One of the major enhancements in release 2.2 of Apache httpd is the addition of the mod_proxy_ajp and mod_proxy_balancer modules to the standard distribution. Gone are the days of needing to build special modules such as Jserv, jk, jk2 and warp to have httpd act as a "front" to a Java (Tomcat) server - the module you need is already there.

Proxy forwarding to a Java Server

Here's an example of a proxied request from Apache httpd on to a server (probably Apache Tomcat) that's running the ajp protocol on port 5090:

ProxyPass /harry ajp://192.168.200.215:5090/latmjdemo
ProxyPassReverse /harry ajp://192.168.200.215:5090/latmjdemo

That's code to be added to the end of your httpd.conf file!

Proxy forwarding to a group of Java Servers

It gets even better ... mod_proxy_balancer lets you define a group of Java servers which you can forward your traffic on to - ideal on a busy site where the background task that's running in Java is a resource hog and needs to be shared between systems. Here's an example of what you would add to httpd.conf:

<Proxy balancer://catbox>
BalancerMember ajp://192.168.200.215:5090/latmjdemo
BalancerMember ajp://192.168.200.214:5009/latmjdemo
</Proxy>
ProxyPass /prince balancer://catbox/

In this example, any references to the web resources on the server under the /prince directory will be forwarded to one of two other machines, on port 5090, and will be directed to the "latmjdemo" web application on there.

On our private Apache Tomcat course yesterday (based on / extended from our public course), we set up exactly this; although a production system would have identical applications on the two servers, we altered the background colour on one of them and it was fascinating to watch different delegates, and different reloads, coming up in either green or white.

More flexibility in forwarding to a group of Servers

The example above uses the default "round robin" scheduler - but there are other facilities available too to help you tune your forwarding. Here's a further example:

<Proxy balancer://kennel>
BalancerMember ajp://192.168.200.219:5009/latmjdemo loadfactor=1
BalancerMember ajp://192.168.200.218:5009/latmjdemo loadfactor=3
BalancerMember ajp://192.168.200.215:5009/latmjdemo status=+h
ProxySet lbmethod=bytraffic
ProxySet stickysession=JSESSIONID
</Proxy>
ProxyPass /corgi balancer://kennel/

In this example, we are forwarding to 2 systems, in a ratio of 1 : 3 and we're allocating traffic based on the traffic quantities coming back from each server rather than the number of requests (so queries that generate a lot of traffic count for more). An extra machine has been designated as "hot swap" if neither of the others is available. Once a visitor is allocated to a particular machine for his forward, he'll continue to be forwarded to that same system while his JSESSIONID cookie remains live.

Some other notes about mod_proxy and family in Apache 2.2:

• ProxyPassMatch is available, which lets you specify a pattern (Regular Expression) for your forwarding - for example, if you wanted to forward all you image requests to an image server:
ProxyPassMatch ^/(.*\.jpg)$ http://images.wellho.net/$1

• mod_rewrite IS aware of mod_proxy_balancer, so that you can rewrite your requests as we do in many parts of our site, and then forward them on to other systems through an appropriate balancer.

• As from Apache 2.2.9, ProxyPassReverse is also mod_proxy_balancer aware.

Further reading ... There's a very good paper by Jim Jagielski - here - on these and more extra facilities. Documentation for mod_proxy_balancer is here, and there's a good description of reverse proxies here.
(written 2008-12-13, updated 2009-05-10)

 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)
  [1377] Load Balancing with Apache mod_jk (httpd/Tomcat) - (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)
  [3018] Tuning Apache httpd and Tomcat to work well together - (2010-10-27)

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)

A608 - Web Application Deployment - Apache httpd mod_proxy
  [1767] mod_proxy and mod_proxy_ajp - httpd - (2008-08-22)
  [2062] Virtual hosting and mod_proxy forwarding of different domains (httpd) - (2009-03-01)
  [2273] Three recent questions on Tomcat Convertors - (2009-07-07)
  [2325] Apache, Tomcat, mod_proxy - (2009-08-06)
  [2482] Load balancing with sticky sessions (httpd / Tomcat) - (2009-10-29)
  [3293] Distributing the server load - yet ensuring that each user return to the same system (Apache httpd and Tomcat) - (2011-05-18)
  [3339] Simplest ever proxy configuration? - (2011-06-28)
  [3680] How can I run multiple web servers behind a single IP address? - (2012-04-02)
  [3892] Distributed, Balanced and Clustered Load Sharing - the difference - (2012-10-13)

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)
  [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)
  [2060] Database connection Pooling, SSL, and command line deployment - httpd and Tomcat - (2009-03-01)
  [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)

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Port and Glasses
Christmas scenes and events
Server - Service - Engine - Host, Tomcat
mod_proxy_ajp and mod_proxy_balancer examples
Predictive Load Balancing - PHP and / or Java
Getting hold of the wrong end of the stick
Quick Summary - PHP installation
Summary of MySQL installation on a Linux system
Lidl opens in Melksham