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 ...
[3256] Displaying a directory or file system tree - Linux
[2896] LAMP - Linux, Apache, MySQL, PHP - install, configure, administer
[2831] Recording (a macro) in vi
[2639] su or su - ... what is the difference?
[2638] Finding what has changed - Linux / Unix
Top or
Show all for Linux and Shell Programming
[3443] Getting more log information from the Apache http web server
- (new - 2011-09-16)[3339] Simplest ever proxy configuration?
[3019] Apache httpd Server Status - monitoring your server
[3018] Tuning Apache httpd and Tomcat to work well together
[3015] Logging the performance of the Apache httpd web server
Top or
Show all for Apache HTTP and Tomcat Servers
[3390] Printing objects in C++
[3388] Templates in C++ - defining a family pattern of methods / functions
[3386] Adding the pieces together to make a complete language - C
[3384] Loops - a comparison of goto, while and for
[3252] C++ - unknown array size, unknown object type. Help!
Top or
Show all for C and C++ Programming
[3503] Melksham House - a community facility
- (new - 2011-10-31)[3502] Sunday Evening ... newest and oldest Melksham pictures
- (new - 2011-10-30)[3500] Campus Shadow - a great responsibility
- (new - 2011-10-30)[3499] Images of Melksham
- (new - 2011-10-29)[3496] Melksham - Business to Business Speed Networking - build ahead for 2012
- (new - 2011-10-26)[3495] Looking forward - Chamber of Commerce has 2012 and beyond on the agenda
- (new - 2011-10-26)[3492] Upcoming events in and about Melksham - more dates for your diary
- (new - 2011-10-22)[3481] Some thoughts in answer to some Melksham Campus questions
- (new - 2011-10-14)[3475] Havant - Shop Frontages.
- (new - 2011-10-12)[3470] Currently pictured in Melksham
- (new - 2011-10-09)[3438] Melksham to become a part of Trowbridge?
- (new - 2011-09-13)[3427] What is on the Melksham Agenda?
- (new - 2011-09-09)[3423] 1 + 1 + 1 + 1 = 12?
- (new - 2011-09-07)[3420] Making best use of the new enthusiasm for Melksham
- (new - 2011-09-05)
Top or
Show all for Melksham
[3460] Flying Colours!
- (new - 2011-09-28)[3419] Data that we use during our training courses, and other training resources
- (new - 2011-09-04)[3401] Open Source Training Schedule - learn a programming language - in Autumn 2011 or 2012
[3391] For programmers who use Internet Explorer as their browser
[3387] Eating out in Melksham - where we like for lunch.
Top or
Show all for Training from Well House Consultants
[3490] How not to call when job seeking ...
- (new - 2011-10-20)[3440] Research is exciting. But should routine be automated?
- (new - 2011-09-14)[3425] Our National Autograss Champion, from Melksham
- (new - 2011-09-08)[3368] Well House Manor hotel, Melksham - recommended to you by our customers
[3354] Sales and Marketing - adding advertising to the product
Top or
Show all for Running a training and hotel company
[3437] Light bulbs
- (new - 2011-09-13)[3373] Another busy Week at Well House Manor ... pictures from the midweek
[3359] Cash and Cheque policy - UK Hotel
[3314] Garden Seating, Well House Manor
[3308] Behind the scenes - setting up a cafe
Top or
Show all for Well House Manor - Hotel and Training Centre
[3491] Who is knocking at your web site door? Are you well set up to deal with allcomers?
- (new - 2011-10-21)[3480] Direct Message: Really horrible blog about you ... a clever phishing trip, said to be from an MP
- (new - 2011-10-14)[3457] Away to train - but still around by video for Melksham meetings
- (new - 2011-09-25)[3402] That spec is a kingfisher ...
[3367] Google +1 - what is it?
Top or
Show all for For the Webmaster, Postmaster and moderator
[3461] From flowchart to program - code design for the newcomer
- (new - 2011-09-29)[3456] Stepping stones - early coding, and writing re-usable code quickly
- (new - 2011-09-24)[3454] Your PHP website - how to factor and refactor to reduce growing pains
- (new - 2011-09-24)[3451] Why would you want to use a Perl hash?
- (new - 2011-09-20)[3366] Specification, Design, Implementation, Testing and Documentation - stages of a (Java) programming project
Top or
Show all for General Programming Topics
[3458] On this day ... one PHP script with three uses
- (new - 2011-09-26)[3453] Reading and using emails including enclosures on your web server.
- (new - 2011-09-23)[3379] Sorting data the way YOU want it sorted
[3210] Catchable fatal error in PHP ... How to catch, and alternative solutions such as JSON
[3197] Finding and diverting image requests from rogue domains
Top or
Show all for PHP - the language and its application
[3497] Not the same language - but based on the same constructs
- (new - 2011-10-27)[3365] Turning bright delegates into bright and knowledgable ones
[3293] Distributing the server load - yet ensuring that each user return to the same system (Apache httpd and Tomcat)
[3048] String handling - from first steps to practical examples
[3047] What is a universal superclass? Java / Perl / Python / Other OO languages
Top or
Show all for Java and the Java Environment
[3498] Which West Wilts Town? A picture quiz for you
- (new - 2011-10-28)[3382] What is the picture?
[3341] Farewell to the Hercules?
[3284] What to do in the evening during a Tcl course
[3267] Reybridge, Easter Monday
Top or
Show all for Around, about and nearby to Wiltshire
[3485] Perl - retrieving and caching web resources
- (new - 2011-10-18)[3449] Apache Internal Dummy Connection - what is it and what should I do with it?
- (new - 2011-09-19)[3446] Awk v Perl
- (new - 2011-09-18)[3445] Perl and CGI - simple form, and monitoring script.
- (new - 2011-09-17)[3444] Take the dog on a lead - do not carry her. Perl references.
- (new - 2011-09-17)[3412] Handling binary data in Perl is easy!
- (new - 2011-08-30)[3411] Single and double quotes strings in Perl - what is the difference?
- (new - 2011-08-30)
Top or
Show all for The Perl Programming Language and its use
[3133] An image from a website that occasionally comes out as hyroglyphics
[3128] How does your browser find out about itself?
[2913] Six languages in one file - an HTML++ web page
[2675] Redirecting to your main domain for correct security keys
[2628] An example of an injection attack using Javascript
Top or
Show all for Client Side Languages (HTML, CSS, Javascript)
[3435] Sorta sorting a hash, and what if an exception is NOT thrown - Ruby
- (new - 2011-09-12)[3432] 3 digit HTTP status codes - what are they, which are most common, which should be a concern?
- (new - 2011-09-11)[3431] Ruby at both extremes of your website
- (new - 2011-09-10)[3430] Sigils - the characters on the start of variable names in Perl, Ruby and Fortran
- (new - 2011-09-10)[3428] How many days to Christmas?
- (new - 2011-09-09)[3426] Automed web site testing scripted in Ruby using watir-webdriver
- (new - 2011-09-09)[3424] Divide 10000 by 17. Do you get 588.235294117647, 588.24 or 588? - Ruby and PHP
- (new - 2011-09-08)[3422] Assigning values to variables within other statements - Ruby
- (new - 2011-09-07)[3421] Ruby off the Rails?
- (new - 2011-09-06)
Top or
Show all for Ruby and Ruby on Rails
[3494] Databases - when to treat the rules as guidelines
- (new - 2011-10-23)[3455] MySQL, MySQLi, PDO or something else - how best to talk to databases from PHP
- (new - 2011-09-24)[3447] Needle in a haystack - finding the web server overload
- (new - 2011-09-18)[3361] Blowing our own trumpet - MySQL resources
[3270] SQL - Data v Metadata, and the various stages of data selection
Top or
Show all for SQL and MySQL
[3448] Checking all the systems on a subnet, using Expect and Tk
- (new - 2011-09-18)[3429] Searching through all the files in or below a directory - Ruby, Tcl, Perl
- (new - 2011-09-09)[3418] Tcl packages, pkg_mkIndex, pkgIndex.tcl -what are they and why use them.
- (new - 2011-09-03)[3417] What is a namespace and why do we need them?
- (new - 2011-09-03)[3416] Storing Tcl source code encoded, and running via your own C program
- (new - 2011-09-02)[3415] User defined sorting and other uses of callbacks in Tcl and Tk
- (new - 2011-09-02)[3414] Passing back multiple results in Tcl - upvar and uplevel
- (new - 2011-09-01)
Top or
Show all for Tcl, Tcl/Tk and Expect
[3396] Tables as Objects in Lua - a gentle introduction to data driven programming
[3395] Parallel but not really parallel. Moving game characters. Coroutines in Lua.
[3142] Private and Public - and things between
[2951] Lots of way of converting 3 letter month abbreviations to numbers
[2949] Lua - variable function arguments and select
Top or
Show all for Programming in Lua
[3489] Python courses and Private courses - gently updating our product to keep it ahead of the game
- (new - 2011-10-20)[3488] Python sets and frozensets - what are they?
- (new - 2011-10-20)[3479] Practical Extraction and Reporting - using Python and Extreme Programming
- (new - 2011-10-14)[3478] Testing your Python classes with the unittest package - how to
- (new - 2011-10-14)[3477] Choosing your Python GUI - wx, Qt, Tk or GTK?
- (new - 2011-10-13)[3476] Tkinter - an easy to use Python Graphic User Interface - introductory examples
- (new - 2011-10-13)[3474] Python Packages - groupings of modules. An introduction
- (new - 2011-10-11)[3472] Static variables in functions - and better ways using objects
- (new - 2011-10-10)[3469] Teaching dilemma - old tricks and techniques, or recent enhancements?
- (new - 2011-10-08)[3468] Python string formatting - the move from % to str.format
- (new - 2011-10-08)[3465] How can I do an FTP transfer in Python?
- (new - 2011-10-05)[3464] Passing optional and named parameters to python methods
- (new - 2011-10-04)[3459] Catching the fishes first?
- (new - 2011-09-27)[3442] A demonstration of how many Python facilities work together
- (new - 2011-09-16)[3441] Pressing ^C in a Python program. Also Progress Bar.
- (new - 2011-09-15)[3439] Python for loops - applying a temporary second name to the same object
- (new - 2011-09-14)[3436] Moving from scripting to Object Orientation in Python
- (new - 2011-09-13)[3433] Exceptions - a fail-safe way of trapping things that may go wrong
- (new - 2011-09-11)
Top or
Show all for Python Programming
[3484] A Melksham Timeline - Domesday to present day
- (new - 2011-10-17)[3473] Picture - Havant Station at Dusk
- (new - 2011-10-11)[3467] Charities v Charitable. The cost of collecting your donation.
- (new - 2011-10-06)[3466] Passing of Steve Jobs - R.I.P.
- (new - 2011-10-06)[3463] Busy weekend of contrasts.
- (new - 2011-10-03)[3450] A threat in the post? Poor marketing practise from Smiletrain?
- (new - 2011-09-19)[3413] If its Sunday, must it be Weymouth?
- (new - 2011-08-30)
Top or
Show all for And also ...
[3501] Santa Claus - on the train from Melksham on 4th December 2011
- (new - 2011-10-30)[3493] West Wilts Rail User Group - Walk yesterday from Bradford-on-Avon to Trowbridge
- (new - 2011-10-23)[3487] Public transport - road and rail
- (new - 2011-10-19)[3486] How important is public transport to people in the Melksham area?
- (new - 2011-10-19)[3483] Canals, watererways in the Melksham area
- (new - 2011-10-16)[3482] Taking a boat down Caen Hill Locks
- (new - 2011-10-15)[3471] Vintage Bus Running Day in Warminster. An example of good bus practise.
- (new - 2011-10-09)[3452] Which or ATOC - who reads train fares right?
- (new - 2011-09-22)[3434] Sundays - and over eating
- (new - 2011-09-11)
Top or
Show all for Public Transport
[3462] Hangers, luggage and possessions
- (new - 2011-10-01)[3369] Local Council leads bans on many activities
[3302] Are you a half full or half empty person?
[3288] Random Questions ...
[3226] A busy morning for railway announcements
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
[3363] Should we take sponsored adverts on our site?
[3342] A Holiday
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?