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The Roles Clusters Play In Distributed Systems
By Peter William Lount

Introduction
Clusters Enable Real-time Collaboration with Vast Numbers of Users

Distributed computing via the Internet enables collaboration between large numbers of people that are dispersed across the globe. It also enables the joining together of vast amounts of computing resources into a vast super computer which can be used to solve problems that require a lot of computational power. Problems are split up into tiny pieces and spread around the computing nodes in the distributed network where the computation is done at the same time (in parallel) and finally the results are returned and integrated into the whole of the solution. This works remarkably well for certain applications while for others it's a puzzle that takes work to find a successful fragmenting and integration strategy.

Interactive collaborative applications that require real-time or near real-time performance in the interaction between the computers in the distributed network face challenges due to limitations of the current technologies deployed that make up the Internet. Interactive distributed collaborative applications are those that need to communicate many "updates" between computers over a network with real-time or near real-time performance. Interactive collaborative applications include but are not limited to simulations, games, live business models and collaborative business and scientific applications.

A typical requirement of interactive collaborative applications is that many update messages, with potentially large amounts of data, must be sent amoung the many computers making up the network. For real-time interaction performance this means many (often as many as possible) update messages need to be sent via the Internet per second. For near real-time interaction performance a few messages per second or every few seconds is acceptable. Collaborative applications that relax this real-time constraint can still be interactive, but become non-real-time in that they only send update messages across the Internet only on an as needed basis, so messages may not occur for many minutes or even longer. Usually these non-real-time applications are not real-time simulations.

The typical performance on the Internet is highly variable and relitively slow for the majority of users even if they have current broadband Internet connections such as ADSL or a Cable Modem. With high speed Internet connection it can easily take anywhere between 30 milli-seconds (ms) and 400 ms to transmit a round trip packet and receive an answer back from one location and another. With round trip packet times of 30 ms to 140 ms being typical within a city and 100 ms to 400 ms being typical between cities across the North American continent you can't get to many back and forth messages send per second. It works out to at best around 30 round trip messages per second in a city and around 2.5 to 10 round trip messages per second between cities. It gets worse when you consider Intenet rush hours, lost packets, errors, and other factors such as processing time to respond to a message.

High speed clusters enable massive simulations to progress in real-time or near real-time. They allow massive collaboration in shared realities to occur using existing technologies. How do they do this? Most of the "shared relality" portions of the simulation gets done on the high speed cluster. A cluster can be made up of massive numbers of low speed commodity computers with high speed network interconnects. The interconnects can range from 100 megabits per second to 1 gigabits per second to 10 gigabits per second any beyond in the future. They can also be exotic direct connections between the main system bus of one computer to another (PCI to PCI). Cross bar type of interconnects.

THe clusters can be at one or more locations. The shared reality simulation occurs within the clusters computing nodes utlizing the high speed network interconnects that are many orders of magnitude faster than connections via the Internet.

In an online massive interactive simulation style game the cluster runs the shared reality portions with the players computers computing their "local point of view" of the reality. The game simulation cluster can be scales to meet the market demand for the game. The players pay a subscription to play the game. As the subscription base grows the cluster will need to grow accordingly.

The hardware in the cluster can be scaled to as many nodes as is needed and practicle given resource and technological limitations.

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