Quality of service


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Once users are provided with the functionality that they require of a service, such as the file service in a distributed system, we can go on to ask about the quality of the service provided. The main nonfunctional properties of systems that affect the quality of the
service experienced by clients and users are reliability, security
that is retrieving a film from a video server and presenting it on the user’s screen. For another at a fixed rate. For example, a movie service might consist of a client program streams of data that are required to be processed or transferred from one process to
time-critical data Some applications, including multimedia applications, handle ability to meet timeliness guarantees, as discussed in the following paragraphs.
responsiveness and computational throughput, but it has been redefined in terms of The performance aspect of quality of service was originally defined in terms of Reliability and security issues are critical in the design of most computer systems.
recognized as a further important aspect of service quality.
to meet changing system configurations and resource availability has been Adaptability performance
satisfactoryresult the successive frames of video need to be displayed to the user within
some specified time limits.
In fact, the abbreviation QoS has effectively been commandeered to refer to the ability of systems to meet such deadlines. Its achievement depends upon the availability of the necessary computing and network resources at the appropriate times. This implies
a requirement for the system to provide guaranteed computing and communication resources that are sufficient to enable applications to complete each task on time (for example, the task of displaying a frame of video).
The networks commonly used today have high performance – for example, BBC iPlayer generally performs acceptably – but when networks are heavily loaded their performance can deteriorate, and no guarantees are provided. QoS applies to operating systems as well as networks. Each critical resource must be reserved by the applications that require QoS, and there must be resource managers that provide guarantees.Reservation requests that cannot be met are rejected.

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Frequently Asked Questions

Ans: oncealment from the user and the application programmer of the separation of components in a distributed system, so that the system is perceived as a whole rather than as a collection of independent components view more..
Ans: he process that manages a shared resource could take one client request at a time. But that approach limits throughput. Therefore services and applications generally allow multiple client requests to be processed concurrently. view more..
Ans: Failures in a distributed system are partial – that is, some components fail while others continue to function. Therefore the handling of failures is particularly difficult. The following techniques for dealing with failures are discussed throughout the book view more..
Ans: Reliability and security issues are critical in the design of most computer systems. The performance aspect of quality of service was originally defined in terms of responsiveness and computational throughput, but it has been redefined in terms of ability to meet timeliness guarantees, as discussed in the following paragraphs view more..
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..
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..
Ans: Physical models consider the types of computers and devices that constitute a system and their interconnectivity, without details of specific technologies. view more..
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..
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..
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..
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..
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..
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..
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..
Ans: We can group organizations that produce software into six major categories. view more..
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..
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..
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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