Distributed Systems

Distributed Systems

A recent trend in computer systems is to distribute computation among several processors. In contrast to the tightly coupled systems, the processors do not share memory or a clock. Instead, each processor has its own local memory. The processors communicate with one another through various communication lines, such as high-speed buses or telephone lines. These systems are usually referred to as loosely coupled systems, or distributed systems.

The processors in a distributed system may vary in size and function. They may include small microprocessors, workstations, minicomputers, and large general-purpose computer systems. These processors are referred to by a number of different names, such as sites, nodes, computers, and so on, depending on the context in which they are mentioned.

There is a variety of reasons for building distributed systems, the major ones being these:

• Resource sharing. If a number of different sites (with different capabilities) are connected to one another, then a user at one site may be able to use the resources available at another. For example, a user at site A may be using a laser printer available only at site B. Meanwhile, a user at B may access a file that resides at A. In general, resource sharing in a distributed system provides mechanisms for sharing files at remote sites, processing information in a distributed database, printing files at remote sites, using remote specialized hardware devices (such as a high-speed array processor), and performing other operations.

• Computation speedup. If a particular computation can be partitioned into a number of sub computations that can run concurrently, then a distributed system may allow us to distribute the computation among the various sites — to run that computation concurrently. In addition, if a particular site is currently overloaded with jobs, some of them may be moved to other, lightly loaded, sites. This movement of jobs is called load sharing.

• Reliability. If one site fails in a distributed system, the remaining sites can potentially continue operating. If the system is composed of a number of large autonomous installations (that is, general-purpose computers), the failure of one of them should not affect the rest. If, on the other hand, the system is composed of a number of small machines, each of which is responsible for some crucial system function (such as terminal character I/O or the file system), then a single failure may effectively halt the operation of the whole system. In general, if sufficient redundancy exists in the system (in both hardware and data), the system can continue with its operation, even if some of its sites have failed.

• Communication. There are many instances in which programs need to exchange data with one another on one system. Window systems are one example, since they frequently share data or transfer data between dis­plays. When many sites are connected to one another by a communication network, the processes at different sites have the opportunity to exchange information. Users may initiate file transfers or communicate with one another via electronic mail. A user can send mail to another user at the same site or at a different site.