CHARACTERIZATION OF DISTRIBUTED SYSTEMS




 

  1. A distributed system is one in which components located at networked computers communicate and coordinate their actions only by passing messages. This definition leads to the following especially significant characteristics of distributed systems: concurrency of components, lack of a global clock and independent failures of components.
  2. We look at several examples of modern distributed applications, including web search, multiplayer online games and financial trading systems, and also examine the key underlying trends driving distributed systems today: the pervasive nature of modern networking, the emergence of mobile and ubiquitous computing, the increasing importance of distributed multimedia systems, and the trend towards viewing distributed systems as a utility.
  3. Resource sharing as the main motivation for constructing distributed systems. Resources may be managed by servers and accessed by clients or they may be encapsulated as objects and accessed by other client objects.
  4. The challenges arising from the construction of distributed systems are the heterogeneity of their components, openness (which allows components to be added or replaced), security, scalability – the ability to work well when the load or the number of users increases – failure handling, concurrency of components, transparency and providing quality of service. Finally, the Web is discussed as an example of a large-scale distributed system and its main features are introduced.

Networks of computers are everywhere. The Internet is one, as are the many networks of which it is composed. Mobile phone networks, corporate networks, factory networks, campus networks, home networks, in-car networks – all of these, both separately and in the combination, share the essential characteristics that make them relevant subjects for study under the heading distributed systems. In this book, we aim to explain the characteristics of networked computers that impact system designers and implementors and to present the main concepts and techniques that have been developed to help in the tasks of designing and implementing systems that are based on them.
We define a distributed system as one in which hardware or software components located at networked computers communicate and coordinate their actions only by passing messages. This simple definition covers the entire range of systems in which networked computers can usefully be deployed. Computers that are connected by a network may be spatially separated by any distance. They may be on separate continents, in the same building or in the same room.
Our definition of distributed systems has the following significant consequences:

  • Concurrency: In a network of computers, concurrent program execution is the norm. I can do my work on my computer while you do your work on yours, sharing resources such as web pages or files when necessary. The capacity of the system to handle shared resources can be increased by adding more resources (for example computers) to the network. The coordination of concurrently executing programs that share resources is also an important and recurring topic.
  • No global clock: When programs need to cooperate they coordinate their actions by exchanging messages. Close coordination often depends on a shared idea of the time at which the programs’ actions occur. But it turns out that there are limits to the accuracy with which the computers in a network can synchronize their clocks – there is no single global notion of the correct time. This is a direct consequence of the fact that the only communication is by sending messages through a network.
  • Independent failures: All computer systems can fail, and it is the responsibility of system designers to plan for the consequences of possible failures. Distributed systems can fail in new ways. Faults in the network result in the isolation of the computers that are connected to it, but that doesn’t mean that they stop running. In fact, the programs on them may not be able to detect whether the network has failed or has become unusually slow. Similarly, the failure of a computer, or the unexpected termination of a program somewhere in the system (a crash), is not immediately made known to the other components with which it communicates. Each component of the system can fail independently, leaving the others still running. The consequences of this characteristic of distributed systems will be a recurring theme throughout the book.

The prime motivation for constructing and using distributed systems stems from a desire to share resources. The term ‘resource’ is a rather abstract one, but it best characterizes the range of things that can usefully be shared in a networked computer system. It extends from hardware components such as disks and printers to software-defined entities such as files, databases and data objects of all kinds. It includes the stream of video frames that emerges from a digital video camera and the audio connection that a mobile phone call represents. The purpose of this chapter is to convey a clear view of the nature of distributed systems and the challenges that must be addressed in order to ensure that they are successful.



Frequently Asked Questions

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Ans: What Is Information Systems Analysis and Design? Information systems analysis and design is a method used by companies ranging from IBM to PepsiCo to Sony to create and maintain information systems that perform basic business functions such as keeping track of customer names and addresses, processing orders, and paying employees. The main goal of systems analysis and design is to improve organizational systems, typically through applying software that can help employees accomplish key business tasks more easily and efficiently. As a systems analyst, you will be at the center of developing this software. view more..
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Ans: A direct measure is obtained by applying measurement rules directly to the phenomenon of interest.For example, by using the specified counting rules, a software program’s “Line of Code” can be measured directly. and sofware reliabity is .... view more..
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Ans: There are three such classes: Process are collection of software related activities. Products are any artifacts, deliverables or documents that result from a process activity view more..
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Ans: concurrency of components, lack of a global clock and independent failures of components and the ability to work well when the load or the number of users increases – failure handling, concurrency of components, transparency and providing quality of service view more..
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Ans: the wide range of applications in use today, from relatively localized systems (as found, for example, in a car or aircraft) to globalscale systems involving millions of nodes, from data-centric services to processorintensive tasks, from systems built from very small and relatively primitive sensors to those incorporating powerful computational elements, from embedded systems to ones that support a sophisticated interactive user experience, and so on. view more..
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Ans: The task of a web search engine is to index the entire contents of the World Wide Web, encompassing a wide range of information styles including web pages, multimedia sources and (scanned) books view more..
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Ans: The growth of the World Wide Web as a repository of information and knowledge; the development of web search engines such as Google and Yahoo to search this vast repository view more..
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Ans: The engineering of MMOGs represents a major challenge for distributed systems technologies, particularly because of the need for fast response times to preserve the user experience of the game. view more..
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Ans: a very different style of underlying architecture from the styles mentioned above (for example client-server), and such systems typically employ what is known as distributed event-based systems. view more..
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Ans: the emergence of ubiquitous computing coupled with the desire to support user mobility in distributed systems view more..
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Ans: The Internet is also a very large distributed system. It enables users, wherever they are, to make use of services such as the World Wide Web, email and file transfer. (Indeed, the Web is sometimes incorrectly equated with the Internet.) view more..
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Ans: Technological advances in device miniaturization and wireless networking have led increasingly to the integration of small and portable computing devices into distributed systems. view more..
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Ans: The crucial characteristic of continuous media types is that they include a temporal dimension, and indeed, the integrity of the media type is fundamentally dependent on preserving real-time relationships between elements of a media type. view more..
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Ans: hysical resources such as storage and processing can be made available to networked computers, removing the need to own such resources on their own. At one end of the spectrum, a user may opt for a remote storage facility for file storage requirements view more..
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Ans: In practice, patterns of resource sharing vary widely in their scope and in how closely users work together. At one extreme, a search engine on the Web provides a facility to users throughout the world, users who need never come into contact with one another directly. At the other extreme, in computer-supported cooperative working (CSCW), a group of users who cooperate directly share resources such as documents in a small, closed group. view more..
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Ans: Data types such as integers may be represented in different ways on different sorts of hardware – for example, there are two alternatives for the byte ordering of integers. These differences in representation must be dealt with if messages are to be exchanged between programs running on different hardware view more..
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Ans: the publication of interfaces is only the starting point for adding and extending services in a distributed system. The challenge to designers is to tackle the complexity of distributed systems consisting of many components engineered by different people. view more..
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Ans: a firewall can be used to form a barrier around an intranet, restricting the traffic that can enter and leave, this does not deal with ensuring the appropriate use of resources by users within an intranet, or with the appropriate use of resources in the Internet, that are not protected by firewalls. view more..



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