1.1 What is a Cluster?

If this were a science fiction novel, then when we refer to clusters, we would be referring to stars. If this were a book on wine, then we would be referring to grapes. The term clusters has been bandied about and used to mean many things in Information Technology. With this in mind, let's define what clusters are. According to one definition in the Merriam Webster Dictionary (the online version of course) a cluster is "a number of similar individuals that occur together." Well, that's not quite right in terms of what a computer cluster is, but it's a good start.

For a more precise definition of what a cluster is, try this: "A cluster is a type of parallel or distributed computer system that forms, to varying degrees, a single, unified resource composed of several interconnected computers. Each interconnected computer has one or more processors, I/O capabilities, an operating-system kernel, and memory."^[1]

What differentiates clustering from distributed computing is that with a cluster, a number of similar computers work together and form a cadre. This unifying relationship becomes the basis for providing the primary themes in clustering: increased application availability (or high availability), load balancing for scalability and performance, and ease of manageability.

This book provides a detailed description of how to create real, single-system image clusters from individual UNIX servers using Compaq's Tru64 UNIX operating system and TruCluster Server software.

^[1]"Clusters: Moving Beyond Failover," by Bruce Walker, August 1998, UNIX Review.com