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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Handling failures / absences of your backend server nicely

It's good for your website to put a professional smile on it's face, even when it's having problems with its backend!

On high traffic sites (for example - TV show and news sites), backend servers can occasionally get overloaded and you'll want your web site visitor to get a message to that effect, rather than a failure. If the feed is a regular news feed, you might even be able to respond with the latest (cached) news if it's less that a few minutes old, and your user will be none the wiser!

Here's a demonstration of one way to do this - setting up a front end Apache httpd to divert requests it would normally pass on to a proxy to a local PHP script if the proxy is not available. Simply add something like
  ErrorDocument 503 "/proxydown.php"
into the httpd.conf file, a file it includes, or an appropriate .htaccess file.

Then within that PHP script, you can force a 200 (OK) header to be returned:
  header("HTTP/1.1 200 OK");
and the PHP script can work out what's running, and what was asked for:
  $requested = $_SERVER["REQUEST_URI"];
  $gotinstead = $_SERVER["PHP_SELF"];
may give when you print out those variables
  You called for /dryfeet but it is not available
  You got instead /proxydown.php

This example is a demonstration which I've written to show the sort of thing that can be done when perhaps you have httpd fronting a number of Tomcat servers, and you want the httpd to catch and control any times that it fails to contact a Tomcat. The complete demonstration is [here] - that's a basic framework into which you can add logging, caching, emailin the system adminsitrator if the problem persists, and lots of other facilities.
(written 2013-02-08)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
H307 - PHP - Web2 and caching
  [1633] Changing a screen saver from a web page (PHP, Perl, OSX) - (2008-05-06)
  [1647] Exchange Rates - PHP with your prices in your users currency - (2008-05-19)
  [1733] memcached - overview, installation, example of use in PHP - (2008-08-02)
  [1812] Starting Ajax - easy example of browser calling up server data - (2008-09-27)
  [1813] Ajax - going Asyncronous and what it means - (2008-09-28)
  [1814] Javascript/HTML example, dynamic server monitor - (2008-09-28)
  [1926] Flash (client) to PHP (server) - example - (2008-12-06)
  [1995] Automated server heartbeat and health check - (2009-01-16)
  [2196] New Example - cacheing results in PHP for faster loading - (2009-05-24)
  [2321] Uploading and Downloading files - changing names (Perl and PHP) - (2009-08-04)
  [2545] Scraping content for your own page via PHP - (2009-12-21)
  [3029] PHP data sources - other web servers, large data flows, and the client (browser) - (2010-11-04)
  [3094] Setting your user_agent in PHP - telling back servers who you are - (2010-12-18)
  [3186] How to add a customised twitter feed to your site - (2011-02-27)
  [3458] On this day ... one PHP script with three uses - (2011-09-26)
  [3955] Building up from a small PHP setup to an enterprise one - (2012-12-16)
  [4055] Using web services to access you data - JSON and RESTful services - (2013-03-29)
  [4075] Further recent PHP examples - (2013-04-28)
  [4106] Web server efficiency - saving repetition through caches - (2013-05-30)
  [4136] How do I post automatically from a PHP script to my Twitter account? - (2013-07-10)
  [4627] Caching results in an object for efficiency - avoiding re-calculation - (2016-01-20)

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)
  [1377] Load Balancing with Apache mod_jk (httpd/Tomcat) - (2007-10-02)
  [1566] Strange behaviour of web directory requests without a trailing slash - (2008-03-06)
  [1939] mod_proxy_ajp and mod_proxy_balancer examples - (2008-12-13)
  [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)
  [3018] Tuning Apache httpd and Tomcat to work well together - (2010-10-27)

A655 - Web Application Deployment - Using Tomcat and Apache httpd Together
  [436] Linking Apache httpd to Apache Tomcat - (2005-09-05)
  [1376] Choosing between mod_proxy and mod_rewrite - (2007-10-02)
  [1383] Monitoring mod_jk and how it is load balancing - (2007-10-07)
  [1549] http, https and ajp - comparison and choice - (2008-02-22)
  [1552] Extra public classes in deploying Apache httpd and Tomcat - (2008-02-24)
  [1771] More HowTo diagrams - MySQL, Tomcat and Java - (2008-08-24)
  [1938] Predictive Load Balancing - PHP and / or Java - (2008-12-13)
  [2482] Load balancing with sticky sessions (httpd / Tomcat) - (2009-10-29)


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Some other Articles
Web and console - same principle, same code - Ruby example
Images of Melksham you havent seen before
Helping search engines with appropriate 400 error codes
9 years, and 4000 articles on
Handling failures / absences of your backend server nicely
Travelogue - from the Netherlands to London via Hook and Harwich
Servlet v JSP (Java Server Page). What is the difference?
Tips on Tomcat - moving applications around
Onward from Brussels
On board Eurostar - a running commentary
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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