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.

This paper explores ways to provide improved consistency for Internet applications that scale to millions of clients. We make four contributions. First, we identify how workloads affect the scalability of cache consistency algorithms. Second, we define two primitive mechanisms, split and join, for growing and shrinking consistency hierarchies, and we present a simple mechanism for implementing them. Third, we describe and evaluate policies for using split and join to address the fault tolerance and performance challenges of consistency hierarchies. Fourth, using synthetic workload and trace-based simulation, we compare various algorithms for maintaining strong consistency in a range of hierarchy configurations. Our results indicate that a promising configuration for providing strong consistency in a WAN is a two-level consistency hierarchy where servers and proxies work to maintain consistency for data cached at clients. Specifically, by adapting to clients' access patterns, two-level...

This article introduces volume leases as a mechanism for providing server-driven cache consistency for large-scale, geographically distributed networks. Volume leases retain the good performance, fault tolerance, and server scalability of the semantically weaker client-driven protocols that are now used on the web. Volume leases are a variation of object leases, which were originally designed for distributed file systems. However, whereas traditional object leases amortize overheads over long lease periods, volume leases exploit spatial locality to amortize overheads across multiple objects in a volume. This approach allows systems to maintain good write performance even in the presence of failures. Using trace-driven simulation, we compare three volume lease algorithms against four existing cache consistency algorithms and show that our new algorithms provide strong consistency while maintaining scalability and faulttolerance. For a trace-based workload of web accesses, we find that volumes can reduce message traffic at servers by 40% compared to a standard lease algorithm, and that volumes can considerably reduce the peak load at servers when popular objects are modified.

In this paper we describe a technique called operation-based update propagation for efficiently transmitting updates to large files that have been modified on a weakly connected client of a distributed file system. In this technique, modifications are captured above the file-system layer at the client, shipped to a surrogate client that is strongly connected to a server, re-executed at the surrogate, and the resulting files transmitted from the surrogate to the server. If re-execution fails to produce a file identical to the original, the system falls back to shipping the file from the client over the slow network. We have implemented a prototype of this mechanism in the Coda File System on Linux, and demonstrated performance improvements ranging from 40 percents to nearly three orders of magnitude in reduced network traffic and elapsed time. We also found a novel use of forward error correction in this context.

Mobile computing environments are characterized by slow wireless links and relatively underprivileged hosts with limited battery powers, predisposed to frequent disconnections. Caching data at the mobile hosts (MHs) in a wireless network helps alleviate problems associated with slow, limited bandwidth wireless links, by reducing latency and conserving bandwidth. Battery power is conserved by reducing the number of up-link requests. A mobile computing environment is a distributed system, thus when data at the server changes, the client hosts must be made aware of this fact in order for them to invalidate their cache otherwise the host would continue to answer queries with the cached values returning incorrect data. The nature of the physical medium coupled with the fact that disconnections from the network are very frequent in mobile computing environments demand a cache invalidation strategy with minimum possible overheads. In this paper, we present a new cache maintenance sche...

this paper we argue that prescient caching and smart scheduling are key techniques for overcoming the network bottleneck.

Recent years have witnessed the rapid evolution of commercially available mobile computing environments. This has given rise to the presence of several viable, but noninteroperable wireless networking technologies - each targeting a niche mobility environment and providing a distinct quality of service. The lack of a uniform set of standards, the heterogeneity in the quality of service, and the diversity in the networking approaches makes it difficult for a mobile computing environment to provide seamless mobility across different wireless networks. Besides, inter-network mobility will typically be accompanied by a change in the quality of service. The application and the environment need to collaboratively adapt their communication and data management strategies in order to gracefully react to the dynamic operating conditions. This paper presents the important challenges in building a mobile computing environment which provides seamless mobility and adaptive computing over commerciall...

Model checking is a proven successful technology for verifying hardware. It works, however, on only finite state machines, and most software systems have infinitely many states. Our approach to applying model checking to software hinges on identifying appropriate abstractions that exploit the nature of both the system, S, and the property, OE, to be verified. We check OE on an abstracted, but finite, model of S. Following this approach we verified three cache coherence protocols used in distributed file systems. These protocols have to satisfy this property: "If a client believes that a cached file is valid then the authorized server believes that the client's copy is valid." In our finite model of the system, we need only represent the "beliefs" that a client and a server have about a cached file; we can abstract from the caches, the files' contents, and even the files themselves. Moreover, by successive application of the generalization rule from predicate logic, we need only conside...

Mobility demands that systems be adaptive.One approach is to make adaptation transparent to applications, allowing them to remain unchanged. An alternative approach views adaptation as a collaborative partnership between applications and the system. This paper is a status report on our research on both fronts. We report on our considerable experience with application-transparent adaptation in the Coda File System. We also describe our ongoing work on application-aware adaptation in Odyssey.

Recent trends have exposed three key problems in today's operating systems. The first is the emergence of I/O latency as the dominant factor in the performance of many applications. The second is the need to cope with mobile communication environments where bandwidth and latency may be highly variable. The third is the importance of search activity to locating files of interest in a distributed system. In this paper we describe a single unifying abstraction called dynamic sets which can offer substantial benefits in the solution of these problems. These benefits include greater opportunity in the I/O subsystem to aggressively exploit prefetching and parallelism, as well as support for associative naming to complement the hierarchical naming in typical file systems. This paper motivates dynamic sets, presents the design of a system that embodies this abstraction, and evaluates a prototype implementation of the system via measurements and an analytical model. This research was supported...