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Theme and Purpose

Systems are distributed for either or both of two main reasons:

  1. An organization and its information systems may be inherently distributed and in connecting its systems into a seamless whole, a distributed system appears.
  2. An organization may take inherently centralized information processing systems and distribute them to achieve higher reliability, availability, safety or performance, or all of the above.

A distributed system consists of a number of components, which are themselves computer systems. The components are connected by some communications medium, usually a sophisticated network. Applications execute by using a number of processes in different component systems. These processes communicate and interact to achieve productive work within the application. A distributed system has a number of advantages over a single computer system:

It can be more fault tolerant. It can be designed so that if one component of the system fails then the others will continue to work. Such a system will provide useful work in the face of quite a large number of failures in individual component systems.

It is more flexible. A distributed system can be made up from a number of different components. Some of these components may be specialized for a specific task, others may be general purpose.

It is easier to extend. More processing, storage or other power can be obtained by increasing the number of components.

It is easier to upgrade. When a single large computer system becomes obsolete all of it has to be replaced in a costly and disruptive operation. A distributed system may be upgraded in increments by replacing individual components without a major disruption, or a large cash injection.

Distributed Systems also introduce several problems not encountered in centralized systems.

They are significantly more complex.

They introduce problems of synchronization between processes.

They introduce problems of maintaining consistency if data.

In general, there is no longer a central management entity in control of the whole system.

next up previous contents
Next: Approach Up: Preface Previous: Preface

Jon Crowcroft
Fri May 10 14:13:51 BST 1996