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 traffic 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 2012-07-27)

 A912 - Web Application Deployment - Upgrading and tuning Tomcat
  [1718] Increasing Java Virtual Machine memory for Tomcat - (2008-07-24)
  [1908] Java CLASSPATH explained - (2008-11-26)
  [2060] Database connection Pooling, SSL, and command line deployment - httpd and Tomcat - (2009-03-01)
  [2082] Jmeter - a first test case - (2009-03-14)
  [3018] Tuning Apache httpd and Tomcat to work well together - (2010-10-27)

A900 - Web Application Deployment - Tomcat - Extra Features
  [934] Clustering, load balancing, mod_rewrite and mod_proxy - (2006-11-21)
  [1006] Apache httpd and Apache Tomcat together tips - (2006-12-24)
  [1121] Sharing the load with Apache httpd and perhaps Tomcat - (2007-03-29)
  [1370] Apache Tomcat Performance Tuning - (2007-09-29)
  [1376] Choosing between mod_proxy and mod_rewrite - (2007-10-02)
  [1383] Monitoring mod_jk and how it is load balancing - (2007-10-07)
  [1896] Virtual Hosts and Virtual Servers - (2008-11-20)
  [1899] Virtual Hosting under Tomcat - an example - (2008-11-22)
  [1939] mod_proxy_ajp and mod_proxy_balancer examples - (2008-12-13)

A657 - Web Application Deployment - Using httpd to front Tomcat
  [576] Why run two different web servers - (2006-01-25)
  [631] Apache httpd to Tomcat - jk v proxy - (2006-03-03)
  [1566] Strange behaviour of web directory requests without a trailing slash - (2008-03-06)
  [1940] URL rewriting with front and back servers - (2008-12-14)
  [1944] Forwarding session and cookie requests from httpd to Tomcat - (2008-12-14)
  [2483] Clustering on Tomcat - (2009-10-30)
  [3999] Handling failures / absences of your backend server nicely - (2013-02-08)

A603 - Web Application Deployment - Further httpd Configuration
  [345] Spotting a denial of service attack - (2005-06-12)
  [466] Separating 'per instance' data from binaries and web sites - (2005-10-16)
  [526] Apache httpd - serving web documents from different directories - (2005-12-12)
  [550] 2006 - Making business a pleasure - (2006-01-01)
  [649] Denial of Service ''attack'' - (2006-03-17)
  [662] An unhelpful error message from Apache httpd - (2006-03-30)
  [755] Using different URLs to navigate around a single script - (2006-06-11)
  [853] To list a directory under httpd on a web server, or not? - (2006-09-02)
  [1009] Passing GET parameters through Apache mod_rewrite - (2006-12-27)
  [1080] httpd.conf or .htaccess? - (2007-02-14)
  [1207] Simple but effective use of mod_rewrite (Apache httpd) - (2007-05-27)
  [1351] Compressing web pages sent out from server. Is it worth it? - (2007-09-14)
  [1355] .php or .html extension? Morally Static Pages - (2007-09-17)
  [1381] Using a MySQL database to control mod_rewrite via PHP - (2007-10-06)
  [1551] Which modules are loaded in my Apache httpd - (2008-02-23)
  [1554] Online hotel reservations - Melksham, Wiltshire (near Bath) - (2008-02-24)
  [1564] Default file (MiMe types) for Apache httpd and Apache Tomcat - (2008-03-04)
  [1619] User and Group settings for Apache httpd web server - (2008-04-22)
  [1636] What to do if the Home Page is missing - (2008-05-08)
  [1707] Configuring Apache httpd - (2008-07-12)
  [1762] WEB-INF (Tomcat) and .htaccess (httpd) - (2008-08-20)
  [1767] mod_proxy and mod_proxy_ajp - httpd - (2008-08-22)
  [1778] Pointing all the web pages in a directory at a database - (2008-08-30)
  [1954] mod_rewrite for newcomers - (2008-12-20)
  [1955] How to avoid duplicating web page maintainance - (2008-12-20)
  [1974] Moving a directory on your web site - (2009-01-03)
  [2272] Monitoring and loading tools for testing Apache Tomcat - (2009-07-07)
  [2478] How did I do THAT? - (2009-10-26)
  [2900] Redirecting a page - silent, temporary or permanent? - (2010-08-03)
  [3133] An image from a website that occasionally comes out as hyroglyphics - (2011-01-14)
  [3449] Apache Internal Dummy Connection - what is it and what should I do with it? - (2011-09-19)
  [3635] Parse error: parse error, unexpected T_STRING on brand new web site - why? - (2012-03-03)
  [3862] Forwarding a whole domain, except for a few directories - Apache http server - (2012-09-17)
  [3955] Building up from a small PHP setup to an enterprise one - (2012-12-16)
  [4001] Helping search engines with appropriate 400 error codes - (2013-02-11)
  [4307] Identifying and clearing denial of service attacks on your Apache server - (2014-09-27)

Back to Choosing between mod_proxy and mod_rewrite

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