This chapter gives reasons for the study of distributed algorithms by briefly introducing the types of hardware and software systems for which distributed algorithms have been developed. By a distributed system we mean all computer applications where several computers or processors cooperate in some way. This definition includes wide-area computer communication networks, but also local-area networks, multiprocessor computers in which each processor has its own control unit, and systems of cooperating processes.

The different types of distributed system and the reasons why distributed systems are used are discussed in Section 1.1. Some examples of existing systems will be given. The main topic of this book, however, is not what these systems look like, or how they are used, but how they can be made to work. And even that topic will be further specialized towards the treatment of the algorithms used in the systems.

Of course, the entire structure and operation of a distributed system is not fully understood by a study of its algorithms alone. To understand such a system fully one must also study the complete architecture of its hardware and software, that is, the partition of the entire functionality into modules. Also, there are many important questions related to properties of the programming languages used to build the software of distributed systems. These subjects will be discussed in Section 1.2.

It is the case, however, that there are already in existence excellent books about distributed systems, which concentrate on the architectural and language aspects; see, e.g., Tanenbaum [Tan96], Sloman and Kramer [SK87], Bal [Bal90], Coulouris and Dollimore [CD88] or Goscinski [Gos91].