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 ...
[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!
[4045] Linux Web Server - User Roles, User Accounts, and shared administration
[3891] The components of an Apache httpd / Tomcat / MySQL stack and what each does
Top or
Show all for Linux and Shell Programming
[4317] Java - an update of the basics
- (new - 2014-11-16)[4277] Sending a message to the server and changing text on a page when a button is pressed
[4178] Where are you? How to write a geosensitive application
[4064] Apache httpd - a robust, open source web server
[3996] Tips on Tomcat - moving applications around
Top or
Show all for Apache HTTP and Tomcat Servers
[4372] Template / design pattern for C++ constructor and accessors
- (new - 2014-12-29)[4355] C++ in 2 days
- (new - 2014-12-18)[4341] Segmentation Fault, Segmentation Violation, Bus Error, Stack Smashing
- (new - 2014-12-04)[4339] Command line and file handling in C
- (new - 2014-12-03)[4340] Simple C structs - building up to full, dynamic example
- (new - 2014-12-03)[4338] Passing arrays into functions in C
- (new - 2014-12-02)[4337] Learning to program sample program - past its prime, but still useful
- (new - 2014-12-02)
Top or
Show all for C and C++ Programming
[4371] Why are people using the TransWilts?
- (new - 2014-12-29)[4362] What a difference a year makes
- (new - 2014-12-23)[4357] Seeing Christmas Coming to Melksham
- (new - 2014-12-20)[4351] A little thing can make a big difference
- (new - 2014-12-13)[4343] Politics and the railway the connects Wiltshire
- (new - 2014-12-06)[4328] Folk music train, Westbury to Swindon round trip, 14th December 2014
- (new - 2014-11-24)[4327] Good, stable, reliable local businesses
- (new - 2014-11-23)[4313] Moving community rail support from amateur to professional
- (new - 2014-11-16)
Top or
Show all for Melksham
[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
- (new - 2014-12-20)[4335] Flexible public courses - residential or commuting, programming newcomer or experienced, C or C++
- (new - 2014-11-30)[4314] PHP training - refreshed modern course, backed up by years of practical experience
- (new - 2014-11-16)[4305] Learning to program in Java - yes, we can help.
Top or
Show all for Training from Well House Consultants
[4373] A long and disappointing evening
- (new - 2014-12-30)[4348] Taking my life in my hands in Swansea
- (new - 2014-12-11)[4321] Are administration / review charges on hotel guests acceptable?
- (new - 2014-11-20)[4302] Sunday is never quiet at Well House Manor
[4293] Certification
Top or
Show all for Running a training and hotel company
[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?
[4166] Pimms and Croquet - Melksham Festival of Food and Drink
Top or
Show all for Well House Manor - Hotel and Training Centre
[4315] Welcoming genuine forum posters quickly - but turning away off topic advertisers
- (new - 2014-11-16)[4303] Libre Office - unable to get past REOPEN WINDOWS? question
[4292] The Horse is back!
[4283] Can a legitimate forum post become illegal a year later?
[4239] Facebook marketing - early experiences
Top or
Show all for For the Webmaster, Postmaster and moderator
[4374] Test driven development, and class design, from first principles (using C++)
- (new - 2014-12-30)[4359] How to avoid too many recalculations within an object
- (new - 2014-12-21)[4346] A behaviour driven example of writing a Java program
- (new - 2014-12-09)[4336] Test Driven Development - a first example of principle in C
- (new - 2014-12-01)[4330] Java - factory method, encapsulation, hashmap example
- (new - 2014-11-27)[4325] Learning to program - what are algorithms and design patterns?
- (new - 2014-11-22)[4320] An example of Model-View-Controller techniques in a Perl / CGI script
- (new - 2014-11-20)[4318] Learning to Program - how we start to teach you at Well House Consultants
- (new - 2014-11-16)
Top or
Show all for General Programming Topics
[4366] Changing what operators do on objects - a comparison across different programming languages
- (new - 2014-12-26)[4356] Object factories in C++, Python, PHP and Perl
- (new - 2014-12-19)[4319] PHP - some quick fixes if scripts have issues, and how to use our support
- (new - 2014-11-19)[4310] Problem ... I want to print a series of numbered forms
[4244] Disambiguation - PHP List
Top or
Show all for PHP - the language and its application
[4350] Keyboard reader for Java programming newcomers
- (new - 2014-12-12)[4347] Arrays in Java - an introduction for newcomers
- (new - 2014-12-10)[4345] Incrementing a variable in Java - Pre and Post Increment
- (new - 2014-12-08)[4334] Splitting out code into name blocks for clarity and reusability
- (new - 2014-11-30)[4332] First Java Application - calculating the weight of a tablecloth
- (new - 2014-11-29)[4326] Learning to program - comments, documentation and test code
- (new - 2014-11-22)[4324] Learning to program - variables and constants
- (new - 2014-11-22)[4323] Learning to program - Loop statements such as while
- (new - 2014-11-22)[4322] Learning to Program - the conditional statement (if)
- (new - 2014-11-21)
Top or
Show all for Java and the Java Environment
[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
[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
[4031] Showing what programming errors look like - web site pitfall
Top or
Show all for The Perl Programming Language and its use
[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)
[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)[4368] Shuffling a list - Ruby and Python
- (new - 2014-12-28)[4294] A bright new gem - updated Ruby training
[4010] Really Simple Rails
Top or
Show all for Ruby and Ruby on Rails
[4007] Which database should I use? MySQL v SQLite
[4006] Ruby / SQLite3 example program, showing JOIN v LEFT JOIN
[3928] Storing your intermediate data - what format should you you choose?
[3494] Databases - when to treat the rules as guidelines
[3455] MySQL, MySQLi, PDO or something else - how best to talk to databases from PHP
Top or
Show all for SQL and MySQL
[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
[4205] Regular Expression Substitution - Tcl
Top or
Show all for Tcl, Tcl/Tk and Expect
[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
[4361] Multiple yields and no loops in a Python generator?
- (new - 2014-12-22)[4360] Python - comparison of old and new string formatters
- (new - 2014-12-22)[4344] Python base and inherited classes, test harness and unit testing - new examples
- (new - 2014-12-07)[4307] Identifying and clearing denial of service attacks on your Apache server
[4298] Python - an interesting application
Top or
Show all for Python Programming
[4364] Christmas day in Melksham
- (new - 2014-12-25)[4363] A quiet evening in Melksham - awaiting Santa
- (new - 2014-12-24)[4349] When I am old, what will worry me?
- (new - 2014-12-11)[4333] Misty Melksham Morning
- (new - 2014-11-30)[4331] Musings on a Welsh town
- (new - 2014-11-28)[4316] Too many Staffies, too many lurchers
- (new - 2014-11-16)
Top or
Show all for And also ...
[4367] A year of rail memories!
- (new - 2014-12-27)[4365] The changing face of Christmas
- (new - 2014-12-26)[4353] Celebration pictures, year 1
- (new - 2014-12-15)[4352] A long day on the trains, but a rewarding one
- (new - 2014-12-14)[4342] My first official measure - Passenger Entrys and Exits - revisited 9 years later
- (new - 2014-12-05)[4312] Officially overcrowded in the first year
- (new - 2014-11-15)
Top or
Show all for Public Transport
[4354] Wiltshire Police - assuming someone is guilty just on the say-so of a member of the public?
- (new - 2014-12-16)[4329] Does Santa Claus need a CRB check?
- (new - 2014-11-25)[4304] Please do not ask me to be the chair!
[4282] On tipping - and the expectation of a tip
[4167] The important customers are the majority, not the celebrity
Top or
Show all for Fun and Flames
[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
[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?