Clustered system is another form of multiprocessor system. This type of system contains multiple processors but the design differs from multiprocessor system. The clustered system consists of two or more individual systems or nodes that are joined (coupled) together. In clustered system, individual systems (or clustered computers) share the same storage and are linked together via local area network (LAN).
Clustering is usually used to provide high availability service. This service will continue even if one or mòre systems in the cluster fail. A layer of cluster software runs on the cluster nodes. Each node can monitor one or more of the other nodes over the LAN. If the monitored machine fails due to some technical fault (or due to other reason), the monitoring machine can take ownership of its storage. The monitoring machine can also restart the applications that were running on the failed machine. The users of the applications see only a brief interruption of service.
Types of Clustered Systems
Like multiprocessor systems, clustered system can also be of two types:
i) Asymmetric Clustered System
ii) Symmetric Clustered System
Asymmetric Clustered System
In asymmetric clustered system, one machine is in hot-standby mode, while the other machine is running the application. The hot-standby host machine does nothing. It only monitors the active server. If the server fails, the hot-standby machine becomes the active server.
Symmetric Clustered System
In symmetric clustered system, two or more hosts (machines) run the applications and monitor each other. This structure or mode is more efficient than asymmetric system, because it uses all the available hardware. This mode is used only if more than one application be available to run.
Other Forms of Clustered Systems
Other forms of clusters include parallel clusters and clustering over a wide-area Other for network (WAN). Parallel cluster allows multiple hosts to access the same data on the shared storage. Most operating systems do not provide support for simultaneous data access by multiple hosts. The parallel clustered systems are usually managed by use of special version of software.
The most no he popular example nple of this software is Oracle Parallel Server. It is a special version of Oracle’s database that has been designed to run on the parallel clusters Each machine runs this software and has full access to all data in the database. To provide this shared access to data, the clustered system must also provide “access control and locking service” to avoid the conflicting operations. This type of service is commonly known as the service Distributed Lock Management (DLM).
