Nomadic Pict: Language and Infrastructure Design for Mobile Computation PaweÃlTomasz Wojciechowski Wolfson College University of Cambridge A dissertation submitted for the degree of Doctor of Philosophy March 2000 ii Abstract Mobile agents — units of executing computation that can migrate between machines — are likely to become an important enabling technology for fu- ture distributed systems. We study the distributed infrastructures required for location-independent communication between migrating agents. These infrastructures are problematic: the choice or design of an infrastructure must be somewhat application-specific — any given algorithm will only have satisfactory performance for some range of migration and communication behaviour; the algorithms must be matched to the expected properties (and robustness demands) of applications and the failure characteristic of the com- munication medium. To study this problem we introduce an agent program- ming language – Nomadic Pict. It is designed to allow infrastructure al- gorithms to be expressed clearly, as translations from a high-level language to a lower level. The levels are based on rigorously-defined process calculi, which provide sharp levels of abstraction. In this dissertation we describe the language and use it to develop a distributed infrastructure for an ex- ample application. The language and examples have been implemented; we conclude with a description of the compiler and runtime system. iv To Maria and ZdzisÃlaw vi Preface Except where otherwise stated in the text, this document is the result of my own work and is not the outcome of work done in collaboration. This dissertation is not substantially the same as any that I have submitted for a degree or diploma or other qualification at any other university. No part of this dissertation has already been or is being concurrently sub- mitted for any such degree, diploma or other qualification. This dissertation does not exceed sixty thousand words, including tables, footnotes and bibliography. This dissertation is copyright °c 2000 by PaweÃlT. Wojciechowski All trademarks used in this dissertation are hereby acknowledged. viii Acknowledgments This research was supported by a scholarship from the Wolfson Foundation. Without this support and Prof. Jerzy Brzezi´nskiat Pozna´nUniversity of Technology, who after my graduation made me want to do research into distributed algorithms, I could not even have gone to England. I am grateful to Ken Moody for introducing me to the problem of locations and failures in process calculi, and for many stimulating and enjoyable dis- cussions. I would also like to thank Ken for supervising my thesis and his continuous support during years of my study. I would like to express my special gratitude to Prof. Robin Milner, whose lectures about the ¼-calculus I was able to attend shortly after coming to Cambridge in the fall of 1995. They gave me a lot of insight into problems which were new to me and appeared to be a great source of inspiration in my research work. I owe especial thanks to Peter Sewell, who not only taught me a great deal about the theory and semantics of programming languages, but also, in the midst of a busy schedule, he took time out to answer all my endless questions and provided me with friendly conversation even as I was depriving him of his work and his peace. Part of this research was done in collaboration with Peter. I consider myself very fortunate, to have been introduced to Benjamin Pierce, a co-author of Pict, while he was visiting the Computer Laboratory at the University of Cambridge in the academic year 1995/96. Some of the early ideas of the nomadic ¼-calculi resulted from the discussions with Benjamin and Peter during our brief meeting at Indiana University in May of 1997. I would also like to thank C´edricFournet and Asis Unyapoth. They have contributed valuable comments on parts of this work at various stages of its completion. I am very grateful too to past and present members of the Opera Group, which is part of the Systems Research Group, and members of the Theory and Semantics Group, who not only showed much interest and understanding of the true nature of my work, but also created an interesting and stimulating environment. x Finally, I would like to thank Ken Moody and Peter Sewell who read what I was scribbling with extraordinary sympathy and care, cheering me on and holding me to the very highest standards. Thanks to the staff in the Computer Laboratory at the University of Cam- bridge for helping me with the administrative aspects of being a PhD student. Thanks to my mates and fellows at Wolfson College and friends from various societies for many enjoyable discussions, social events, and sharing time in Cambridge. More personally, thanks to my family and best friends, especially to Ania, for her friendship and happiest memories. Finally, all the time I was at home, and away, I was kept upright by my parents, who reconciled themselves with patience and good grace to a son who chose to live far away, and always looked after my well-being — showing so much kindness and helping me in various important ways. Publications Aspects of the work described in this dissertation feature in the following publications: ² PaweÃl T. Wojciechowski and Peter Sewell. Nomadic Pict: Language and Infrastructure Design for Mobile Agents. In IEEE Concurrency. The Computer Society’s Systems Magazin, April-June 2000. This is an extended version of the paper below. ² PaweÃl T. Wojciechowski and Peter Sewell. Nomadic Pict: Language and Infrastructure Design for Mobile Agents. This appeared in the Proceedings of ASA/MA’99 (First International Symposium on Agent Systems and Applications/Third International Symposium on Mobile Agents), Palm Springs, CA, USA, October 1999.1 ² Peter Sewell, PaweÃl T. Wojciechowski, and Benjamin C. Pierce. Location-Independent Communication for Mobile Agents: A Two- Level Architecture. In Henri E. Bal, Boumediene Belkhouche, and Luca Cardelli, editors, Internet Programming Languages (ICCL’98 Work- shop), volume 1686 of Lecture Notes in Computer Science, pages 1–31. Springer, 1999. Also appeared as Technical Report 462, Computer Laboratory, University of Cambridge, April 1999. ² Peter Sewell, PaweÃlT. Wojciechowski, and Benjamin C. Pierce. Loca- tion Independence for Mobile Agents. This appeared in the Proceedings of ICCL’98 Workshop on Internet Programming Languages, Chicago, USA, May 1998. It is largely superseded by the paper above. 1Awarded Best Paper Overall of Conference xii Contents 1 Introduction 3 1.1 Mobility and Location Independence . 4 1.1.1 Process Migration . 6 1.1.2 Distributed Objects . 8 1.1.3 Mobile Agents . 12 1.2 Thesis Contribution . 13 1.2.1 Observations . 13 1.2.2 Problem Statement . 14 1.2.3 Project Foundations . 15 1.2.4 Contribution . 16 2 Model of Mobile Computation 19 2.1 Asynchronous ¼-Calculus . 20 2.1.1 Syntax . 21 2.1.2 Informal Semantics . 22 2.1.3 Operational Semantics . 24 2.2 Nomadic ¼-Calculus . 27 2.2.1 Low-Level Calculus . 28 2.2.2 High-Level Calculus . 34 2.2.3 Reduction Semantics . 35 2.3 Related Models . 39 2.3.1 Related Calculi . 39 2.3.2 I/O Automata . 41 2.3.3 Mobile UNITY . 43 2.3.4 Brief Comparison . 46 3 Programming Language 51 3.1 Motivations . 51 3.1.1 Mobility in a Wide-Area Network . 52 3.1.2 Verification of Mobile Computation . 53 3.1.3 Infrastructure Design and Specification . 56 3.2 Nomadic Pict . 57 3.2.1 Language Principles . 57 3.2.2 Low-Level Language . 58 3.2.3 High-Level Language . 60 3.2.4 Examples and Idioms . 61 3.3 Related Languages . 66 xiv CONTENTS 3.3.1 Facile . 67 3.3.2 The Join Language . 71 4 Infrastructure for Location-Independent Communication 77 4.1 Algorithms . 77 4.1.1 Central Server . 79 4.1.2 Forwarding Pointers . 81 4.1.3 Broadcast . 82 4.1.4 Group Communication . 84 4.1.5 Hierarchical Directory . 85 4.1.6 Arrow Directory . 86 4.2 Example Translations in Nomadic Pict . 87 4.2.1 A Central-Server Infrastructure Translation . 88 4.2.2 A Forwarding-Pointers Infrastructure Translation . 94 4.3 Alternative Descriptions . 100 5 Infrastructure Design for Mobile Agents 103 5.1 Resource Monitoring . 104 5.1.1 Migration and Communication Pattern . 104 5.1.2 Example Infrastructure . 105 5.2 Mobile Devices . 106 5.2.1 Example Infrastructure . 106 5.3 Information Retrieval . 108 5.3.1 Migration and Communication Pattern . 109 5.3.2 Example Infrastructure . 109 5.4 Fault-Tolerance . 111 5.4.1 Mobile Agent Support for Checkpointing . 112 5.5 Large-Scale Parallel Computation . 113 5.5.1 Migration and Communication Pattern . 113 5.5.2 Example Infrastructure . 114 5.6 Event-Driven Mobility . 115 5.6.1 Mobile Agent Support for Events . 117 5.6.2 Migration and Communication Pattern . 118 5.6.3 Example Infrastructure . 118 6 The PA Application and Infrastructure Design 121 6.1 Application . 122 6.1.1 High-Level Architecture . 122 6.1.2 Migration and Communication Pattern . 123 6.2 Design of Appropriate Infrastructure . 124 6.2.1 Example Infrastructure: The QSC Algorithm . 126 6.2.2 Disconnected Operation: The QSCD Algorithm . 131 6.2.3 Wide-Area Architecture: The FQSC Algorithm . 141 7 Nomadic Pict Implementation 151 7.1 Architecture of the Compiler . 151 7.1.1 Compilation Phases . 153 7.1.2 Architecture-Independent Core Language . 155 7.2 Architecture of the Runtime System . 156 CONTENTS xv 7.2.1 Virtual Machine and Execution Fairness . 156 7.2.2 Interaction with an Operating System and User . 158 7.2.3 I/O Server and Trader Service . 158 8 Conclusions and Future Work 161 A Syntax 165 A.1 Lexical Rules .