Distributed data processing is fast becoming a reality. Businesses want to have it for many reasons, and they often must have it in order to stay competitive. While much of the infrastructure for distributed data processing is already in place (e.g., modern network technology), there are a number of issues which still make distributed data processing a complex undertaking: (1) distributed systems can become very large involving thousands of heterogeneous sites including PCs and mainframe server machines � (2) the state of a distributed system changes rapidly because the load of sites varies over time and new sites are added to the system� (3) legacy systems need to be integrated|such legacy systems usually have not been designed for distributed data processing and now need to interact with other (modern) systems in a distributed environment. This paper presents the state of the art of query processing for distributed database and information systems. The paper presents the \textbook " architecture for distributed query processing and a series of techniques that are particularly useful for distributed database systems. These techniques include special join techniques, techniques to exploit intra-query parallelism, techniques to reduce communication costs, and techniques to exploit caching and replication of data. Furthermore, the paper discusses di erent kinds of distributed systems such as client-server, middleware (multi-tier), and heterogeneous database systems and shows how query processing works in these systems. Categories and subject descriptors: E.5 [Data]:Files � H.2.4 [Database Management Systems]: distributed databases, query processing � H.2.5 [Heterogeneous Databases]: data translation General terms: algorithms � performance Additional key words and phrases: query optimization � query execution � client-server databases � middleware � multi-tier architectures � database application systems � wrappers� replication � caching � economic models for query processing � dissemination-based information systems 1

With the immense popularity of the Web,the world is witnessing an unprecedented demand for data services. At the same time, the Internet is evolving towards an information super-highway that incorporates a wide mixture of existing and emerging communication technologies, including wireless, mobile, and hybrid networking. Taking advantage of these new technologies, we are proposing a hybrid scheme which effectively combines broadcast for massive data dissemination and unicast for individual data delivery. In this paper, we describe a technique that uses the broadcast medium for storage of frequently requested data, and an algorithm that continuously adapts the broadcast content to match the hot-spot of the database. We show that the hot-spot can be accurately obtained by monitoring the "broadcast misses" observed through direct requests. This is a departure from other broadcast-based systems which rely on efficient scheduling based on precompiled user profiles. We also show that the pro...

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The emergence of powerful portable computers, along with advances in wireless communication technologies, has made mobile computing a reality. Among the applications that are finding their way to the market of mobile computingthose that involve data managementhold a prominent position. In the past few years, there has been a tremendous surge of research in the area of data management in mobile computing. This research has produced interesting results in areas such as data dissemination over limited bandwith channels, location-dependent querying of data, and advanced interfaces for mobile computers. This paper is an effort to survey these techniques and to classify this research in a few broad areas. Index Terms---Mobile computing, databases, data dissemination, bandwith, location-dependent queries, interfaces, transaction management. ------------------------------ ###p### ------------------------------ 1INTRODUCTION OBILE computing has become a reality thanks to the convergen...

Broadcast is becoming an increasingly attractive data dissemination method for large client populations. In order to effectively utilize a broadcast medium for sucha service, it is necessary to have efficient, on-line scheduling algorithms that can balance individual and overall performance, and can scale in terms of data set sizes, client populations, and broadcast bandwidth. We propose an algorithm, called RxW, that provides good performance across all of these criteria and that can be tuned to trade o# average and worst case waiting time. Unlike previous work on lowoverhead scheduling, the algorithm does not use estimates of the access probabilities of items, but rather, it makes scheduling decisions based on the current queue state, allowing it to easily adapt to changes in the intensity and distribution of the workload. We demonstrate the performance advantages of the algorithm under a range of scenarios using a simulation model and present analytical results that describe the intrinsic behavior of the algorithm.

Mobile computing has the potential for managing information globally. Data management issues in...

Abstract — Recent advances in telecommunications have enabled the deployment of broadcast-based wide-area information services that provide on-demand data access to very large client populations. In order to effectively utilize a broadcast medium for such a service, it is necessary to have efficient, on-line scheduling algorithms that can balance individual and overall performance, and can scale in terms of data set sizes, client populations, and broadcast bandwidth. In this study we introduce a parameterized algorithm that provides good performance across all of these criteria and can be tuned to emphasize either average or worst case waiting time. Unlike previous work on low overhead scheduling, the algorithm is not based on estimates of the access probabilities of items, but rather, it makes scheduling decisions based on the current queue state, allowing it to easily adapt to changes in the intensity and distribution of the workload. We examine the performance of the algorithm using a simulation model.

The dramatic improvements in global interconnectivity due to intranets, extranets, and the Internet has led to an explosion in the number and variety of new data-intensive applications. Along with the proliferation of these new applications have come increased problems of scale. This is demonstrated by frequent delays and service disruptions when accessing networked data sources. Recently, push-based techniques have been proposed as a solution to scalability problems for distributed applications. This paper argues that push indeed has its place, but that it is just one aspect of a much larger design space for distributed information systems. We propose the notion of a Dissemination-Based Information System (DBIS) which integrates a variety of data delivery mechanisms and information broker hierarchies. We discuss the properties of such systems and provide some insight into the architectural imperatives that will influence their design. The DBIS framework can serve as the basis for deve...

An important issue in the dissemination of time-varying web data such as sports scores and stock prices is the maintenance of temporal coherency. In the case of servers adhering to the HTTP protocol, clients need to frequently pull the data based on the dynamics of the data and a user's coherency requirements. In contrast, servers that possess push capability maintain state information pertaining to clients and push only those changes that are of interest to a user. These two canonical techniques have complementary properties with respect to the level of temporal coherency maintained, communication overheads, state space overheads, and loss of coherency due to (server) failures. In this paper, we show how to combine push- and pull-based techniques to achieve the best features of both approaches. Our combined technique tailors the dissemination of data from servers to clients based on (i) the capabilities and load at servers and proxies, and (ii) clients' coherency requirements. Our experimental results demonstrate that such adaptive data dissemination is essential to meet diverse temporal coherency requirements, to be resilient to failures, and for the efficient and scalable utilization of server and network resources.

this paper. article began by declaring: "Remember the browser war between Netscape and Microsoft? Well forget it. The Web browser itself is about to croak. And good riddance.". While the article was certainly provocative and clearly overstated, the argument it made was simply that push technology would change the Web from a passive library of information into a networked, immersive medium for information and entertainment delivery. Despite this simple message, the article seemed to epitomize both the promise of push technology and the potential for overselling its virtues.