Clustered system, which gathers together multiple CPUs they are composed of two or more individual systems—or nodes—joined together. Such systems are considered loosely coupled.
Each node may be a single processor system or a multicore system. We should note that the definition of clustered is not concrete; many commercial packages wrestle to define a clustered system and why one form is better than another.
The generally accepted definition is that clustered computers share storage and are closely linked via a local-area network LAN or a faster interconnect, such as InfiniBand
Clustering is usually used to provide high-availability service—that is, service will continue even if one or more systems in the cluster fail. Generally, we obtain high availability by adding a level of redundancy in the system.
A layer of cluster software runs on the cluster nodes. Each node can monitor one or more of the others (over the LAN). If the monitored machine fails, the monitoring machine can take ownership of its storage and restart the applications that were running on the failed machine. The users and clients of the applications see only a brief interruption of service