A single instance of Apache httpd can handle a very large number of simple requests for files to be served ... but a single instance of Apache Tomcat can handle far fewer requests for applications to be run. After all, there's a big difference between just handing someone a piece of data and having to work out something complicated to answer a question.

And so with users wishing to have a single point of contact for a web site, it makes sense to run an instance of httpd which serves out all the simple files, but passes requests on to multiple instances of Tomcat when it gets something that needs a bit of work doing. There are no fewer that three ways of doing this - mod_proxy_balancer, mod_rewrite and mod_jk. And I've got examples of the configuration files for mod_jk here today.

Here's an example of the extra configuartion needed in the Apache httpd configuration file - (/usr/local/apache2/conf/httpd.conf)

LoadModule jk_module modules/mod_jk.so
JkWorkersFile /usr/local/apache2/conf/jkworkers.properties
JkMount /latmjdemo* catkin

And here's the jkworkers.properties file:

worker.list=catkin
worker.oak.port=8009
worker.oak.host=192.168.200.1
worker.oak.lbfactor=5
worker.elm.port=8009
worker.elm.host=192.168.200.158
worker.elm.lbfactor=15
worker.catkin.type=lb
worker.catkin.balanced_workers=oak,elm
worker.catkin.sticky_session=1

Traffic is forwarded to a Tomcat server called "Oak" on 192.168.200.1, or a Tomcat server called "Elm" on 192.168.200.158, with that latter getting 3 forwards for every one passed to Oak.

The "sticky_session" is worth comment. Rather than randomly forwarding tarffic to either server, httpd will forward users who already have sessions established to the same system right through their session. That way, a multiple page process (such as an on line ordering system) can easily be implemented without the need for a lot of extra code to share work-in-progress data between the various Tomcat server.

In order for sticky sessions to work, you need to configure your jvmRoute in Tomcat to reflect the server name; all of this covered on our Deploying Java Applications on Linux course.
(written 2007-10-02, updated 2010-05-28)

 A603 - Web Application Deployment - Further httpd Configuration
  [3449] Apache Internal Dummy Connection - what is it and what should I do with it? - (2011-09-19)
  [3133] An image from a website that occasionally comes out as hyroglyphics - (2011-01-14)
  [2900] Redirecting a page - silent, temporary or permanent? - (2010-08-03)
  [2478] How did I do THAT? - (2009-10-26)
  [2272] Monitoring and loading tools for testing Apache Tomcat - (2009-07-07)
  [2060] Database connection Pooling, SSL, and command line deployment - httpd and Tomcat - (2009-03-01)
  [1974] Moving a directory on your web site - (2009-01-03)
  [1955] How to avoid duplicating web page maintainance - (2008-12-20)
  [1954] mod_rewrite for newcomers - (2008-12-20)
  [1939] mod_proxy_ajp and mod_proxy_balancer examples - (2008-12-13)
  [1778] Pointing all the web pages in a directory at a database - (2008-08-30)
  [1767] mod_proxy and mod_proxy_ajp - httpd - (2008-08-22)
  [1762] WEB-INF (Tomcat) and .htaccess (httpd) - (2008-08-20)
  [1707] Configuring Apache httpd - (2008-07-12)
  [1636] What to do if the Home Page is missing - (2008-05-08)
  [1619] User and Group settings for Apache httpd web server - (2008-04-22)
  [1566] Strange behaviour of web directory requests without a trailing slash - (2008-03-06)
  [1564] Default file (MiMe types) for Apache httpd and Apache Tomcat - (2008-03-04)
  [1554] Online hotel reservations - Melksham, Wiltshire (near Bath) - (2008-02-24)
  [1551] Which modules are loaded in my Apache httpd - (2008-02-23)
  [1381] Using a MySQL database to control mod_rewrite via PHP - (2007-10-06)
  [1355] .php or .html extension? Morally Static Pages - (2007-09-17)
  [1351] Compressing web pages sent out from server. Is it worth it? - (2007-09-14)
  [1207] Simple but effective use of mod_rewrite (Apache httpd) - (2007-05-27)
  [1121] Sharing the load with Apache httpd and perhaps Tomcat - (2007-03-29)
  [1080] httpd.conf or .htaccess? - (2007-02-14)
  [1009] Passing GET parameters through Apache mod_rewrite - (2006-12-27)
  [934] Clustering, load balancing, mod_rewrite and mod_proxy - (2006-11-21)
  [853] To list a directory under httpd on a web server, or not? - (2006-09-02)
  [755] Using different URLs to navigate around a single script - (2006-06-11)
  [662] An unhelpful error message from Apache httpd - (2006-03-30)
  [649] Denial of Service ''attack'' - (2006-03-17)
  [631] Apache httpd to Tomcat - jk v proxy - (2006-03-03)
  [550] 2006 - Making business a pleasure - (2006-01-01)
  [526] Apache httpd - serving web documents from different directories - (2005-12-12)
  [466] Separating 'per instance' data from binaries and web sites - (2005-10-16)
  [345] Spotting a denial of service attack - (2005-06-12)

A912 - Web Application Deployment - Upgrading and tuning Tomcat
  [3018] Tuning Apache httpd and Tomcat to work well together - (2010-10-27)
  [2082] Jmeter - a first test case - (2009-03-14)
  [1908] Java CLASSPATH explained - (2008-11-26)
  [1718] Increasing Java Virtual Machine memory for Tomcat - (2008-07-24)

A657 - Web Application Deployment - Using httpd to front Tomcat
  [2483] Clustering on Tomcat - (2009-10-30)
  [1944] Forwarding session and cookie requests from httpd to Tomcat - (2008-12-14)
  [1940] URL rewriting with front and back servers - (2008-12-14)
  [576] Why run two different web servers - (2006-01-25)

A900 - Web Application Deployment - Tomcat - Extra Features
  [1899] Virtual Hosting under Tomcat - an example - (2008-11-22)
  [1896] Virtual Hosts and Virtual Servers - (2008-11-20)
  [1383] Monitoring mod_jk and how it is load balancing - (2007-10-07)
  [1376] Choosing between mod_proxy and mod_rewrite - (2007-10-02)
  [1370] Apache Tomcat Performance Tuning - (2007-09-29)
  [1006] Apache httpd and Apache Tomcat together tips - (2006-12-24)

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