Disconnected operation is a mode of operation that enables a client to continue accessing critical data during temporary failures of a shared data repository. An important, though not exclusive, application of disconnected operation is in supporting portable computers. In this paper, we show that disconnected operation is feasible, efficient and usable by describing its design and implementation in the Coda File System. The central idea behind our work is that caching of data, now widely used for performance, can also be exploited to improve availability.

In this paper we present qualitative and quantitative data on file access in a mobile computing environment. This information is based on actual usage experience with the Coda File System over a period of about two years. Our experience confirms the viability and effectiveness of disconnected operation. It also exposes certain deficiencies of the current implementation of Coda, and identifies new functionality that would enhance its usefulness for mobile computing. The paper concludes with a description of what we are doing to address these issues. This work was supported by the Advanced Research Projects Agency (Avionics Laboratory, Wright Research and Development Center, Aeronautical Systems Division(AFSC), U.S. Air Force, Wright-Patterson AFB under Contract F33615-90-C-1465, Arpa Order No. 7597), the National Science Foundation (Grant ECD 8907068), IBM, Digital Equipment Corporation, and Bellcore. The views and conclusions contained in this document are those of the authors and sho...

In this paper we present results from a six-month empirical study of the high availability aspectsof the Coda File System. We report on the service failures experienced by Coda clients, and show that such failures are masked successfully. We also explorethe effectiveness and resource costs of key aspects of server replication and disconnected operation, the two high availability mechanisms of Coda. Wherever possible, we compare our measurements to simulation-based predictions from earlier papers and to anecdotal evidence from users. Finally, we explore how users take advantage of the support provided by Coda for mobile computing.

CONTENTS DEDICATIONS iii LIST OF FIGURES vi SUMMARY vii CHAPTER I. INTRODUCTION 1 1.1 Thesis Statement 1 1.2 Problems With Current Solutions 2 1.3 Contributions 6 1.4 Overview of Remainder of Paper 7 II. RELATED WORK 9 2.1 Connectivity 9 2.2 Platform Assumptions 11 2.3 Application-Specific Effort 12 2.4 Data Integration 13 2.5 Summary 17 III. APPLICATIONS 18 3.1 Integrating Mobile Information into Desktop Applications 18 3.2 Collaboration among mobile devices 32 3.3 Summary 36 IV. ARCHITECTURE 37 4.1 Goals 37 4.2 Overview 39 4.3 Newton Overview 40 4.4 LlamaServer 43 4.5 MobileConnect 49 4.6 DesktopConnect 52 4.7 Intercomponent Communication 56 4.8 Summary 67 V. DESIGN DECISIONS, HURDLES, AND LIMITATIONS 68 5.1 Some information is kept permanently on the mobile device 69 v 5.2 Mobile devices always connected 71 5.3 Centralized server instead of point to point communications 73 5.4 Client applications explicitly written to handle mobile data 74 5.5 Assumes a homogeneous mobile

Distributed system are becoming increasingly important in day to day computing tasks. This paper describes the difficulties inherent in measuring distributed systems, and enumerates four goals for such measurement: longevity, flexibility, fault-tolerance, and unintrusiveness. It then describes the Coda File System, a research system that has been deployed in a moderately-sized user community for four years, and actively measured for three and a half years. The architecture of this measurement framework is described in detail, with an eye toward examining how well it meets the four goals. The paper concludes with the lessons to be taken from this experience, both those that were foreseen as well as those that were learned along the way. In an effort to help teach these lessons, we have made the Coda source code, along with the measurement framework, freely available.