The Well House Newsletter - Tuesday, 1st September 2015
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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)

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[1461] Python - input v raw input
[1448] Question on division (Java) - Also Perl, PHP, Python ...
[1442] Reading a file multiple times - file pointers
[1430] Integer v float - Python
[1375] Python v Ruby
[1348] Screw it or Glue it? Access to Object variables - a warning
[1340] Tk locks up - 100% c.p.u. on a simple program (Tcl, Perl, Python)
[1336] Ignore case in Regular Expression
[1310] Callbacks - a more complex code sandwich
[1306] Python class rattling around
[1305] Regular expressions made easy - building from components
[1304] Last elements in a Perl or Python list
[1236] Trying things in Python
[1220] for loop - how it works (Perl, PHP, Java, C, etc)
[1217] What are factory and singleton classes?
[1202] Returning multiple values from a function (Perl, PHP, Python)
[1201] No switch in Python
[1195] Regular Express Primer
[1181] Good Programming practise - where to initialise variables
[1163] A better alternative to cutting and pasting code
[1149] Turning objects into something you can store - Pickling (Python)
[1148] Python decorators - wrapping a method call in extra code
[1146] __new__ v __init__ - python constructor alternatives?
[1145] Using a list of keys and a list of values to make a dictionary in Python - zip
[1144] Python dictionary for quick look ups
[1140] Python GTK - Widget, Packing, Event and Feedback example
[1136] Buffering output - why it is done and issues raised in Tcl, Perl, Python and PHP
[1134] Function / method parameters with * and ** in Python
[1110] Python - two different splits
[1043] Sending an email from Python
[1042] Nested exceptions in Python
[1036] Python Qt, wX, TkInter, and Jython - training??
[970] String duplication - x in Perl, * in Python and Ruby
[964] Practical polymorphism in action
[959] It's the 1st, not the 1nd 1rd or 1th.
[956] Python security - trouble with input
[955] Python collections - mutable and imutable
[954] Splitting Pythons in Bradford
[950] Python and the Magic Roundabout
[949] Sludge off the mountain, and Python and PHP
[945] Code quality counts
[943] Matching within multiline strings, and ignoring case in regular expressions
[913] Python - A list of methods
[912] Recursion in Python
[909] Python is like a narrowboat
[903] Pieces of Python
[901] Python - listing out the contents of all variables
[900] Python - function v method
[899] Python - extend v append on a list
[846] Is Perl being replaced by PHP and Python?
[836] Build on what you already have with OO
[835] Python - when to use the in operator
[834] Python makes University Challenge
[831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl
[821] Dynamic functions and names - Python
[788] New - Conditional expressions in Python 2.5
[775] Do not duplicate your code
[773] Breaking bread
[753] Python 3000 - the next generation
[749] Cottage industry or production line data handling methods
[748] Getting rid of variables after you have finished with them
[745] Python modules. The distribution, The Cheese Shop and the Vaults of Parnassus.
[712] Why reinvent the wheel
[710] Linux training Glasgow, Python programming course Dundee
[672] Keeping your regular expressions simple
[668] Python - block insets help with documentation
[663] Python to MySQL
[657] The ternary operator in Python
[656] Think about your design even if you don't use full UML
[633] Copying a reference, or cloning
[629] Choosing the right language
[595] Add a friendly front end with Tk
[561] Python's Generator functions
[560] The fencepost problem
[496] Python printf
[477] Class, static and unbound variables
[463] Splitting the difference
[444] Database or Progamming - which to learn first?
[433] FTP - how to make the right transfers
[426] Robust checking of data entered by users
[418] Difference between import and from in Python
[386] What is a callback?
[383] Overloading of operators on standard objects in Python
[382] Central London Courses - Perl, PHP, Python, Tcl, MySQL
[381] Exceptions in Python
[380] Bridging to the customer requirement
[353] Wimbledon Neck
[340] Code and code maintainance efficiency
[328] Making programs easy for any user to start
[324] The backtick operator in Python and Perl
[317] Programming languages - a comparison
[316] Development Environments
[308] Call by name v call by value
[303] Lambdas in Python
[299] What - no switch or case statement?
[296] Using a Python dictionary as a holder of object attributes
[294] Python generator functions, lambdas, and iterators
[239] What and why for the epoch
[237] Crossfertilisation, PHP to Python
[236] Tapping in on resources
[235] Preparation for a day's work
[208] Examples - Gadfly, NI Number, and Tcl to C interface
[190] Python engines
[183] The elegance of Python
[114] Relative or absolute milkman
[105] Distance Learning
[103] Can't resist writing about Python
[61] Python is a fabulous language
[16] Python training
Top or Show all for Python Programming

And also ...
[4507] In favour of adoption rather than puppy purchase - dogs!
[4490] What and where are the Azores?
[4489] Election results - what if we had a party list system?
[4488] Election day, 2015
[4485] Misusing statistics? - the seedy side of election campaigning
Top or Show all for And also ...

Public Transport
[4511] The Royal Albert Bridge as you do not normally see it - (new - 2015-07-12)
[4496] Sunday train times - 17th May to 6th September 2015
[4495] Swindon to Westbury train services - Saturdays from 23rd May 2015
[4486] Newark to New York - the PATH suburban railway
[4478] Home in Melksham to Gatwick Airport - easy by public transport
Top or Show all for Public Transport

Fun and Flames
[4392] Interview conditions,and other instructions to staff
[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
[4509] Throwing a die - exercise in Ruby - (new - 2015-07-01)
[4508] Melksham to London by train - dont buy more than you need
[4484] TransAtlantic crossing - the starter
[4242] A busy January
[4237] Why the long gap?
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?

go to Top or other editions July 2015May 2015March 2015January 2015November 2014September 2014July 2014May 2014March 2014 • or current edition


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