In computers, clustering is the use of multiple computers, typically PCs or UNIX workstations, multiple storage devices, and redundant interconnections, to form what appears to users as a single highly available system. Cluster computing can be used for load balancing as well as for high availability. Advocates of clustering suggest that the approach can help an enterprise achieve 99.999 availability in some cases. One of the main ideas of cluster computing is that, to the outside world, the cluster appears to be a single system.
A common use of cluster computing is to load balance traffic on high-traffic Web sites. A Web page request is sent to a "manager" server, which then determines which of several identical or very similar Web servers to forward the request to for handling. Having a Web farm (as such a configuration is sometimes called) allows traffic to be handled more quickly.
Clustering has been available since the 1980s when it was used in DEC's VMS systems. IBM's Sysplex is a cluster approach for a mainframe system. Microsoft, Sun Microsystems, and other leading hardware and software companies offer clustering packages that are said to offer scalability as well as availability. As traffic or availability assurance increases, all or some parts of the cluster can be increased in size or number.
Cluster computing can also be used as a relatively low-cost form of parallel processing for scientific and other applications that lend themselves to parallel operations. An early and well-known example was the Beowulf project in which a number of off-the-shelf PCs were used to form a cluster for scientific applications.