Tomcat sticky session

Common web architecture is Web – WAS – (DB). For example, Apache as Web Server and Tomcat as WAS. In this case, mod_jk is is used to connect from Apache to Tomcat. The following is Web to WAS load balancing architecture.

With this architecture, multiple http request is sent to Tomcat#1 and Tomcat#2 in round robin. If the server application use servlet session, the following scenario happens.

step 1) A client connects to Tomcat#1. Tomcat#1 creates a session

step 2) the client connects to Tomcat#2 with previous session. But Tomcat#2 does not know the session, therefore previous status at Tomcat#1 can not be continued (when session cluster is not enabled)

To resolve this problem, Tomcat and Apache supports sticky session. With sticky session, mod_jk routes the request to the Tomcat which generated the session.  For the previous scenario, step2 goes to Tomcat#1 although mod_jk is working in load balancing.

How to setup sticky session

1) To name each Tomcat instance

Inside server.xml, instance name must be defined.

<Engine name="Catalina" defaultHost="localhost" jvmRoute="test1">

Each instance must have unique name.

2) To enable mod_jk

To enable mod_jk in Apache http server, the following config is needed.

httpd.conf
… Include conf/mod_jk.conf …

mod_jk.conf
LoadModule jk_module modules/mod_jk.so    JkWorkersFile conf/workers.properties    JkLogFile “|/usr/local/httpd-2.2.27/bin/rotatelogs -l /usr/local/httpd-2.2.27/logs/mod_jk_test.log.%Y%m%d 86400”    JkLogLevel info    JkShmFile logs/mod_jk_test.shm    JkMountFile conf/uri.properties    JkWatchdogInterval 60

• mod_jk directives

workers.properties
worker.list=test_worker worker.test_worker.type=lb worker.test_worker.balance_workers=test1, test2 worker.test1.host=host_ip worker.test1.port=8009 worker.test1.type=ajp13 worker.test2.host=host_ip worker.test2.port=8009 worker.test2.type=ajp13

• test_worker, test1 and test2 are worker name.
• test_worker is load balancer. It balances test1 and test2.
• test1 and test2 are ajp worker which point to each Tomcat instance.
• ajp worker name must be same with jvmRoute in Tomcat server.xml
• More options

uri.properties
/*=test_worker

• This file defines which worker is used for which url pattern. This example shows that all requests are handled by test_worker (the name must be declared in workers.properties)

How sticky session works

1. If jvmRoute attribute is set, Tomcat generates a session id with the following format
   session_random_value.jvmRouteValue
   (ex. 9E4E7AE669AEFDF68CC03A35E4636646.test1)
2. This session id is returned to client as a cookie
   ex) Cookie : JSESSIONID=9E4E7AE669AEFDF68CC03A35E4636646.test1
3. Next time, the client connects with the cookie
4. mod_jk parses JSESSIONID and extracts server name (for this example, test1)
5. mod_jk sends the request to the same named worker (which is defined at workers.properties)

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