Sách CHAPTER 1- INTRODUCTION TO DISTRIBUTED SYSTEMS : Communication Theory Basics

Introduction
A distributed system consists of a collection of autonomous computers linked
by a computer network and equipped with distributed system software.
Distributed system software enables computers to co-ordinate their activities
and to share resources such as computer hardware, software & data. Users of a
distributed system should perceive a single, integrated computing facility even
though the system is implemented using many computers in different locations.
The development of the distributed systems followed the emergence of high-
speed local area networks at the beginning of the 1970s. Recently the availability
of high-performance personal computers, workstations and server computers has
resulted in a major shift towards distributed systems. This caused a drawback in
centralized & multi-user computing systems. The trend of distributed systems has
been increased by the development of distributed system software, designed to
support the development of distributed applications.

1.1 shows the components of a distributed system based on a local area network.
Here there is no central computer. Each user is provided with a computer
sufficiently powerful to run their programs, and there are many types of servers
that provide access to a variety of shared system facilities and resources. These
facilities include filing, printing, wide-area communication and user authentication.
The programs running on users' workstations act as clients of the servers, obtaining
whatever access they require to the servers' resources. The number of servers can
be incremented to support the workload. For example, even for a small-distributed
system several file servers are often included, because the frequency of access to
files by client programs would swamp the processing capacity of a single server
computer. This simple client-server model has been the basis for much of the
distributed system developments that are undertaken.

Examples for distributed systems
1. Distributed UNIX system - The UNIX system can exploit the resources of
many computers, offering facilities and performance that exceeds the performance
of a single multi user system. Although the development of distributed UNIX
system offers obvious commercial and technical advantages, UNIX was originally
designed to meet a restricted range of requirements in an era when the limitations of
computer hardware restricted the scope of the designers.
2. Commercial Applications - Many commercial data processing and
information systems involve data communication. Examples include airline
reservation systems, networks operated by banks to support their branches and to
operate their automatic teller machines, the system used in supermarkets to co-ordinate
their stock control, delivery and sales check-out. Most current implementations of




P1. Distributed Systems