{"id":78,"date":"2009-02-16T14:16:45","date_gmt":"2009-02-16T21:16:45","guid":{"rendered":"http:\/\/www.zulutown.com\/blog\/?p=78"},"modified":"2009-02-16T14:16:45","modified_gmt":"2009-02-16T21:16:45","slug":"java-ee-load-balancing-with-tomcat-and-apache","status":"publish","type":"post","link":"https:\/\/www.zulutown.com\/blog\/2009\/02\/16\/java-ee-load-balancing-with-tomcat-and-apache\/","title":{"rendered":"Java EE Load Balancing with Tomcat and Apache"},"content":{"rendered":"

This tutorial explains how to configure an Apache HTTPD<\/em> server to map a specific path on a series of load-balanced Apache Tomcat<\/em>.
\nThe first step is to define the Virtual Host<\/em> in the Apache configuration files.
\nIn this case the root directory (on file system) of the site is located in \/path\/to\/your\/site\/<\/code>, the name of the site is www.yoursite.com<\/code> and the path where the Tomcat servers may be reached is \/javaee<\/code>.<\/p>\n

In few words, an URL like http:\/\/www.yoursite.com\/home.html<\/code> is mapped on the file \/path\/to\/your\/site\/home.html<\/code>.<\/p>\n

An URL like http:\/\/www.yoursite.com\/javaee\/hello.jsp<\/code> is mapped to the hello.jsp<\/code> file contained in javaee.war<\/code> application deployed on all the Tomcat servers defined in the load balanced cluster<\/em>.<\/p>\n

The configuration of the Apache virtual host:<\/p>\n

<VirtualHost *>\r\n\tServerAdmin webmaster@localhost\r\n\tServerName www.yoursite.com\r\n\tDocumentRoot \/path\/to\/your\/site\/\r\n\t<Directory \/path\/to\/your\/site\/>\r\n\t\tOptions MultiViews\r\n\t\tAllowOverride All\r\n\t\tOrder allow,deny\r\n\t\tallow from all\r\n\t<\/Directory>\r\n\r\n\tErrorLog \/var\/log\/yoursite-error.log\r\n\r\n\tLogLevel warn\r\n\r\n\tCustomLog \/var\/log\/yoursite-access.log combined\r\n\r\n    <Proxy balancer:\/\/tomcatservers>\r\n\tBalancerMember ajp:\/\/tomcatserver.yoursite.com:8009 route=tomcatA retry=60\r\n        BalancerMember ajp:\/\/tomcatserver.yoursite.com:8010 route=tomcatB retry=60\r\n\tBalancerMember ajp:\/\/tomcatserver.yoursite.com:8011 route=tomcatC retry=60\r\n    <\/Proxy>\r\n\r\n    <Location \/javaee>\r\n\tAllow From All\r\n        ProxyPass balancer:\/\/tomcatservers\/javaee stickysession=JSESSIONID nofailover=off\r\n    <\/Location>\r\n\r\n<\/VirtualHost><\/pre>\n
The most important settings are Proxy<\/code> and Location<\/code>.
\nIn Proxy<\/code> it’s defined a load balancer<\/em> made with 3 tomcat servers and an URL is assigned to the balancer, in this case balancer:\/\/tomcatservers<\/code>.<\/p>\n
The balancer has three members, everyone with its own URL based on the ajp<\/code> protocol. In this case Apache will connect to the Tomcat servers on their AJP connectors<\/em> (an alternative would be to use their HTTP connectors<\/em>).<\/p>\n
The Tomcat servers run on the tomcatserver.yoursite.com<\/code> hostname and each of them opens its own AJP connector on a different port: the first on 8009<\/code> (the default one), the second on 8010<\/code>, the third on 8011<\/code> (obviously if they run on the same hostname\/IP they must bind to different ports).<\/p>\n
Each Tomcat is identified by a route name: tomcatA<\/code>, tomcatB<\/code> and tomcatC<\/code>. The importance of it will be explained later.<\/p>\n
In the Location<\/code> section, a specific path \/javaee<\/code> of the virtual host is mapped on the previously defined balancer balancer:\/\/tomcatservers\/javaee<\/code>. So when someone asks for http:\/\/www.yoursite.com\/javaee\/hello.jsp<\/code> the virtual host will request that JSP to a randomly chosen Tomcat in the balancer members.<\/p>\n
What’s the stickysession<\/code> attribute? It’s a very useful configuration parameter used in conjunction with the route<\/code> attributes, defined before.<\/p>\n
As probably every Java EE (or Web) developer should know, while browsing on a server, it keeps trace of some data about the browsing session in a server-side HttpSession<\/em> object. For example an ecommerce web application needs to store somewhere the information about the shopping cart of non registered users.<\/p>\n
How the server can associate the remote session data with the specific navigation session? This is done through a cookie (or via a GET parameter in the URL) that gives to the server the session ID<\/em> value.<\/p>\n
In Java EE applications, the cookie name to identify the sessions is JSESSIONID<\/code>.<\/p>\n
This is closely related to the management of the load balancing between the Tomcat servers.<\/p>\n
If Apache picked randomly one of the Tomcat to handle a single request and if the next request from the same user\/browser was forwarded by the balancer to another Tomcat<\/em> in that battery, things wouldn’t work correctly.<\/p>\n
Each Tomcat doesn’t know anything of the existence of other Tomcat in that balancer configuration and especially a single Tomcat server cannot access the information of http sessions<\/em> handled by another Tomcat.<\/p>\n
In few words, when a Tomcat is chosen to handle the first request from a user\/browser, it’s absolutely required that, to keep valid session data, the same Tomcat must be used to handle the following requests coming from that browser\/user.<\/p>\n
If not, on each request, the session data would be lost and simple tasks, such as building a shopping cart would result impossible.<\/p>\n
So, it’s required to tell to Apache what is the session cookie name: JSESSIONID<\/code> and which is the identifier of the routes to each single tomcat Server: tomcatA<\/code>, tomcatB<\/code>, tomcatC<\/code>.<\/p>\n
In this way, Apache will append to the end of the cookie value the information about the route to the specific Tomcat.<\/p>\n
\"Java
Java EE Tomcat Load Balancing<\/p><\/div>\n
Finally, the last thing to set-up Apache, is obviously to add to it the  modules required by the previous configuration:<\/p>\n