The Well House Newsletter - Sunday, 1st March 2015
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Public Transport Training from Well House Consultants And also ... Tcl, Tcl/Tk and Expect Programming in Lua Python Programming Well House Manor - Hotel and Training Centre Apache HTTP and Tomcat Servers The Perl Programming Language and its use Ruby and Ruby on Rails PHP - the language and its application C and C++ Programming Linux and Shell Programming Melksham SQL and MySQL For the Webmaster, Postmaster and moderator Java and the Java Environment Fun and Flames Running a training and hotel company Around, about and nearby to Wiltshire Client Side Languages (HTML, CSS, Javascript) Keynote Articles General Programming Topics
Keynote article ...

Clustering on Tomcat

Subject: Clustering, using Apache http server (version 2.2.14 in my example) with mod_proxy_balancer as the front load splitter and Apache Tomcat 6.0.20 as the replicated application engine. [[Tip should also work for other recent 2.2.x and 6.0.x versions]]

Background

This is a follow on article from Load balancing with sticky sessions (httpd / Tomcat), where I looked at sharing out the application work between a number instances of Tomcat from an Apache http server (httpd) that did the bookkeeping. In a nutshell, the Apache http server sent new arrivals to a 'random' Tomcat, and then used sticky sessions so that - when a visitor came back for their subsequent visit in the same series of accesses - they would always talk to the same Tomcat and could continue their conversation with the server having full knowledge of the position to date.

The balancer alone is a good solution as far as it goes but:
• What happens if the Tomcat that has been stuck to goes out of service?
• What happens if you have such a lot of traffic that you need to replicate your httpd front end?
• What happens if your httpd fails?
• What is you don't actually want to use sessions, but still need what appears to be a single Tomcat?

One possible option to addressing some of these is to use the clustering capability of Tomcat, which I'll describe below. But you should first consider if you really need the extra step:
(a) can I accept that a session will be lost on the rare occasions that a Tomcat goes offline?
(b) is writing to a backend database going to preserve sufficient information anyway?
and if the answer to either is "yes", you probably do NOT need to cluster.

How does clustering work?

You run your web application on a series of identical (or rather "near identical" - the IP address will differ!) servers. With clustering turned on, each of the servers in the cluster is broadcasting (via multicast) any changes made in sessions, cookies, etc to any other listening cluster members on that same multicast address. So that when a visitor comes back for his / her next access, all the machines know what's been going on and can knowledgeably handle the request, even if the original machine isn't available.

You can turn clustering on in Apache Tomcat 6.0.20 simply by uncommenting the line in the default server.xml file that relates to it:
  <Cluster className = "org.apache.catalina.ha.tcp.SimpleTcpCluster"/>
and restating your Tomcat. Older versions of Tomcat (such as 5.5) had a long configuration section listing the ports, replication time, IP addresses to use, trigger files all of which are important but none of which actually needs to be changed from default in the current release that's the target of this article.

Once you have turned clustering on (yes, it's now that simple), your machines will be communicating ... it's rather like starting a rumor in an office - before you know it, EVERYONE who's around has heard the rumor.

Clustering with the balancer

If you have already implemented balancing with sticky sessions (as covered in the preceeding article), turning on clustering will cause the data to be shared around. Most of the time the data passed around will not be used - it will ONLY form a backup of the session, to be used if the balancer is unable to reach the sticky machine because it has done down or been taken out of service.

With sticky sessions activated, even a second front-end Apache http server won't cause a switch from one Tomcat to another unless a fail-over occurs, as the jvmroute is a part of the cookie so either (any) of the httpd front ends will correctly forward to the original Tomcat. And if you have an intelligent hardware load balancer, that too will be able to forward consistently and the the clustering will remain merely as a backup.

If you disable sticky sessions on your balancer, the metrics will change. Forwarding will now be at shared to each of the Tomcats in the balanced group / cluster group (take care that all members of the balance group are included in the cluster!) and so the visitor will get to a differnt back end box each time. But that's now perfectly fine, as they're sharing the data between them so will all know about the originator.

Testing if your cluster is working

Ironically, clustering and balancing is designed to be transparent, so how do you test whether it's working?

My first simple 'trick' is to change the background colour of the pages returned from each cluster member so that "if it's orange it must be Holt" and "if it's blue it must be Chippenham" (our servers are names after local towns and villages!). Going a little further, you can edit your servlet / JSP to return the name of the current host. In Java, the following line:
  String myname = InetAddress.getLocalHost().getHostName();
will return you the local name of your computer, so that you can then echo the name.

On last Tuesday's course, I took our sample "Barman" script that remembers how many drinks you've had in a session (visit counter!) and extended it into a "Pub Watch" script, where each of the barman communicates with his colleagues in neighboring pubs to keep track of who's out on the town, and how much they have had to drink in each establishment.

If you click on the links in the previous paragraph, you can download the source code for "Barman" and "PubWatch" and try the code out for yourself. Using the balancer manage that I introduced at the end of yesterday's article, you can open and close individual pubs and see how their customers go elsewhere for their next drink, and you can turn sticky sessions off in the balancer and see how faithful customers will then hit the road and go to a different pub each time for their next drink.

Some notes on clustering

1. The machines in the cluster communicate through multicast, so must be on the same subnet.

2. It's a good idea for the subnet you use to have plenty of capacity if your environment is busy, and for it to be firmly behind a strong firewall from your own company's general user traffic, let alone the Internet

3. If you have multiple Tomcat clusters on the same subnet, you'll need to configure one of the clusters away from the default settings - otherwise they'll end up as being one big cluster (you'll find the word 'tribe' creaping in here!)

At present, we mention clustering on our public deploying apache httpd and Tomcat course. Only a small proportion of our delegate want to go 'that far', and for newcomers who hadn't done any web server work when they first came along a couple of days earlier, it would be just too much for the one session.

An extra day on the end of a Tomcat course, coverage in a private course, or a special session set up for the purpose ... all are possible to help you learn how clustering and balancing work. We'll have a network of computers set aside at our training centre for the purpose of setting up a test case, experimenting with configurations, seeing what happens when machines are switched on and off. Something you wouldn't dare so with your own production environment, and might be reluctant to do even on your development of test networks (that's even assuming that you do HAVE multiple machines at the development or test level).
(this article written on 2009-10-30)

Other articles ...

Linux and Shell Programming
[4405] Backup procedures - via backup server - (new - 2015-01-24)
[4400] Commenting out an echo killed my bash backup script - (new - 2015-01-19)
[4259] Upgrading our training systems to all the current stable versions
[4063] Backups by crossover between network centres - setting up automatic scp transfers
[4056] An overpractical test of our backup strategy!
Top or Show all for Linux and Shell Programming

Apache HTTP and Tomcat Servers
[4434] Public training courses - upcoming dates - (new - 2015-02-21)
[4432] Java web application for teaching - now with sessions and clustering / load balancing demonstrations - (new - 2015-02-20)
[4431] A Java servlet that is also a stand alone program. And a server that is also a web client. - (new - 2015-02-19)
[4412] Java -making sure you have the right versions - (new - 2015-02-02)
[4404] Which (virtual) host was visited? Tuning Apache log files, and Python analysis - (new - 2015-01-23)
Top or Show all for Apache HTTP and Tomcat Servers

C and C++ Programming
[4377] Designing a base class and subclasses, and their extension, in C++ - (new - 2015-01-01)
[4372] Template / design pattern for C++ constructor and accessors - (new - 2014-12-29)
[4355] C++ in 2 days
[4341] Segmentation Fault, Segmentation Violation, Bus Error, Stack Smashing
[4339] Command line and file handling in C
Top or Show all for C and C++ Programming

Melksham
[4435] Images of our rail promotion campaign - (new - 2015-02-21)
[4424] Looking Forward - TransWilts Community Rail Partnership and TransWilts CIC - (new - 2015-02-12)
[4423] Campus - about the logos / way marks you have been choosing from - (new - 2015-02-11)
[4418] Melksham Community Campus - which of these potential signs do you like? - (new - 2015-02-07)
[4399] The Campus in Melksham - name, logo, and livery choice - (new - 2015-01-18)
[4397] TransWilts / Press and Publicity report for AGM / 30th January 2015 - (new - 2015-01-17)
[4371] Why are people using the TransWilts? - (new - 2014-12-29)
Top or Show all for Melksham

Training from Well House Consultants
[4416] Challenging the IT course business model - (new - 2015-02-05)
[4415] Buffet review - taking the price into account - (new - 2015-02-05)
[4375] Final examples for 2014 - and a look at our 2015 training course options - (new - 2014-12-31)
[4358] A brilliant finish to 2014 training and business guests, and a look to 2015
[4335] Flexible public courses - residential or commuting, programming newcomer or experienced, C or C++
Top or Show all for Training from Well House Consultants

Running a training and hotel company
[4443] Seventh stay away this year - and it's still only February! - (new - 2015-02-25)
[4373] A long and disappointing evening - (new - 2014-12-30)
[4348] Taking my life in my hands in Swansea
[4321] Are administration / review charges on hotel guests acceptable?
[4302] Sunday is never quiet at Well House Manor
Top or Show all for Running a training and hotel company

Well House Manor - Hotel and Training Centre
[4379] Well House Consultants / Well House Manor - Prices for 2015 - (new - 2015-01-01)
[4299] The Best Bed and Breakfast in Melksham?
[4264] Weekend breaks by train to Melksham, Wiltshire
[4175] An alternative format for a new train service timetable
[4174] What would the extra trains from Melksham mean to your journey?
Top or Show all for Well House Manor - Hotel and Training Centre

For the Webmaster, Postmaster and moderator
[4426] FileMaker Day to Unix Time conversion - (new - 2015-02-15)
[4403] The unbalanced relationship between customer and provider - (new - 2015-01-21)
[4385] A booking that looks too good to be true? It probably is too good to be true! - (new - 2015-01-05)
[4376] Well House Consultants, Well House Manor, First Great Western Coffee shop, TransWilts / 2014 web site reports - (new - 2015-01-01)
[4315] Welcoming genuine forum posters quickly - but turning away off topic advertisers
Top or Show all for For the Webmaster, Postmaster and moderator

General Programming Topics
[4430] The spirit of Java - delegating to classes - (new - 2015-02-18)
[4421] How healthy are the stars of stage and screen? - (new - 2015-02-09)
[4410] A good example of recursion - a real use in Python - (new - 2015-02-01)
[4401] Selecting RECENT and POPULAR news and trends for your web site users - (new - 2015-01-19)
[4396] Java Utility class - flexible replacement for array. Also cacheing in objects and multiple catch clauses example. - (new - 2015-01-16)
[4394] Philosophy behind object design - and how I applied in to a Java example - (new - 2015-01-14)
[4391] Refactoring Perl applications to give them a rosy future - (new - 2015-01-11)
[4387] Regression Testing my website - Cucumber and Watir - (new - 2015-01-07)
[4380] Behaviour Driven Development / Ruby and Cucumber - (new - 2015-01-02)
[4374] Test driven development, and class design, from first principles (using C++) - (new - 2014-12-30)
Top or Show all for General Programming Topics

PHP - the language and its application
[4437] Adding a PHP build option, rotating an image based on camera data, and a new look at thumbnails in PHP - (new - 2015-02-22)
[4366] Changing what operators do on objects - a comparison across different programming languages
[4356] Object factories in C++, Python, PHP and Perl
[4319] PHP - some quick fixes if scripts have issues, and how to use our support
[4310] Problem ... I want to print a series of numbered forms
Top or Show all for PHP - the language and its application

Java and the Java Environment
[4428] Using the lead - passing arrays and other collections in Java - (new - 2015-02-16)
[4427] Java example - for loop and conditionals from course exercise - (new - 2015-02-16)
[4422] Objects - from physical to virtual or abstract - Java - (new - 2015-02-10)
[4420] Flexibility in input - read from file, web resource or keyboard - (new - 2015-02-08)
[4419] Java Inheritance example - group of classes - step by step - (new - 2015-02-08)
[4414] Java - converting an integer to a fixed length string - (new - 2015-02-04)
[4413] Binomial Coefficient (Pascal Triangle) objects in Java - (new - 2015-02-03)
[4393] Splitting Java strings and extracting numbers - (new - 2015-01-13)
Top or Show all for Java and the Java Environment

Around, about and nearby to Wiltshire
[4265] A lovely meal in Swindon - just a short walk from the station
[4233] Open for the new year - Imber
[3989] Plenty to see and do - even in Winter - on a Well House Manor weekend
[3884] Pictures - across the park, French Weir and River Tone, Taunton
[3800] Fancy a weekend away? Try Well House Manor in Melksham, Wiltshire
Top or Show all for Around, about and nearby to Wiltshire

The Perl Programming Language and its use
[4395] Preparing data through a little bit of Perl - (new - 2015-01-15)
[4301] Perl - still a very effective language indeed for extracting and reporting
[4296] Polishing the Perl courses - updated training
[4100] Perl Dancer - from installation to your first real application
[4099] Perl Dancer - a Perl Framework - Installation and first test
Top or Show all for The Perl Programming Language and its use

Client Side Languages (HTML, CSS, Javascript)
[4037] Cascading Style Sheets and formatting your web page
[4036] HTML tags uses in these blog articles
[4035] Special characters in HTML
[4034] The VERY basics of a web page ... and web site
[3532] Sharing the user experience - designing a form with the customer in mind
Top or Show all for Client Side Languages (HTML, CSS, Javascript)

Ruby and Ruby on Rails
[4402] Finding sum, minimum, maximum and average in Python (and Ruby) - (new - 2015-01-19)
[4389] Cucumber example - test::unit, scenario outlines, datafile driven test - (new - 2015-01-09)
[4388] Global Regular Expression matching in Ruby (using scan) - (new - 2015-01-08)
[4386] Guide exercise to help you learn Gherkin, Cucumber and Rspec - (new - 2015-01-06)
[4384] Installing Cucumber on Ubuntu - cannot load such file -- mkmf (LoadError) message - (new - 2015-01-04)
[4383] Improved test in Cucumber with RSpec - (new - 2015-01-03)
[4382] Second step Cucumber and Gherkin - beyond Hello World - (new - 2015-01-03)
[4381] Installing Cucumber (Ruby) - (new - 2015-01-03)
[4370] Conditionals, loops and methods in Ruby - a primer with simple examples - (new - 2014-12-29)
[4369] Ruby - the second rung of learning the language - (new - 2014-12-28)
Top or Show all for Ruby and Ruby on Rails

SQL and MySQL
[4436] Accessing a MySQL database from Python with mysql.connector - (new - 2015-02-21)
[4406] Fixing damaged MySQL tables - Error 1712 and Error 2013 - (new - 2015-01-25)
[4390] Checking MySQL database backups have worked (not failed) - (new - 2015-01-10)
[4007] Which database should I use? MySQL v SQLite
[4006] Ruby / SQLite3 example program, showing JOIN v LEFT JOIN
Top or Show all for SQL and MySQL

Tcl, Tcl/Tk and Expect
[4398] Accessing variables across subroutine boundaries - Perl, Python, Java and Tcl - (new - 2015-01-18)
[4258] Keeping you Tkinter display up to date while monitoring
[4209] Lists in Tcl - fundamentals in a commented source code example
[4208] Tcl + Tk (Wish) - an introduction and revision example
[4207] Exception handling in Tcl
Top or Show all for Tcl, Tcl/Tk and Expect

Programming in Lua
[4297] Lua courses - an update
[4274] String formatting in Lua - string.format as a wrapper for sprintf
[4273] Dot or Colon separator between table name and member in Lua - what is the difference?
[4272] Lazy operators in Lua - what they mean, and examples
[4271] Line, block and nested comments - Lua compared to other languages
Top or Show all for Programming in Lua

Python Programming
[4445] Graphing presentations in Python - huge data, numpy and matplotlib - (new - 2015-02-28)
[4444] Elements of an exception in Python - try, except, else, finally - (new - 2015-02-28)
[4442] Mutable v Immuatble objects in Python, and the implication - (new - 2015-02-24)
[4441] Reading command line parameters in Python - (new - 2015-02-23)
[4440] A first graph with Matplotlib in Python - (new - 2015-02-22)
[4439] Json is the new marshall, pickle and cPickle / Python - (new - 2015-02-22)
[4438] Loving programming in Python - and ready to teach YOU how - (new - 2015-02-22)
[4409] Setting up and using a dict in Python - simple first example - (new - 2015-01-30)
[4408] Additional Python courses added to our schedule - (new - 2015-01-29)
[4407] Python - even named code blocks are objects - (new - 2015-01-28)
Top or Show all for Python Programming

And also ...
[4433] Different views of a Welsh Valley - but headed home - (new - 2015-02-20)
[4429] Wind Turbines - beauty or menace? - (new - 2015-02-17)
[4417] Crisp morning, fast run - (new - 2015-02-06)
[4411] Location, location location. And a chance of a giggle! - (new - 2015-02-01)
[4364] Christmas day in Melksham
Top or Show all for And also ...

Public Transport
[4425] A Welsh valley - what the transport looks like in 2015 - (new - 2015-02-14)
[4378] What FGW passengers want to talk about / and PHP programming to find out - (new - 2015-01-01)
[4367] A year of rail memories!
[4365] The changing face of Christmas
[4353] Celebration pictures, year 1
Top or Show all for Public Transport

Fun and Flames
[4392] Interview conditions,and other instructions to staff - (new - 2015-01-12)
[4354] Wiltshire Police - assuming someone is guilty just on the say-so of a member of the public?
[4329] Does Santa Claus need a CRB check?
[4304] Please do not ask me to be the chair!
[4282] On tipping - and the expectation of a tip
Top or Show all for Fun and Flames

Keynote Articles
[2483] Clustering on Tomcat
[2384] Looking ahead to the Autumn season of training and accommodation
[2144] Looking for a career change - Physician to Web Site Designer
[1955] How to avoid duplicating web page maintainance
[1857] November and December Public Course Schedule
Top or Show all for Keynote Articles

Unclassfied Articles
[4242] A busy January
[4237] Why the long gap?
[4127] Exploring the area ... Ynys, near Harlech
[4054] On Salford Docks - mind over matter?
[4047] Looking for Creative Design, Catering and Transport inputs to help with Melksham Campus
Top or Show all for unclassified

Public Transport Training from Well House Consultants And also ... Tcl, Tcl/Tk and Expect Programming in Lua Python Programming Well House Manor - Hotel and Training Centre Apache HTTP and Tomcat Servers The Perl Programming Language and its use Ruby and Ruby on Rails PHP - the language and its application C and C++ Programming Linux and Shell Programming Melksham SQL and MySQL For the Webmaster, Postmaster and moderator Java and the Java Environment Fun and Flames Running a training and hotel company Around, about and nearby to Wiltshire Client Side Languages (HTML, CSS, Javascript) Keynote Articles General Programming Topics

A little more about this newsletter ...

At Well House Consultants, we run niche IT training courses ... and we run a hotel for delegates on those courses and other visitors to Melksham too. And we make a lot of friends - have a lot of ambassadors with whom we want to keep in touch. So every day Graham (that's me, writing this piece) puts together an article or two which might include the latest sample programs that I've written during the current course, new information about Well House Manor - our business hotel, tips on search engine optimisation, announcements of upcoming public courses, pictures of local places, and even (on occasions) rants and whimsical pieces to keep those friends up to date and in touch. The feeds are available directlt via the Blog - "The Horse's Mouth", they're on our Twitter Feed and you can find me at my LinkedIn profile. But most people just want to look us up occasionally - every month or two, and then to catch up on the latest news just for their particular subjects of interest ... and that's what this newsletter is about

You'll find above the titles of ALL the new articles written in the last two months, listed by major subject area, and showing as (new) with their date of publication. You'll find additional articles in each category too - topping each category up to a minimum of five articles. And you'll find a link at the end of each section which lets you expand that section to show the titles of every article that's been published in that section. After all, "the old ones are often the best ones", aren't they?

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