java - Tomcat load balancer solutions

Question

I'm looking for a good load balancer to use with Tomcat. Our application doesn't store anything in the session context so it isn't important to redirect to same server for the same user. I'd simply like something that can queue requests round-robin style or based on each server's inidividual load. I'd also like to be able to add application servers to those available to handle requests without having to restart the load balancer. We're running the application on linux if that matters.

Answer 1

If all you need is a software load balancer on linux use Apache Webserver2, Mod-Jk and Tomcat Clustering:

At your Webserver:

1. Install apache2 and modjk:

   sudo apt-get install apache2 libapache2-mod-jk
   sudo a2enmod jk
   

2. Create a workers.properties file available to your apache2. In some cases it's automatically created in your /etc/apache2 directory. This file is holding the lb config, node names, ips and ports of your Tomcat servers, i.e.:

   worker.list=balancer,lbstats
   
   #node1
   worker.node1.type=ajp13
   worker.node1.host=YOUR_TOMCAT-NODE-IP
   worker.node1.port=YOUR_TOMCAT-NODE-AJP-PORT
   worker.node1.lbfactor=10
   
   #more nodes here ... (change name in between)
   
   #lb config
   worker.balancer.type=lb
   #turn off sticky session
   worker.balancer.sticky_session=0
   
   #add all defined node names to this list (node1, node2, ...):
   worker.balancer.balance_workers=node1
   
   #lb status information (optional)
   worker.lbstats.type=status
   

3. Create a Mod-Jk section in your apache2 config file, if it has not been created automatically.

   JkWorkersFile   /etc/apache2/workers.properties
   JkLogFile       /var/log/apache2/mod_jk.log
   JkShmFile       /tmp/jk-runtime-status
   JkLogLevel      info
   

4. Mount your application to the load balancer (apache2 config file):

   JkMount /My-Java-App-Name       balancer
   JkMount /My-Java-App-Name/*     balancer
   
   JkMount /modjkstatus lbstats
   

At your Tomcat servers:

5. Install tomcat (using the tarball package, imho, way better then the apt verison). Change server.xml:

   1. disable the http connectors in server.xml (by commenting them out).

   2. enable AJP/1.3 connector and set the port you defined in workers.properties for this node.

   3. add jvmRoute with the right node name to the "Engine" element:

      <Engine jvmRoute="node1" ...

   4. add a "Cluster" element for simplest configuration

        <Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster" />
      

6. Deploy your application to all tomcats and add a distributable element to your applications web.xml.

      <distributable/>
   

7. Make sure the webserver can access the ajp ports on your tomcat servers and no one else can.

8. Start the webserver and the tomcats one after another and check the logs (/var/log/apache2/mod_jk.log, too).

9. Access your app: http://mywebserver.com/MyApp

10. Check (and deny access to) the lb status page http://mywebserver.com/modjkstatus