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Higher-Order Concurrency: Expressiveness andDecidability Results

Jorge A. Perez P.

Technical Report UBLCS-2010-07

March 2010

Department of Computer ScienceUniversity of BolognaMura Anteo Zamboni 740127 Bologna (Italy)

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Dottorato di Ricerca in InformaticaUniversita di Bologna e PadovaINF/01 INFORMATICACiclo XXII

Higher-Order Concurrency: Expressiveness andDecidability Results

Jorge A. Perez P.March 2010

Coordinatore: Tutore:Prof. Simone Martini Prof. Davide Sangiorgi

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Higher-Order Concurrency: Expressiveness and Decidability Results

Higher-order process calculi are formalisms for concurrency in which processes can be passedaround in communications. Higher-order (or process-passing) concurrency is often presentedas an alternative paradigm to the first order (or name-passing) concurrency of the -calculusfor the description of mobile systems. These calculi are inspired by, and formally close to, the-calculus, whose basic computational step -reduction involves term instantiation.The theory of higher-order process calculi is more complex than that of first-order processcalculi. This shows up in, for instance, the definition of behavioral equivalences. A long-standing approach to overcome this burden is to define encodings of higher-order processesinto a first-order setting, so as to transfer the theory of the first-order paradigm to the higher-order one. While satisfactory in the case of calculi with basic (higher-order) primitives, thisindirect approach falls short in the case of higher-order process calculi featuring constructsfor phenomena such as, e.g., localities and dynamic system reconfiguration, which are frequentin modern distributed systems. Indeed, for higher-order process calculi involving little morethan traditional process communication, encodings into some first-order language are difficultto handle or do not exist. We then observe that foundational studies for higher-order processcalculi must be carried out directly on them and exploit their peculiarities.This dissertation contributes to such foundational studies for higher-order process calculi.We concentrate on two closely interwoven issues in process calculi: expressiveness and decid-ability. Surprisingly, these issues have been little explored in the higher-order setting. Ourresearch is centered around a core calculus for higher-order concurrency in which only theoperators strictly necessary to obtain higher-order communication are retained. We developthe basic theory of this core calculus and rely on it to study the expressive power of issuesuniversally accepted as basic in process calculi, namely synchrony, forwarding, and polyadiccommunication.

Keywords: concurrency theory, process calculi, higher-order communication, expressiveness,decidability.

Acknowledgments

My greatest debt is to Davide Sangiorgi. Having him as supervisor has been truly inspiring.His careful supervision has influenced enormously my way of doing (and approaching) research.His continuous support and patience during these three years were fundamental to me. I amstill amazed by the fact that Davide had always time for me, not only for scientific discussionsbut also for sorting out everyday issues. I am most grateful to him for his honest and directadvice, and for the liberty that he gave me during my studies.I also owe much to Camilo Rueda and Frank D. Valencia. I do not forget that it was Camilowho introduced me to research, thus giving me an opportunity that most people in his positionwould have refused. Even if my PhD studies were not directly related to his research interests,Camilo was always there, interested in my progresses, encouraging me with his support andfriendship. Frank not only introduced me to concurrency theory; he also gave me constantadvise and support during my PhD studies and long before. Frank had a lot to do with mecoming to Bologna, and that I will never forget.There is no way in which I could have completed this dissertation by myself. It has been apleasure to collaborate with extremely talented people, to whom I am deeply grateful: CinziaDi Giusto, Ivan Lanese, Alan Schmitt, Gianluigi Zavattaro. Thank you for your kindness,generosity and, above all, for your patience.Many thanks to Uwe Nestmann and Nobuko Yoshida for having accepted to review thisdissertation. Thanks also to the members of my internal committee (commissione), CosimoLaneve and Claudio Sacerdoti-Coen. I am indebted to Simone Martini, the coordinator of thePhD program, for all his constant availability and kindness.Many people proof-read parts of this dissertation, and provided me with constructive crit-icisms. I am grateful to all of them for their time and availability: Jesus Aranda, AlbertoDelgado, Cinzia Di Giusto, Daniele Gorla, Julian Gutierrez, Hugo A. Lopez, Claudio Mezzina,Margarida Piriquito, Frank D. Valencia. A special thanks goes to Daniele Varacca, who suf-fered an early draft of the whole document and provided me with insightful remarks. Alongthese years I have benefited a lot from discussions with/comments from a lot of people. Iam most grateful for their positive attitude towards my work: Jesus Aranda, Ahmed Bouaj-jani, Gerard Boudol, Santiago Cortes, Rocco De Nicola, Daniele Gorla, Matthew Hennessy,

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Thomas Hildebrandt, Kohei Honda, Roland Meyer, Fabrizio Montesi, Camilo Rueda, Jean-Bernard Stefani, Frank D. Valencia, Daniele Varacca, Nobuko Yoshida.During 2009 I spent some months visiting Alan Schmitt in the SARDES team at INRIAGrenoble - Rhone-Alpes. The period in Grenoble was very enriching and productive; a sub-stantial part of this dissertation was written there. I am grateful to Alan and to Jean-BernardStefani for the opportunity of working with them and for treating me as another member ofthe team. I would like to thank Diane Courtiol for her patient help with all the administrativeissues during my stay, and to Claudio Mezzina (or the tiny little Italian with a pony tail,as he requested to be acknowledged) for being such a friendly office mate. I also thank theINRIA Equipe Associee BACON for partially supporting my visit.I would like to express my appreciation to the University of Bologna - MIUR for supportingmy studies through a full scholarship. Thanks also to the administrative staff in the Departmentof Computer Science, for their help and kindness in everyday issues.I am most proud to be part of a small group of Colombians doing research abroad. Weall share many things: we