The increasing ability to interconnect computers through internetworking,wireless networks, high-bandwidth satellite, and cable networks has spawned a new class of information-centered applications based on data dissemination. These applications employ broadcast to deliver data to very large client populations. We have proposed the Broadcast Disks paradigm [Zdon94, Acha95b] for organizing the contents of a data broadcast program and for managing client resources in response to such a program. Our previous workon Broadcast Disks focused exclusively on the "push-based" approach, where data is sent out on the broadcast channel according to a periodic schedule, in anticipation of client requests. In this paper, we study how to augment the push-only model with a "pull-based" approach of using a backchannel to allow clients to send explicit requests for data to the server. We analyze the scalability and performance of a broadcast-based system that integrates push and pull and study the impac...

Moving object environments are characterized by large numbers of moving objects and numerous concurrent continuous queries over these objects. Efficient evaluation of these queries in response to the movement of the objects is critical for supporting acceptable response times. In such environments the traditional approach of building an index on the objects (data) suffers from the need for frequent updates and thereby results in poor performance. In fact, a brute force, no-index strategy yields better performance in many cases. Neither the traditional approach, nor the brute force strategy achieve reasonable query processing times. This paper develops novel techniques for the efficient and scalable evaluation of multiple continuous queries on moving objects. Our solution leverages two complimentary techniques: Query Indexing and Velocity Constrained Indexing (VCI). Query Indexing relies on i) incremental evaluation; ii) reversing the role of queries and data; and iii) exploiting the relative locations of objects and queries. VCI takes advantage of the maximum possible speed of objects in order to delay the expensive operation of updating an index to reflect the movement of objects. In contrast to an earlier technique [29] that requires exact knowledge about the movement of the objects, VCI does not rely on such information. While Query Indexing outperforms VCI, it does not efficiently handle the arrival of new queries. Velocity constrained indexing, on the other hand, is unaffected by changes in queries. We demonstrate that a combination of Query Indexing and Velocity Constrained Indexing enables the scalable execution of insertion and deletion of queries in addition to processing ongoing queries. We also develop several optimizations and present a detaile...

Information dissemination is a powerful mechanism for finding information in wide-area environments. An information dissemination server accepts long-term user queries, collects new documents from information sources, matches the documents against the queries, and continuously updates the users with relevant information. This paper is a retrospective of the Stanford Information Filtering Service (SIFT), a system that as of April 1996 was processing over 40,000 worldwide subscriptions and over 80,000 daily documents. The paper describes some of the indexing mechanisms that were developed for SIFT, as well as the evaluations that were conducted to select a scheme to implement. It also describes the implementation of SIFT, and experimental results for the actual system. Finally, it also discusses and experimentally evaluates techniques for distributing a service such as SIFT for added performance and availability. Note to Referees: This paper contains material from three earlier...

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

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...

Caching frequently accessed data items on the client side is an effective technique for improving performance in a mobile environment. Classical cache invalidation strategies are not suitable for mobile environments due to frequent disconnections and mobility of the clients. One attractive cache invalidation technique is based on invalidation reports (IRs). However, the IR-based cache invalidation solution has two major drawbacks, which have not been addressed in previous research. First, there is a long query latency associated with this solution since a client cannot answer the query until the next IR interval. Second, when the server updates a hot data item, all clients have to query the server and get the data from the server separately, which wastes a large amount of bandwidth. In this paper, we propose an IR-based cache invalidation algorithm, which can significantly reduce the query latency and efficiently utilize the broadcast bandwidth. Detailed analytical analysis and simulation experiments are carried out to evaluate the proposed methodology. Compared to previous IR-based schemes, our scheme can significantly improve the throughput and reduce the query latency, the number of uplink request, and the broadcast bandwidth requirements.

A crucial consideration in environments where data is broadcast to clients is the low bandwidth available for clients to communicate with servers. Advanced applications in such environments do need to read data that is mutually consistent aswell as current. However, given the asymmetric communication capabilities and the needs of clients in mobile environments, traditional serializability-based approaches are too restrictive, unnecessary, and impractical. We thus propose the use of a weaker correctness criterion called update consistency and outline mechanisms based on this criterion that ensure (1) the mutual consistency of data maintained by the server and read by clients, and (2) the currency of data read by clients. Using these mechanisms, clients can obtain data that is current and mutually consistent "off the air", i.e., without contacting the server to, say, obtain locks. Experimental results show a substantial reduction in response times as compared to existing (serializability-based) approaches. A further attractive feature of the approach is that if caching is possible at a client, weaker forms of currency can be obtained while still satisfying the mutual consistency of data.

Recently, push-based delivery has attracted considerable attention as a means of disseminating information to large client populations in both wired and wireless settings. In this paper, we address the problem of ensuring the consistency and currency of client read-only transactions in the presence of updates. To this end, additional control information is broadcast. A suite of methods is proposed that vary in the complexity and volume of the control information transmitted and subsequently differ in response times, degrees of concurrency, and space and processing overheads. The proposed methods are combined with caching to improve query latency. The relative advantages of each method are demonstrated through both simulation results and qualitative arguments. Read-only transactions are processed locally at the client without contacting the server and thus the proposed approaches are scalable, i.e., their performance is independent of the number of clients.

In this paper, we present Bit-Sequences (BS), an adaptive cache invalidation algorithm for client/server mobile environments. The algorithm uses adaptable mechanisms to adjust the size of the invalidation report to optimize the use of a limited communication bandwidth while retaining the effectiveness of cache invalidation. The proposed BS algorithm is especially suited for dissemination-based (or "server-push" based) nomadic information service applications. The critical aspect of our algorithm is its self-adaptability and effectiveness, regardless of the connectivity behavior of the mobile clients. The performance of BS is analyzed through a simulation study that compares BS's effectiveness with that of a hypothetical optimal cache invalidation algorithm. 1 Introduction In mobile and wireless environments, caching of frequently-accessed data is critical for reducing contention on the narrow bandwidth channels. Classical cache invalidation strategies in these environments are likely ...

This paper examines the construction of distributed information systems that incorporate the ability to push data out to clients (i.e., dissemination) in addition to having clients pull data from servers. One key issue that distinguishes such dissemination-based information systems (DBIS) from more traditional ones is communications asymmetry. Asymmetry arises in many new applications due to both physical characteristics such as network bandwidths, as well as to workload characteristics. We outline a number of different data delivery mechanisms, and then focus on one particular dissemination mechanism that we have developed, called Broadcast Disks. Finally, we discuss issues that arise in the design of a DBIS architecture that can provide many different modes of data delivery. 1 Introduction Ongoing advances in communications including the proliferation of the Internet and intranets, the development of mobile and wireless networks, and the impending availability of high bandwidth link...