Distributed Computing 1

A distributed computer system (DCS) is a collection of computers connected by a communications subnet and logically integrated in varying degrees by a distributed operating system and/or distributed database system. Each computer node may be a uniprocessor, or multiprocessor, or multicomputer. The communications subnet may be a widely geographically dispersed collection of communication processors or a local area network. Typical applications that use distributed computing include e-mail, teleconferencing, electronic funds transfers, multi-media telecommunications, command and control systems, and support for general purpose computing in industrial and academic settings. The widespread use of distributed computer systems is due to the price-performance revolution in microelectronics, the development of cost e ective and e cient communication subnets [4] (which is itself due to the merging of data communications and computer communications), the development of resource sharing software, and the increased user demands for communication, economical sharing of resources, and productivity. A DCS potentially provides signi cant advantages, including good performance, good reliability, good resource sharing, and extensibility [35, 41]. Potential performance enhancement is due to multiple processors and an e cient subnet, as well as avoiding contention and bottlenecks that exist in uniprocessors and multiprocessors. Potential reliability improvements are due to the data and control redundancy possible, the geographical distribution of the system, and the ability for hosts and communication processors to perform mutual inspection. With the proper subnet, distributed operating system [46], and distributed database [85], it is possible to share hardware and software resources in a cost e ective manner, increasing productivity and lowering costs. Possibly the most important potential advantage of a DCS is extensibility. Extensibility is the ability to easily adapt to both short and long term changes without signi cant disruption of the system. Short term changes include varying workloads and host or subnet failures or additions. Long term changes are associated with major modi cations to the requirements or content of the system.