URL from your web browser, ‘http://tomcat-A:8080/’ or ‘http://tomcat-B:8080/’.

g. To auto start Tomcat during boot time, configure it as mentioned below.

(i) Copy the following content into a new file by the name ‘tomcat’ under ‘/etc/init.d’.

#!/bin/bash

#

# description: Start up the Tomcat servlet engine.

RETVAL=$?

CATALINA_HOME="/webserver/active-tomcat"

case "$1" in

start)

if [ -f $CATALINA_HOME/bin/startup.sh ];

then

echo $"Starting Tomcat"

$CATALINA_HOME/bin/startup.sh

fi

;;

stop)

if [ -f $CATALINA_HOME/bin/shutdown.sh ];

then

echo $"Stopping Tomcat"

$CATALINA_HOME/bin/shutdown.sh

fi

;;

restart)

$0 stop

sleep 5

$0 start

;;

*)

echo $"Usage: $0 {start|stop|restart}"

exit 1

;;

esac

exit $RETVAL

(ii) Save the file above.

(iii) Change the permission of this script and make it executable. Run the following command to do so.

chmod 754 /etc/init.d/tomcat

(iv) Enable the script to start in the run levels 3, 4, and 5, by executing the following command.

chkconfig -s tomcat 345

3. Apache Configuration:

You can have Apache configured on a separate server as shown in the diagram above. The Apache version used here is 2.2.10.

Firstly, ensure that Apache service always runs in the desired run level. To verify this, execute the following command.

chkconfig --list | grep apache2

If this results in ‘off’ status for all the run levels, then turn on ‘apache2’ to run always in run level 3, 4, and 5, by executing the following command.

chkconfig -s apache2 345

Start the Apache service by the running the following command.

/etc/init.d/apache2 start

We use Apache's 'mod_proxy_balancer' module to configure the load balancer. This configuration is useful for applications that are stateless and therefore do not require clustering or sticky-sessions.

We need to initially load the 'mod_proxy' and 'mod_proxy_balancer' module. To use the Balancer Manager (for finding out load balancing statistics), the 'mod_status' module is loaded as well.

Execute the following commands to load the above modules.

a2enmod proxy

a2enmod proxy_balancer

a2enmod status

To verify whether the modules are loaded, run the following command.

apache2ctl -t -D DUMP_MODULES

In the output you should be able to locate the above loaded modules.

Later in the Apache2 configuration file, '/etc/apache2/httpd.conf', append the following lines to configure the balancer.

# Load Balancing Tomcat Servers on tomcat-A and tomcat-B
<IfModule mod_proxy_balancer.c>
  <Location "/balancer-manager">
    SetHandler balancer-manager
  </Location>


  <Proxy balancer://mycluster>
    BalancerMember http://tomcat-A:8080/ loadfactor=1
    BalancerMember http://tomcat-B:8080/ loadfactor=1
  </Proxy>


  ProxyPass /lb-test balancer://mycluster
</IfModule>

Now restart Apache to start the load balancer.

/etc/init.d/apache2 restart

The service should be up and running without any errors.

4. Verifying the Load Balancer

To verify the working of the load balancer, change the ‘/webserver/active-tomcat/webapps/ROOT/index.html’ configuration file in Tomcat on both the servers.

For the configuration file on tomcat-A update the following line (~ line 97),

Apache Tomcat

as,

Apache Tomcat - tomcat-A

Similarly, for the configuration file on tomcat-B update the same line as,

Apache Tomcat - tomcat-B

Restart tomcat web server on both the servers, by executing the following commands from their ‘bin’ folders.

./shutdown.sh

./startup.sh

Tomcat should be up and running on port 8080 on the respective server. Access the Tomcat URL on tomcat-A ‘http://tomcat-A:8080/’, from your web browser. This should show up ‘Apache Tomcat - tomcat-A’ on the index page. Similarly when you access ‘http://tomcat-B:8080/’, from your web browser, the index page should show up ‘Apache Tomcat - tomcat-B’.

Access the following URL to verify the load balancer statistics, ‘http://apache-proxy/balancer-manager’. This should show column ‘Factor’ set as ‘1’ and ‘Method’ as ‘byrequests’, that means the load should be evenly balanced in a round-robin fashion. The ‘Elected’ column shows the value as ‘0’ after Apache is configured for Load Balancing and restarted.

Access the balancer URL to verify the Tomcat Load Balancing between ‘tomcat-A’ and ‘tomcat-B’, ‘http://apache-proxy/lb-test/’. Keep refreshing this web page; you will notice that the browser alters displaying pages containing ‘Apache Tomcat - tomcat-A’ and ‘Apache Tomcat - tomcat-B’. To understand the statistics of the load balancer, again access the balancer statistics page, ‘http://apache-proxy/balancer-manager’. This will now show values in the ‘Elected’ column. The value will either be the same for both the worker URL’s or one less than the other. This indicates that the load is getting evenly balanced across both the servers.

Now, stop the Tomcat web server on ‘tomcat-B’, and now check the balancer URL, ‘http://apache-proxy/lb-test/’. The page will display only ‘Apache Tomcat - tomcat-A’, because ‘tomcat-B’ is no more serving the web page. Also verify the balancer statistics page, ‘http://apache-proxy/balancer-manager’. This will show the ‘Status’ column for ‘tomcat-B’ worker URL as ‘Err’, that means the worker is not serving the pages on that URL. If you keep refreshing the balancer URL again and again, the statistics page will show that ‘tomcat-A’ will always get ‘Elected’ to serve the pages as its value keeps increasing.

If you start Tomcat on ‘tomcat-B’ again, the balancer setup will be back to normal and serve as expected.