Currently, a variety of information retrieval systems are available to potential users. These services are provided by commercial enterprises (such as Dow Jones [6] and The Source [7]), while others are research efforts (the Boston Community Information System [S]). While in many cases these systems are accessed from personal computers, typically no advantage is taken of the computing resources of those machines (such as local processing and storage). In this paper we explore the possibility of using the user’s local storage capabilities to cache data at the user’s site. This would improve the response time of user queries albeit at the cost of incurring the overhead required in maintaining multiple copies. In order to reduce this overhead it may be appropriate to allow copies to diverge in a controlled fashion. This would not only make caching less costly, but would also make it possible to propagate updates to the copies more efficiently, for example, when the system is lightly loaded, when communication tariffs are lower, or by batching updates together. Just as importantly, it also makes it possible to access the copies even when the communication lines or the central site are down. Thus, we introduce the notion of quasi-copies, which embodies the ideas sketched above. We also define the types of deviations that seem useful, and discuss the available implementation strategies.

This paper is an answer to the question: "What is unique and conceptually different about mobile computing? " The paper begins by describing a set of constraints intrinsic to mobile computing, and examining the impact of these constraints on the design of distributed systems. Next, it summarizes the key results of the Coda and Odyssey systems. Finally, it describes the research opportunities in five important topics relevant to mobile computing: caching metrics, semantic callbacks and validators, resource revocation, analysis of adaptation, and global estimation from local observations. 1.2. The Need for Adaptation Mobility exacerbates the tension between autonomy and interdependence that is characteristic of all distributed systems. The relative resource poverty of mobile elements as well as their lower trust and robustness argues for reliance on static servers. But the need to cope with unreliable and low-performance networks, as well as the need to be sensitive to power consumption argues for self-reliance. 1.

Server replication is an approach often used to improve the ability of a service to handle a large number of clients. One of the important factors in the efficient utilization of replicated servers is the ability to direct client requests to the best server, according to some optimality criteria. In this paper we target an environment in which servers are distributed across the Internet, and clients identify servers using our application-layer anycasting service. Our goal is to allocate servers to clients in a way that minimizes a client's response time. To that end, we develop an approach for estimating the performance that a client would experience when accessing particular servers. Such information is maintained in a resolver that clients can query to obtain the identity of the server with the best response time. Our performance collection technique combines server push with client probes to estimate the expected response time. A set of experiments is used to demonstrate the propert...

Abstract--Server replication improves the ability of a service to handle a large number of clients. One of the important fac-tors in the efficient utilization of replicated servers is the ability to direct client requests to the "best " server, according to some optimality criteria. In the anycasting communication paradigm, a sender communicates with a receiver chosen from an anycast group of equivalent receivers. As such, anycasting is well suited to the problem of directing clients to replicated servers. This paper examines the definition and support of the anycasting paradigm at the application layer, providing a service that uses an anycast resolver to map an anycast domain name and a selection criteria into an IP address. By realizing anycasting in the appli-cation layer, we achieve flexibility in the optimization criteria and ease the deployment of the service. As a case study, we examine the performance of our system for a key service: replicated web servers. To this end, we develop an approach for estimating the response time that a client will experi-ence when accessing given servers. Such information is maintained in the anycast resolver that clients query to obtain the identity of the server with the best estimated response time. Our performance collection technique combines server push with resolver probes to estimate the expected response time without undue overhead. Our experiments show that selecting a server using our architecture and estimation technique can improve the client response time by a factor of two over nearest server selection and by a factor of four over random server selection. Index Terms--Anycasting, replication, server selection. I.

This survey concerns the role of data structures for compactly storing and representing various types of information in a localized and distributed fashion. Traditional approaches to data representation are based on global data structures, which require access to the entire structure even if the sought information involves only a small and local set of entities. In contrast, localized data representation schemes are based on breaking the information into small local pieces, or labels, selected in a way that allows one to infer information regarding a small set of entities directly from their labels, without using any additional (global) information. The survey focuses on combinatorial and algorithmic techniques, and covers complexity results on various applications, including compact localized schemes for message routing in communication networks, and adjacency and distance labeling schemes.

Abstract This paper is a survey of the current state of the art in the design and implementation of distributed file systems. It consists of four major parts: an overview of background material, case studies of a number of contemporary file systems, identification of key design techniques, and an examination of current research issues. The systems surveyed are Sun NFS, Apollo Domain, Andrew, IBM AIX DS, AT&T RFS, and Sprite. The coverage of background material includes a taxonomy of file system issues, a brief history of distributed file systems, and a summary of empirical research on file properties. A comprehensive bibliography forms an important of the paper. Copyright (C) 1988,1989 M. Satyanarayanan The author was supported in the writing of this paper by the National Science Foundation (Contract No. CCR-8657907), Defense Advanced Research Projects Agency (Order No. 4976, Contract F33615-84-K-1520) and the IBM Corporation (Faculty Development Award). The views and conclusions in t...

The anycasting communication paradigm is designed to support server replication by allowing applications to easily select and communicate with the "best" server, according to some performance or policy criteria, in a group of content-equivalent servers. We examine the definition and support of the anycasting paradigm at the application layer, providing a service that maps anycast domain names into one or more IP addresses using anycast resolvers. In addition to being independent from network-layer support, our definition includes the notion of filters, functions that are applied to groups of addresses to affect the selection process. We consider both metric-based filters (e.g., server response time) and policy-based filters. An expanded version of this work can be found as a technical report. 1 . 1 Introduction The Internet is increasingly being viewed as providing services, and not just connectivity. As this view becomes more prevalent, it becomes important to provide, within the ...

Portable computers are now common, a fact that raises the possibility that file service clients might move on a regular basis. This new development requires rethinking some features of distributed file system design. We argue that existing approaches to file replica management would not cope well with the likely behavior of mobile clients, and we present our solution: a lazy "server-based" update operation. This operation facilitates fast, scalable, and highly fault-tolerant implementations of both read and write operations in the usual case. To cope with the weak semantics of the update operation, we propose a new file system service interface that allows applications to opt for "UNIX semantics" by use of a slower, less fault-tolerant read operation. 1 Introduction This work investigates how to maintain replicas in a distributed file system, especially one supporting mobile clients. While the topic of replica management within file systems has received so much attention that one mig...

This paper reasons about naming systems as specialized inference mechanisms, It describes a preference)-zierarch.v that can be used to specify the structure of a naming system’s inference mechanism and defines criteria by which different naming systems can be evaluated, For example, the preference hierarchy allows one to compare naming systems based on how dkcrzmznating they are and to identify the class of names for which a given naming system is sound and complete. A study of several example naming systems demonstrates how the prefer-ence hierarchy can be used as a formal tool for designing naming systems. Categories and Subject Descriptors: H.2.3 [Database Management]: Languages—query lan-guages; H.2.4 [Database Management]: Systems—query processing; H.3.3 [Information

this paper, we present three adaptive cache invalidation report methods, in which the server broadcasts different invalidation reports according to the update and query rates/patterns and client disconnection time while spending little uplink cost. Simulation results show that the adaptive invalidation methods are efficient in improving mobile caching and reducing the uplink and downlink costs without degrading the system throughput.