Introduction




 

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Systems that are intended for use in real-world environments should be designed to function correctly in the widest possible range of circumstances and in the face of many possible difficulties and threats.
Difficulties and threats for distributed systems • Here are some of the problems that the designers of distributed systems face.
Widely varying modes of use: The component parts of systems are subject to wide variations in workload – for example, some web pages are accessed several million times a day. Some parts of a system may be disconnected, or poorly connected some of the time – for 
example when mobile computers are included in a system. Some applications have special requirements for high communication bandwidth and low latency – for example, multimedia applications. Wide range of system environments: A distributed system must accommodate heterogeneous hardware, operating systems and networks. The networks may differ widely in performance – wireless networks operate at a fraction of the speed of local networks. Systems of widely differing scales, ranging from tens of computers to millions of computers, must be supported.
Internal problems: Non-synchronized clocks, conflicting data updates and many modes of hardware and software failure involving the individual system components.
External threats: Attacks on data integrity and secrecy, denial of service attacks. The discussion and examples of Chapter 1 suggest that distributed systems of different types share important underlying properties and give rise to common design problems. In this
chapter we show how the properties and design issues of distributed systems can be captured and discussed through the use of descriptive models. Each type ofmodel is intended to provide an abstract, simplified but consistent description of a
relevant aspect of distributed system design:
Physical models are the most explicit way in which to describe a system; they capture the hardware composition of a system in terms of the computers (and other devices, such as mobile phones) and their interconnecting networks.
Architectural models describe a system in terms of the computational and communication tasks performed by its computational elements; the computational elements being individual computers or aggregates of them supported by appropriate network interconnections.
Fundamental models take an abstract perspective in order to examine individual aspects of a distributed system. In this chapter, we introduce fundamental models that examine three important aspects of distributed systems: interaction models, which
consider the structure and sequencing of the communication between the elements of the system; failure models, which consider the ways in which a system may fail to
operate correctly and; security models, which consider how the system is protected against attempts to interfere with its correct operation or to steal its data.

Topics You May Be Interested In
System Development Life Cycle (sdlc) Physical Models
Direct And Indirect Measures, Reliability Outsourcing-systems Acquisition
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Transparency Building The Baseline Project Plan


Frequently Asked Questions

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Ans: Physical models consider the types of computers and devices that constitute a system and their interconnectivity, without details of specific technologies. view more..
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Ans: Resource sharing is the main motivating factor for constructing distributed systems. Resources such as printers, files, web pages or database records are managed by servers of the appropriate type. For example, web servers manage web pages and other web resources. Resources are accessed by clients – for example, the clients of web servers are generally called browsers. view more..
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Ans: The Web began life at the European centre for nuclear research (CERN), Switzerland, in 1989 as a vehicle for exchanging documents between a community of physicists connected by the Internet [Berners-Lee 1999]. A key feature of the Web is that it provides a hypertext structure among the documents that it stores, reflecting the users’ requirement to organize their knowledge. view more..
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Ans: The discussion and examples of Chapter 1 suggest that distributed systems of different types share important underlying properties and give rise to common design problems. In this chapter we show how the properties and design issues of distributed systems can be captured and discussed through the use of descriptive models view more..
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Ans: A physical model is a representation of the underlying hardware elements of a distributed system that abstracts away from specific details of the computer and networking technologies employed. view more..
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Ans: Major concerns are to make the system reliable, manageable, adaptable and cost-effective. The architectural design of a building has similar aspects – it determines not only its appearance but also its general structure and architectural style (gothic, neo-classical, modern) and provides a consistent frame of reference for the design view more..
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Ans: From a system perspective, the answer is normally very clear in that the entities that communicate in a distributed system are typically processes, leading to the prevailing view of a distributed system as processes coupled with appropriate interprocess communication paradigms view more..
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Ans: ion for a given problem domain. This is a large topic, and many architectural patterns have been identified for distributed systems. In this section, we present several key architectural patterns in distributed systems, including layering and tiered architectures and the related concept of thin clients (including the specific mechanism of virtual network computing). We also examine web services as an architectural pattern and give pointers to others that may be applicable in distributed systems. view more..
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Ans: As mentioned in the introduction, networks are everywhere and underpin many everyday services that we now take for granted: the Internet and the associated World Wide Web, web search, online gaming, email, social networks, eCommerce, etc. To illustrate this point further, consider Figure 1.1 , which describes a selected range of key commercial or social application sectors highlighting some of the associated established or emerging uses of distributed systems technology. view more..
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Ans: If another organization develops or runs a computer application for your organization, that practice is called outsourcing. Outsourcing includes a spectrum of working arrangements view more..
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Ans: We can group organizations that produce software into six major categories. view more..
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Ans: Once you have decided to purchase off-the-shelf software rather than write some or all of the software for your new system, how do you decide what to buy? Several criteria need consideration, and special ones may arise with each potential software purchase. view more..
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Ans: Reuse is the use of previously written software resources in new applications. Because so many bits and pieces of applications are relatively generic across applications, it seems intuitive that great savings can be achieved in many areas if those generic bits and pieces do not have to be written anew each time they are needed. view more..
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Ans: Pine Valley Furniture (PVF) Company manufactures high-quality wood furniture and distributes it to retail stores within the United States. Its product lines include dinette sets, stereo cabinets, wall units, living room furniture, and bedroom furniture. view more..
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Ans: During project initiation the project manager performs several activities that assess the size, scope, and complexity of the project, and establishes procedures to support subsequent activities. view more..
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Ans: The next step in the project management process is project planning. Project planning involves defining clear, discrete activities and the work needed to complete each activity within a single project. view more..
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Ans: Project execution puts the baseline project plan into action. view more..
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Ans: The focus of project closedown is to bring the project to an end. Projects can conclude with a natural or unnatural termination. view more..



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