Distributed Systems

Definition of a distributed system

Various definitions of distributed systems have been given in the literature, none of them satisfactory, and none of them in agreement with any of the others. For our purposes it is sufficient to give a loose characterization:


A distributed system is a collection of independent computers that appears to its users as a single coherent system.

This definition has several important aspects. The first one is that a distributed system consists of components (i.e., computers) that are au- tonomous. A second aspect is that users (be they people or programs) think they are dealing with a single system. This means that one way or the other the autonomous components need to collaborate. How to establish this col- laboration lies at the heart of developing distributed systems. Note that no assumptions are made concerning the type of computers. In principle, even within a single system, they could range from high-performance mainframe computers to small nodes in sensor networks. Likewise, no assumptions are made on the way that computers are interconnected. We will return to these aspects later in this chapter.

Instead of going further with definitions, it is perhaps more useful to concentrate on important characteristics of distributed systems. One important characteristic is that differences between the various computers and the ways in which they communicate are mostly hidden from users. The same holds for the internal organization of the distributed system. Another important characteristic is that users and applications can interact with a distributed system in a consistent and uniform way, regardless of where and when interaction takes place.
In principle, distributed systems should also be relatively easy to expand or scale. This characteristic is a direct consequence of having independent computers, but at the same time, hiding how these computers actually take part in the system as a whole. A distributed system will normally be continuously available, although perhaps some parts may be temporarily out of order. Users and applications should not notice that parts are being

replaced or fixed, or that new parts are added to serve more users or applications.

In order to support heterogeneous computers and networks while offer- ing a single-system view, distributed systems are often organized by means of a layer of software–that is, logically placed between a higher-level layer consisting of users and applications, and a layer underneath consisting of operating systems and basic communication facilities, as shown in Figure 1.1 Accordingly, such a distributed system is sometimes called middleware.

Figure 1.1: A distributed system organized as middleware. The middleware layer extends over multiple machines, and offers each application the same interface.

Figure 1.1 shows four networked computers and three applications, of which application B is distributed across computers 2 and 3. Each appli- cation is offered the same interface. The distributed system provides the means for components of a single distributed application to communicate with each other, but also to let different applications communicate. At the same time, it hides, as best and reasonable as possible, the differences in hardware and operating systems from each application.

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