Distributed, Balanced and Clustered Load Sharing

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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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))

Distributed, Balanced and Clustered Load Sharing - the difference

If one web server isn't enough to handle all your traffic, you can share the load. But you need to be careful that you "maintain state" for your visitors if you're running applications that involve a series of forms / inputs that follow on from each other.

Specialist hardware load sharing devices are available ... though often they turn out to be dedicated computers running Linux and a specialised piece of software. Such devices sit at the public entrance to your website - your front door - and distribute all incoming traffic around as appropriate. If you don't want to go to the expense of running a hardware load sharing device, you'll often find that a web server such as Apache httpd, running mod_proxy and perhaps mod_proxy_balancer will do just as good a job for you. After all, a restaurant with 10 waiters / waitresses only needs one maitre d'hotel to seat people and to check them out at the end, so you'll only need one front door server even if you have a lot of web servers doing the real work behind it.

There are a number of ways of sharing the load of a single website across multiple servers.

In a distributed setup, certain folders / subdirectories are forwarded to one second tier server, other folders to another second tier server, and so on.

In a balanced setup, NEW connections are forwarded based on a balancing algorithm to a series of near-identical second tier servers. The balancing may be as simple as "round robin" where each second tier server takes its turn to get new clients, or the top (load balancing) server may look at queue length, or even how quickly each server in the second tier is responding as it makes its choice. RETURNING (old) connections are passed back to the same server that previous requests were passed to, so that a user's series of forms will all be handled by the same machine.

In a clustered setup, all connections are forwarded based on a balancing algorithm. However, in this case no account is taken of whether or not the connection is a new one, and requests can end up at any of the second tier servers. However, in a cluster, each second tier server broadcasts (multicasts) after each request is handled to copy the user's current data to all servers. That way, whichever server is next selected, the data will be on hand for it to work correctly.

(written 2012-10-13, updated 2012-10-20)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles

A690 - Web Application Deployment - Clustering and load balancing
  [934] Clustering, load balancing, mod_rewrite and mod_proxy - (2006-11-21)
  [1121] Sharing the load with Apache httpd and perhaps Tomcat - (2007-03-29)
  [1771] More HowTo diagrams - MySQL, Tomcat and Java - (2008-08-24)
  [1993] Load Balancing - Hardware or Software? - (2009-01-15)
  [1995] Automated server heartbeat and health check - (2009-01-16)
  [2059] Sharing the load between servers - httpd and Tomcat - (2009-02-28)
  [2482] Load balancing with sticky sessions (httpd / Tomcat) - (2009-10-29)
  [2483] Clustering on Tomcat - (2009-10-30)
  [3293] Distributing the server load - yet ensuring that each user return to the same system (Apache httpd and Tomcat) - (2011-05-18)
  [3339] Simplest ever proxy configuration? - (2011-06-28)
  [4432] Java web application for teaching - now with sessions and clustering / load balancing demonstrations - (2015-02-20)

A608 - Web Application Deployment - Apache httpd mod_proxy
  [631] Apache httpd to Tomcat - jk v proxy - (2006-03-03)
  [1006] Apache httpd and Apache Tomcat together tips - (2006-12-24)
  [1376] Choosing between mod_proxy and mod_rewrite - (2007-10-02)
  [1566] Strange behaviour of web directory requests without a trailing slash - (2008-03-06)
  [1767] mod_proxy and mod_proxy_ajp - httpd - (2008-08-22)
  [1939] mod_proxy_ajp and mod_proxy_balancer examples - (2008-12-13)
  [1944] Forwarding session and cookie requests from httpd to Tomcat - (2008-12-14)
  [2062] Virtual hosting and mod_proxy forwarding of different domains (httpd) - (2009-03-01)
  [2273] Three recent questions on Tomcat Convertors - (2009-07-07)
  [2325] Apache, Tomcat, mod_proxy - (2009-08-06)
  [3680] How can I run multiple web servers behind a single IP address? - (2012-04-02)

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The components of an Apache httpd / Tomcat / MySQL stack and what each does

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Public Transport across Wiltshire - a new map

Some other Articles

An email marathon
Flowchart to program - learning to program with Well House
A wet Saturday
Public Transport across Wiltshire - a new map
Distributed, Balanced and Clustered Load Sharing - the difference
The components of an Apache httpd / Tomcat / MySQL stack and what each does
Here comes Santa - on the train from Melksham to Swindon on 2nd December 2012
Melksham Chamber of Commerce - whence in 2013 and beyond?
What a difference a year made - Melksham Campus
Inheritance, Composition and Associated objects - when to use which - Python example

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