Tomcat High Availability & Clustering

Tomcat High Availability & Clustering

 

Tomcat servers need to be clustered. This requirement is for constant uptime.

 

The following solution brings up 2 Tomcat servers and one Apache server. The customer web-sites will contact the Apache web server. The Apache web server will divert the traffic to a Tomcat server.

 

Here I describe the method and some reference web pages:

 

1.      Install 2 copies of Tomcat server, on the same host or different hosts, depending on your requirements. Here I have given the example of installing on the same host. I named the servers tomcat1 and tomcat2.

 

2.      Open server.xml files for tomcat1 and tomcat2. Change the following line in tomcat1/conf/server.xml to:

 

    <Connector port="8091" maxHttpHeaderSize="8192"

               maxThreads="150" minSpareThreads="25" maxSpareThreads="75"

               enableLookups="false" redirectPort="8443" acceptCount="100"

               connectionTimeout="20000" disableUploadTimeout="true" />

 

3.      Change the following line in tomcat2/conf/server.xml to:

    <Connector port="8092" maxHttpHeaderSize="8192"

               maxThreads="150" minSpareThreads="25" maxSpareThreads="75"

               enableLookups="false" redirectPort="8443" acceptCount="100"

               connectionTimeout="20000" disableUploadTimeout="true" />

 

4.      Also on the second server, change the port in the server entry at the top. For example:

<Server port="8006" shutdown="SHUTDOWN">

 

5.      Download sources of mod_jk from the following URL:

http://tomcat.apache.org/connectors-doc/

 

Build mod_jk. If required download the following rpms:

apr-devel-1.2.7-10.i386.rpm

apr-util-devel-1.2.7-3.i386.rpm

 

 

6.      Modify Apache’s httpd.conf to add following line:

LoadModule jk_module   modules/mod_jk.so

 

7.      Edit the workers.properties file inside tomcat1/conf directory.

Set workers.tomcat_home=<tomcat installation directory>

This is where the conf files are. This has to be set only once.

 

8.      Set workers.java_home=/usr/java/jdk1.5.0_10

This is the Java Home directory.

 

9.      Set worker.list=tomcat1, tomcat2, loadbalancer

Think of loadbalancer as the root node for tomcat1 and tomcat2 as worker nodes.

 

10.  Set the following properties for the first worker thread:

worker.tomcat1.port=8009

worker.tomcat1.host=192.168.1.101

worker.tomcat1.type=ajp13

worker.tomcat1.lbfactor=1

Note that the port is the AJP port, not the default tomcat port.

 

11.  Set the following properties for the second worker thread:

worker.tomcat2.port=8010

worker.tomcat2.host=192.168.1.101

worker.tomcat2.type=ajp13

worker.tomcat2.connection_pool_timeout=600

worker.tomcat2.socket_keepalive=1

worker.tomcat2.socket_timeout=60

worker.tomcat2.lbfactor=1

Note that the port is the AJP port, not the default tomcat port.

 

12.  Set the following properties for the loadbalancer thread:

worker.loadbalancer.type=lb

worker.loadbalancer.balance_workers=tomcat1, tomcat2

 

13.  Set the following properties for the status thread:

# Add the status worker to the worker list

worker.list=jkstatus

 

# Define a 'jkstatus' worker using status

worker.jkstatus.type=status

 

14.  Edit the Apache’s httpd.conf file and add the following lines:

 

# JkWorkersFile must be full pathname of the file you created above

# in my case it was tomcat1/conf/workers.properties

JkWorkersFile workers.properties

 

# LogLevel could be info, error, debug etc. debug seems to work best

JkLogLevel debug

JkLogFile /etc/httpd/logs/mod_jk.log

 

# Pathname of the redirection

# This means localhost/servlets-examples/* will go to tomcat[12]

JkMount /servlets-examples/* loadbalancer

 

# Add the jkstatus mount point

JkMount /jkmanager/* jkstatus

 

JkMountCopy off

 

15.  Having configured Apache as our dispatcher, we now need to configure the Tomcat workers.

Make sure the following line is uncommented in your tomcat1/conf/server.xml file:

    <Connector port="8009"

               enableLookups="false" redirectPort="8443" protocol="AJP/1.3" />

 

Make sure the following line is uncommented in your tomcat2/conf/server.xml file:

    <Connector port="8010"

               enableLookups="false" redirectPort="8443" protocol="AJP/1.3" />

 

16.  Edit the Engine entry to have a jvmRoute matching the tomcatId entry in workers.properties:

For tomcat1/conf/server.xml file:

<Engine name="Catalina" defaultHost="localhost" jvmRoute="tomcat1">

 

For tomcat2/conf/server.xml file:

<Engine name="Catalina" defaultHost="localhost" jvmRoute="tomcat2">

 

17.  Identify Tomcat server – create two different files called me.html inside tomcat1/webapps/servlets-examples and tomcat2/webapps/servlets-examples.

 

18.  Make sure you restart Apache server and the Tomcat servers before you test.

Load http://host_ip_address/servlets-examples/me.html in browser. Reload it, it should load balancer between the servers.

 

19.  For troubleshooting open /etc/httpd/logs/mod_jk.log file and try to understand what the error could be.

 

20.  Note:

 

a.      Set export JRE_HOME=/usr/java/jdk1.5.0_10 before you start Tomcat server.

b.      To stop Tomcat, kill the Java process, don’t do shutdown, that didn’t work for me.

 

Phew!

 

References in order of relevance

1.      http://www.javaworld.com/javaworld/jw-12-2004/jw-1220-tomcat.html?page=1

2.      http://publib.boulder.ibm.com/iseries/v5r2/ic2924/info/rzaie/rzaiemod_jk.htm#jkworkersfile

3.      http://tomcat.apache.org/connectors-doc/generic_howto/workers.html

4.      http://tomcat.apache.org/connectors-doc/