1. Other software requirement

We use Apache web server to work with tomcat servers. Apache web server can be downloaded from http://www.apache.org

Besides, JK 1.2 library (mod_jk) is used and it can be downloaded from http://jakarta.apache.org/site/downloads/downloads_tomcat.html

For more information about how to install Apache and JK 1.2 library, please refer to my previous tutorials ?Install and Use Apache Web Server? and ?Use Apache with Web Application Server?.

In this article, we use two tomcat servers. In our machine, we created two tomcat installations.

2. Create Load Balance For Tomcat

In previous tutorial ?Use Apache with Web Application Server?, we have demonstrated that how Apache and Tomcat integrated.

We summarize the steps:

1. Create a workers.properties file to describe tomcat servers.

2. Add entries in Tomcat configuration file.

3. Add entries in Apache configuration (httpd.conf) file.

4. Start/restart Tomcat and Apache.

Now we will use the same steps to create load balance for tomcat server.

2.1 Create a workers.properties file

In the workers.properties file, we added a loadbalancer worker, and the worker.list now point to loadbalancer

List 1. workers.properties file

workers.tomcat_home=C:\jakarta-tomcat-5
workers.java_home=D:\jdk1.5.0
ps=\
worker.list=loadbalancer
worker.first.port=8007
worker.first.host=localhost
worker.first.type=ajp13
worker.first.lbfactor=1
worker.second.port=8009
worker.second.host=localhost
worker.second.type=ajp13
worker.second.lbfactor=1
worker.second.cachesize=5
worker.loadbalancer.type=lb
worker.loadbalancer.balanced_workers=first, second

Explanation

worker.list=loadbalancer

Apache uses loadbalancer to handle JSPs. Then loadbalancer delegates this job to registered workers (first and second).

2.2 Add entries in Tomcat configuration file

In C:\ jakarta-tomcat-5.5.9\conf\server.xml, we change port ?8080? to ?8081?, change ?8443? to ?8463?, so the two Tomcat server would not have conflicts in ports.

Also we change ?8009? to ?8007?, so load balance work can send servlet request to this port.

   <!-- Define an AJP 1.3 Connector on port 8007 -->
    <Connector port="8007" 
               enableLookups="false" redirectPort="8463" protocol="AJP/1.3" />

    <!-- You should set jvmRoute to support load-balancing via AJP ie :-->
    <Engine name="Standalone" defaultHost="localhost" jvmRoute="first">

should replace the original Engine definition.

We only need change the other Tomcat settings at C:\ jakarta-tomcat-5.5.9\conf\server.xml the following line

    <!-- You should set jvmRoute to support load-balancing via AJP ie :-->
    <Engine name="Standalone" defaultHost="localhost" jvmRoute="second">

2.3 Add entries in Apache configuration file

In httpd.conf file, we need add following lines:

List 2. Alias

Alias /jsp-examples/ "C:/jakarta-tomcat-5/webapps/jsp-examples/"
<Directory "C:/jakarta-tomcat-5/webapps/jsp-examples/">
    Options +Indexes FollowSymLinks
    AllowOverride None
    Order allow,deny
    Allow from all
</Directory>

We add alias definition in httpd.conf. This can ensure Apache server take care of the static files such as html and text files in C:/jakarta-tomcat-5/webapps/jsp-examples, where tomcat was installed.

Note that the load balance did not affect Apache server reading static contents, i.e., Apache web server reads html and text files.

We add alias definition in httpd.conf. This can ensure Apache server take care of the static files such as html and text files in C:/jakarta-tomcat-5/webapps/jsp-examples, where tomcat was installed.

List 3. Added to httpd.conf

<Location /*/WEB-INF/*>
	AllowOverride None
	Deny from all
</Location>

Above we don?t allow WEB-INF folder accessed from web server, because this folder has configuration files and other files that need to be hidden from web users.

List 4. Add to httpd.conf

 # Load mod_jk
LoadModule jk_module "C:/Program Files/Apache Group/Apache2/modules/mod_jk-1.2.13-apache-2.0.54.so"
# Configure mod_jk
JkWorkersFile "C:\jakarta-tomcat-5\conf\workers.properties"
JkLogFile "C:\jakarta-tomcat-5\logs\mod_jk.log"
JkLogLevel info
JkMount /jsp-examples/*.jsp loadbalancer
JkMount /jsp-examples/servlet/* loadbalancer

JkMount /jsp-examples/*.jsp loadbalancer
JkMount /jsp-examples/servlet/* loadbalancer

Save httpd.conf in Apache.

Explanation

If you compare the changes in httpd.conf with what we did in ?Use Apache with Web Application Server? article, we can see JSP/servlets are directed to loadbalancer worker, instead of individual workers. And List 1 workers.properties has loadbalancer worker assigning the job of processing jsp/servlets to individual workers (Tomcat application servers).

2.4 Start/restart Tomcat and Apache

Now we start the two Tomcat servers, and the Apache Server,

Type: http://localhost/jsp-examples/

We see Apache presents this static page. Click a JSP link, the two Tomcats server would be assigned JSP page by loadbalancer that we have set up.

To test that Tomcat servers process JSP/servlets in turn. We wrote the following pages:

List 5. load.jsp saved to ?C:\jakarta-tomcat-5.5.9\webapps\jsp-examples?

<html>
	<title>From Tomcat Server 1</title>
	<font color=blue size=+1>Hello from Tomcat Server 1</font>
</html>

List 6. load.jsp saved to ?C:\jakarta-tomcat-5\webapps\jsp-examples?

<html>
	<title>From Tomcat Server 2</title>
	<font color=red size=+1>Hello from Tomcat Server 2</font>
</html>

We save List 5 as load.jsp to the folder ?C:\jakarta-tomcat-5.5.9\webapps\jsp-examples?.
And we save List 6 as load.jsp to the folder ?C:\jakarta-tomcat-5\webapps\jsp-examples?.

Note: for the purpose of demonstration, we use different load.jsp files in two Tomcat server. In true production of load balanced Tomcat servers, we should use exact files in load balancing directories.

If we open two IE 6 windows, each type: http://localhost/jsp-examples/load.jsp

We see two windows are different?the first window comes from Tomcat server 1, and the second comes from another Tomcat server.