Load Balancer HOW-TO

 

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Using Apache 2 with mod_proxy and mod_rewrite

You can use Apache 2, with mod_proxy and mod_rewrite, to load balance requests to a set of Tomcat servers, using the HTTP protocol rather than AJP.

Edit $CATALINA_HOME/conf/server.xml, and find the Engine element and add an attribute called jvmRoute. Set its value to LB1. 

<Engine name="MainEngine" defaultHost="localhost" jvmRoute="LB1">

Do the same for each server in the group, giving each a different ID: LB2, LB3, ...

First we must create a rewrite map (for mod_rewrite) (balancing.conf): 

LB1  tomcat1.foo.com:8080
LB2  tomcat2.foo.com:8080
LB3  tomcat3.foo.com:8888
ALL  tomcat1.foo.com:8080|tomcat2.foo.com:8080|tomcat3.foo.com:8888

This means each worker is associated with a specific server and port. The name associated with each node must match the one used for the jvmRoute attribute specified on the Engine element of each node's server.xml.

In httpd.conf: 

RewriteMap SERVERS rnd:/usr/local/httpd/conf/servers.conf

<Location /webapp>
    RewriteEngine On

    RewriteCond "%{HTTP_COOKIE}"                  "(^|;\s*)jsessionid=\w*\.(\w+)($|;)"
    RewriteRule "(.*)"                            "http://${SERVERS:%2}%{REQUEST_URI}"  [P,L]
    RewriteRule "^.*;jsessionid=\w*\.(\w+)($|;)"  "http://${SERVERS:$1}%{REQUEST_URI}"  [P,L]
    RewriteRule "(.*)"                            "http://${SERVERS:ALL}%{REQUEST_URI}" [P,L]
</Location>

The first entry means that we want to load the server map which we created. The "ALL" entry in the server map will affect those users who do not yet have a session ID. Meaning the session cookie which Tomcat creates will have the LBx id in it, the other entries will govern which host we're directed to.

The rewrite rules govern those users with a cookie. Apache will rewrite the URL and point you to one of the instances of Tomcat. This is vodoo but it should work.

 

Using the balancer webapp

Tomcat 5.0.15 and later ships with a webapp named balancer. This is a simple implemention of a rules-based load balancer. It was not designed as a replacement for other load-balancing mechanisms used for high traffic environments. Rather, it is a simple, pure Java, easily extensible, and fast way to direct traffic among multiple servers.

Although balancer ships with Tomcat, it is not Tomcat-specific and runs on other containers without any modification. The balancer webapp requires a Servlet Specification 2.3 or later container if you wish to use a filter to redirect traffic. If you wish to redirect traffic using a servlet, you may use any servlet container.

 

Sample Configuration

The default balancer installation uses a single filter, BalancerFilter, mapped to all requests (url-pattern /*). The filter reads its rules from the location specified in the balancer deployment descriptor (web.xml file). The default rules are:

  • Redirect requests with News in the URL to www.cnn.com
  • Redirect requests with a parameter named paramName whose value is paramValue to www.yahoo.com.
  • Redirect all other requests to jakarta.apache.org.
Therefore, when you install tomcat, start it, and point your browser to http://localhost:8080/balancer, you will be redirected to http://jakarta.apache.org. If you point your browser to http://localhost:8080/balancer/News you will be redirected to http://www.cnn.com. The request for http://localhost:8080/balancer/BlahBlah?paramName=paramValue will be redirected to http://www.yahoo.com.

 

Balancer Rules

A Rule in the balancer system is a combination of a request matching criterion and a redirection URL for matching requests. Rules implement the org.apache.webapp.balancer.Rule interface.

The balancer distribution contains a number of useful rules. The framework is also designed for easy extensibility so that you can write your own rules quickly. Rules should be JavaBeans (public no-args constructor, public setter method setXXX for property xxx), as they are instantiated by Jakarta Commons Digester. Feel free to inquire on the tomcat-user mailing list regarding the availability of rules or the inclusion of your rules in the distribution.

Rules are assembled into RuleChains. Each BalancerFilter (or Servlet/JSP) refers to one RuleChain when making its redirection decisions. Note that you are not restricted to having one filter mapped to /* as done in the sample configuration. You can configure as many filters as desired, using the full filter mapping possibilities defined in the Servlet Specification. Each filter will have its own RuleChain.

 

How it Works

  1. You write a rules configuration file containing various rules and redirection locations.

  2. You define the balancer filter in your web.xml, mapping it as desired (/* is a common use-case) and configuring it with your rules configuration file.

  3. The server is started, initializing the filter.

  4. A request comes into the server. The filter consults its rule chain to determine where to redirect the request. Rules are consulted in the order in which they are defined in the rules configuration file. The first matching rule will stop the evaluation and cause the request to be redirected.