Self-stabilizing extensions for meassage-passing systems

Summary A self-stabilizing program eventually resumes normal behavior even if excution begins in, an abnormal initial state. In this paper, we explore the possibility of extending an arbitrary program into a self-stabilizing one. Our contributions are: (1) a formal definition of the concept of one program being aself-stabilizing extension of another; (2) a characterization of what properties may hold in such extensions; (3) a demonstration of the possibility of mechanically creating such extensions. The computtional model used is that of an asynchronous distributed message-passing system whose communication topology is an arbitrary graph. We contrast the difficulties of self-stabilization in thismodel with those of themore common shared-memory models. Shmuel Katz received his B.A. in Mathematics and Englisch Literature from U.C.L.A., and his M.Sc. and Ph.D. in Computer Science (1976) from the Weizmann Institute in Rechovot, Israel. From 1976 to 1981 he was a research at the IBM Israel Scientific Center. Presently, he is an Associate Professor in the Computer Science Department at the Technion in Haifa, Israel. In 1977–78 he visited for a year at the University of California, Berkeley, and in 1984–85 was at the University of Texas at Austin. He has been a consultant and vistor at the MCC Software Technology Program, and in 1988–89 was a visiting scientist at the IBM Watson Research Center. His research interests include the methodology of programming, specification methods, program verification and semantics, distributed programming, data structure, and programming languages. Kenneth J. Pery has performed research in the area of distributed computing since obtaining Masters and Doctorate degrees in Computer Science from Cornell Univesity. His current interest is in studying problems of a partical nature in a formal context. He was graduated from Princeton University in 1979 with a B.S.E. degree in Electrical Engineering and Computer Science.The Research of this author was partially supported by Research Grant 120-749 and the Argentinian Research Fund at the Technion     

Verification of distributed programs using representative interleaving sequences

Summary We present a formal proof method for distributed programs. The semantics used to justify the proof method explicitly identifies equivalence classes of execution sequences which are equivalent up to permuting commutative operations. Each equivalence class is called an interleaving set or a run. The proof rules allow concluding the correctness of certain classes of properties for all execution sequences, even though such properties are demonstrated directly only for a subset of the sequences. The subset used must include a representative sequence from each interleaving set, and the proof rules, when applicable, guarantee that this is the case. By choosing a subset with appropriate sequences, simpler intermediate assertions can be used than in previous formal approaches. The method employs proof lattices, and is expressed using the temporal logic ISTL. Shmuel Katz received his B.A. in Mathematics and English Literature from U.C.L.A., and his M.Sc. and Ph.D. in Computer Science (1976) from the Weizmann Institute in Rechovot, Israel. From 1976 to 1981 he was at the IBM Israel Scientific Center. Presently, he is on the faculty of the Computer Science Department at the Technion in Haifa, Israel. In 1977–1978 he visited for a year at the University of California, Berkeley, and in 1984–1985 was at the University of Texas at Austin. He has been a consultant and visitor at the MCC Software Technology Program, and in 1988–1989 was a visiting scientist at the I.B.M. Watson Research Center. His research interests include the methodology of programming, specification methods, program verification and semantics, distributed programming, data structures, and programming languages. Doron Peled was born in 1962 in Haifa. He received his B.Sc. and M.Sc. in Computer Science from the Technion, Israel in 1984 and 1987, respectively. Between 1987 and 1991 he did his military service. He also completed his D.Sc. degree in the Technion during these years. Dr. Peled was with the Computer Science department at Warwick University in 1991–1992. He is currently a member of the technical staff with AT & T Bell Laboratories. His main research interests are specification and verification of programs, especially as related to partial order models, fault-tolerance and real-time. He is also interested in semantics and topology.This research was carried out while the second author was at the Department of Computer Science, The Technion, Haifa 32000, Israel     

One-bit algorithms

Many algorithms in distributed systems assume that the size of a single message depends on the number of processors. In this paper, we assume in contrast that messages consist of a single bit. Our main goal is to explore how the one-bit translation of unbounded message algorithms can be sped up by pipelining. We consider two problems. The first is routing between two processors in an arbitrary network and in some special networks (ring, grid, hypercube). The second problem is coloring a synchronous ring with three colors. The routing problem is a very basic subroutine in many distributed algorithms; the three coloring problem demonstrates that pipelining is not always useful. Amotz Bar-Noy received his B.Sc. degree in Mathematics and Computer Science in 1981, and his Ph.D. degree in Computer Science in 1987, both from the Hebrew University of Jerusalem, Israel. Between 1987 and 1989 he was a post-doctoral fellow in the Department of Computer Science at Stanford University. He is currently a visiting scientist at the IBM Thomas J. Watson Research Center. His current research interests include the theoretical aspects of distributed and parallel computing, computational complexity and combinatorial optimization. Joseph (Seffi) Naor received his B.A. degree in Computer Science in 1981 from the Technion, Israel Institute of Technology. He received his M.Sc. in 1983 and Ph.D. in 1987 in Computer Science, both from the Hebrew University of Jerusalem, Israel. Between 1987 and 1988 he was a post-doctoral fellow at the University of Southern California, Los Angeles, CA. Since 1988 he has been a post-doctoral fellow in the Department of Computer Science at Stanford University. His research interests include combinatorial optimization, randomized algorithms, computational complexity and the theoretical aspects of parallel and distributed computing. Moni Naor received his B.A. in Computer Science from the Technion, Israel Institute of Technology, in 1985, and his Ph.D. in Computer Science from the University of California at Berkeley in 1989. He is currently a visiting scientist at the IBM Almaden Research Center. His research interests include computational complexity, data structures, cryptography, and parallel and distributed computation.Supported in part by a Weizmann fellowship and by contract ONR N00014-85-C-0731Supported by contract ONR N00014-88-K-0166 and by a grant from Stanford's Center for Integrated Systems. This work was done while the author was a post-doctoral fellow at the University of Southern California, Los Angeles, CAThis work was done while the author was with the Computer Science Division, University of California at Berkeley, and Supported by NSF grant DCR 85-13926     

Modelling knowledge and action in distributed systems

We present a formal model that captures the subtle interaction between knowledge and action in distributed systems. We view a distributed system as a set ofruns, where a run is a function from time toglobal states and a global state is a tuple consisting of anenvironment state and alocal state for earch process in the system. This model is a generalization of those used in many previous papers.Actions in this model are associated with functions from global states to global states. Aprotocol is a function from local states to actions. We extend the standard notion of a protocol by definingknowledge-based protocols, ones in which a process' actions may depend explicitly on its knowledge. Knowledge-based protocols provide a natural way of describing how actions should take place in a distributed system. Finally, we show how the notion of one protocolimplementing another can be captured in our model. Joseph Y. Halpern received a B.Sc. in mathematics from the University of Toronto in 1975, and a Ph.D. in mathematics from Harvard University in 1981. In between, he spent two years as the head of the Mathematics Department at Bawku Secondary School, in Ghana. After a year as a visiting scientist at MIT, he joined IBM in 1982. He is currently the manager of the Mathematics and Related Computer Science Department at the IBM Almaden Research Center, and a consulting professor in the Computer Science Department at Stanford. His major research interests are reasoning about knowledge, distributed computation, and logics of programs. He was program chairman and organizer of the first conference of Theoretical Aspects of Reasoning About Knowledge, program chairman of the Fifth ACM Symposium on Principles of Distributed Computing, and was the co-recipient (with Ronald Fagin) of the MIT Publisher's Prize for the Best Paper Paper at the 1985 International Joint Conference on Artificial Intelligence. Ronald Fagin is manager of the Foundations of Computer Science group at the IBM Almaden Research Center. He received his B.A. degree in mathematics from Dartmouth College in 1967 and his Ph.D. in mathematics, specializing in mathematical logic, from the University of California at Berkeley in 1973. He joined IBM in 1973 at the Thomas J. Watson Research Center. In 1975, he transferred to the San Jose Research Laboratory (now the IBM Almaden Research Center) where most of his research has centered on applications of logic to computer science. In particular, he has done research on the theory of relational databases and, more recently, on theories of knowledge and belief. He has received three IBM Outstanding Innovation Awards for his contributions to relational database theory, extendible hashing, and reasoning about knowledge. He was co-recipient (with Joe Halpern) of the MIT Press Publisher's Prize for the Best Paper at the 1985 International Joint Conference on Artificial Interlligence.Some material in this paper appeared in preliminary form in Halpern and Fagin (1985). An abridged version of the paper appeared in Vogt F (ed) Proceeding of Concurrency 88 (Lecture Notes in Computer Science Vol. 335) Springer-Verlag, 1988, pp 18–32     

Self-stabilization of dynamic systems assuming only read/write atomicity

Summary Three self-stabilizing protocols for distributed systems in the shared memory model are presented. The first protocol is a mutual-exclusion prootocol for tree structured systems. The second protocol is a spanning tree protocol for systems with any connected communication graph. The thrid protocol is obtianed by use offair protoco combination, a simple technique which enables the combination of two self-stabilizing dynamic protocols. The result protocol is a self-stabilizing, mutualexclusion protocol for dynamic systems with a general (connected) communication graph. The presented protocols improve upon previous protocols in two ways: First, it is assumed that the only atomic operations are either read or write to the shared memory. Second, our protocols work for any connected network and even for dynamic network, in which the topology of the network may change during the excution. Shlomi Dolev received his B.Sc. in Civil Engineering and B.A. in Computer Science in 1984 and 1985, and his M.Sc. and Ph.D. in computer Sciene in 1989 and 1992 from the Technion Israel Institute of Technology. He is currently a post-dotoral fellow in the Department of Computer Science at Texas A & M Univeristy. His current research interests include the theoretical aspects of distributed computing and communcation networks. Amos Israeli received his B.Sc. in Mathematics and Physics from Hebrew University in 1976, and his M.Sc. and D.Sc. in Computer Science from the Weizmann Institute in 1980 and the Technion in 1985, respectively. Currently he is a sensior lecturer at the Electrical Engineering Department at the Technion. Prior tot his he was a postdoctoral fellow at the Aiken Computation Laboratory at harvard. His research interests are in Parellel and Distributed Computing and in Robotics. In particular he has worked on the design and analysis of Wait-Free and Self-Stabilizing distributed protocols. Shlomo Moran received his B.Sc. and D.Sc. degrees in matheamtics from Technion, Israel Institute of Technology, Haifa, in 1975 and 1979, respectively. From 1979 to 1981 he was assistant professors and a visiting research specialist at the University of Minnesota, Minneapolis. From 1981 to 1985 he was a senior lecturer at the Department of Computer Science. Technion, and from 1985 to 1986 he visted at IBM Thoas J. Watson Research Center, Yorktown Heights. From 1986 to 1993 he was an associated professor at the Department of Computer Science, Technin. in 1992–3 he visited at AT & T Bell Labs at Murray Hill and at Centrum voor Wiskunde en Informatica, Amsterdam. From 1993 he is a full professor at the Department of Computer Science, Technion. His researchinterests include distributed algorithm, computational complexity, combinatorics and grapth theory.Part of this research was supported in part by Technion V.P.R. Funds — Wellner Research Fund, and by the Foundation for Research in Electronics, Computers and Communictions, administrated by the Israel Academy of Sciences and Humanities.     

Fairness and hyperfairness in multi-party interactions

Summary In this paper, a new fairness notion is proposed for languages withmulti-party interactions as the sole interprocess synchronization and communication primitive. The main advantage of this fairness notion is the elimination of starvation occurring solely due to race conditions (i.e., ordering of independent actions). Also, this is the first fairness notion for such languages which is fully adequate with respect to the criteria presented in [2]. The paper defines the notion, proves its properties, and presents examples of its usefulness. Orna Grumberg received her B.Sc. degree, M.Sc. and Ph.D. in the Computer Science Department at the Technion—Israel Institute of Technology. Since 1984 she is a faculty member in the Computer Science Department at the Technion. Her research interests include verification of distributed systems, computer-aided verification, model checking, temporal logics and automata. Paul Attie received a B.A. degree in engineering science from the University of Oxford, and an M.Sc. degree in computer science from the University of London. Since 1986, Paul has been with the Microelectronics and Computer Technology Corporation, where he is currently a member of technical staff. He is also a candidate for the Ph.D. in computer science degree at the University of Texas at Austin. His research interests include temporal logic, fairness, algebraic process theory, formal semantics, and concurrent program verification.The photograph and autobiography of Dr. Nissim Francez were published in Volume 2, Issue No. 4, 1988 on page 226     

Strong stable properties in distributed systems

Summary A stable property in a distributed system is a global property which once true, remains true forever. This paper refines this notion by formally introducing the concept ofstrong stable properties. A strong stable property has the nice property that it can be correctly evaluated on the consistent part of uncoordinated snapshots. Termination and deadlock are shown to be strong stable properties, whereas distributed garbage is not. We also show how to derive a simple generic algorithm for the detection of a strong stable property. The generic algorithm is illustrated by two examples: termination detection and deadlock detection. Incidentally the paper presents a very simple algorithm for termination detection. Andre Schiper has been a professor of Computer Science at EPFL (Federal Institute of Technology in Lausanne, Switzerland) since 1985, leading the Operating Systems laboratory. He graduated in Physics from the Federal Institute of technology in Zürich and received his Ph.D. in Computer Science from EPFL in 1980. In 1981–82 he spent one year at the University of Rennes, France. From 1983 to 1985, he was professor at the Engineering School in Yverdon, Switzerland. Between 1989 and 1991 André Schiper was head of the Department of Computer Science of EPFL, and during the academic year 1992–93 he was on sabbatical leave at Cornell University, Ithaca (NY). His research interests are in the areas of operating systems, distributed and fault-tolerant distributed systems, and parallelism. He is currently involved in the European Esprit project BROADCAST whose objective is the design and implementation of large scale distributed computing systems. Alain Sandoz graduated in Mathematics from the University of Neuchâtel, Switzerland, in 1984 and in Computer Science from the Federal Institute of Technology in Lausanne, Switzerland, in 1988. He received his Ph.D. in Computer Science from the Federal Institute of Technology in Lausanne in 1992. His dissertation was concerned with modelling causal relationships between transactions in distributed and replicated database systems. From 1992 to 1994 he was involved in research on fault-tolerant and large scale distributed computing systems. He is currently working on the development of information systems for the Swiss government.     

Static extensivity analysis for λ-definable functions over lattices

We employ a static analysis to examine the extensivity (∀x:x≤f(x)) of functions defined over lattices in a λ-calculus augmented with lattice operations. The need for such a verification procedure has arisen in our work on a generator system (called Zoo) of static program-analyzers. The input to Zoo is a static analysis specification that consists of lattice definitions and function definitions over the lattices. Once the extensivity of the functions is ascertained, the generated analyzer is guaranteed to terminate when the lattices have finite-heights. The extensivity analysis consists of a sound syntax-driven deductive rules whose satisfiability check is done by a constraint solving procedure. Hyunjun Eo: He is a Ph.D. candidate of Computer Science Dept. at KAIST (Korea Advanced Institute of Science and Technology). He received his B.S. and M.S. in Computer Science from KAIST in 1996 and 1998, respectively. For 1998–2003, he was a research assistant of the National Creative Research Initiative Center for Research On Program Analysis System. His research interest has been on static program analysis, program logics, and higher-order and typed languages. He is currently working on developing a tool for automatic generation of program analyzers. Kwangkeun Yi, Ph.D.: His research interest has been on semantic-based program analysis and systems application of language technologies. After his Ph.D. from University of Illinois at Urbana-Champaign he joined the Software Principles Research Department at Bell Laboratories, where he worked on various static analysis approaches for higher-order and typed programming languages. For 1995–2003, he was a faculty member in the Department of Computer Science, Korea Advanced Institute of Science and Technology. Since Fall 2003, he has been a faculty member in the School of Computer Science and Engineering, Seoul National University. Kwang-Moo Choe, Ph.D.: He is a professor of Computer Science at Korea Advanced Institute of Science and Technology. He received his B.S. from Seoul National University in 1976, and his M.S. and Ph.D. from Korea Advanced Institute of Science and Technology in 1978 and 1984, respectively. For 1985–1986, he was a technical staff of AT&T Bell Labs at Murray Hill. His research interest is formal language theory, parallel evaluation of logic programs, and optimizing compilers.     

Temporal disjunctive logic programming

In this paper we introduce the logic programming languageDisjunctive Chronolog which combines the programming paradigms of temporal and disjunctive logic programming. Disjunctive Chronolog is capable of expressing dynamic behaviour as well as uncertainty, two notions that are very common in a variety of real systems. We present the minimal temporal model semantics and the fixpoint semantics for the new programming language and demonstrate their equivalence. We also show how proof procedures developed for disjunctive logic programs can be easily extended to apply to Disjunctive Chronolog programs. Manolis Gergatsoulis, Ph.D.: He received his B.Sc. in Physics in 1983, the M.Sc. and the Ph.D. degrees in Computer Science in 1986 and 1995 respectively all from the University of Athens, Greece. Since 1996 he is a Research Associate in the Institute of Informatics and Telecommunications, NCSR ‘Demokritos’, Athens. His research interests include logic and temporal programming, program transformations and synthesis, as well as theory of programming languages. Panagiotis Rondogiannis, Ph.D.: He received his B.Sc. from the Department of Computer Engineering and Informatics, University of Patras, Greece, in 1989, and his M.Sc. and Ph.D. from the Department of Computer Science, University of Victoria, Canada, in 1991 and 1994 respectively. From 1995 to 1996 he served in the Greek army. From 1996 to 1997 he was a visiting professor in the Department of Computer Science, University of Ioannina, Greece, and since 1997 he is a Lecturer in the same Department. In January 2000 he was elected Assistant Professor in the Department of Informatics at the University of Athens. His research interests include functional, logic and temporal programming, as well as theory of programming languages. Themis Panayiotopoulos, Ph.D.: He received his Diploma on Electrical Engineering from the Department of Electrical Engineering, National Technical Univesity of Athens, in 1984, and his Ph.D. on Artificial Intelligence from the above mentioned department in 1989. From 1991 to 1994 he was a visiting professor at the Department of Mathematics, University of the Aegean, Samos, Greece and a Research Associate at the Institute of Informatics and Telecommunications of “Democritos” National Research Center. Since 1995 he is an Assistant Prof. at the Department of Computer Science, University of Piraeus. His research interests include temporal programming, logic programming, expert systems and intelligent agent architectures.     

Some impossibility results in interprocess synchronization

Summary In this paper we construct a formal specification of the problem of synchronizing asynchronous processes under strong fairness. We prove that strong interaction fairness is impossible for binary (and hence for multiway) interactions and strong process fairness is impossible for multiway interactions. Yih-Kuen Tsay received his B.S. degree form National Taiwan University in 1984 and his M.S. degree from UCLA in 1989. He is currently a Ph.D. candidate in the UCLA Computer Science Department. His research interests include distributed algorithms, fault-tolerant systems, and specification and verification of concurrent programs. Rajive L. Bagrodia received the B. Tech. degree in Electrical Engineering from the Indian Institute of Technology, Bombay in 1981 and the M.A. and Ph.D. degrees in Computer Science from the University of Texas at Austin in 1983 and 1987 respectively. He is currently an Assistant Professor in the Computer Science Department at UCLA. His research interests include parallel languages, distributed algorithms, parallel simulation and software design methodologies. He was selected as a 1991 Presidential Young Investigator by NSF.This research was partially supported by NSF PYI Award number ASC9157610 and by ONR under grant N00014-91-J1605     

An improved upperbound for distributed election in bidirectional rings of processors

We present a distributed algorithm for electing a leader (i. e., breaking symmetry) in bidirectional rings ofN processors with no global sense of orientation, that uses at most 1.44 ...N logN+O(N) messages in the worst case.Jan van Leeuwen received his M. Sc. degree in 1969 (cum laude) and the Ph.D. degree in 1972 from the University of Utrecht, Utrecht, The Netherlands. He held a postdoctorate fellowship in computer science at the University of California at Berkeley (1972–1973), visiting assistant professorship in computer science at the State University of New York at Buffalo (1973–1974, 1975–1976), and a visiting associate professorship in computer science at The Pennsylvania State University, University Park (1976–1977). In 1977 he was appointed Associate Professor of Computer Science at the University of Utrecht and became head of the new Department of Computer Science at this university. He is presently Full Professor of Computer Science. Dr. van Leeuwen is active in many disciplines within computer science. His primary research interests are fundamental studies in varied areas of computer science, viz. the analysis and complexity of computer algorithms, in both a theoretical and an applied sense (e. g. data structures, machine models, VLSI, parallel and distributed computing, and cryptography).Richard B. Tan is an Associate Professor of Mathematics and Computer Science at the University of Sciences and Arts of Oklahoma. He spends his summers at the University of Utrecht, the Netherlands. His research interests are in distributed computation and graph algorithms. He received the B. Sc. in Physics from Beloit College, WI., the M.S. in Computer Science and the Ph.D. (in 1980) in Mathematics from the University of Oklahoma.This work was done while the second author was visiting the University of Utrecht, supported by a grant of the Netherlands Organization for the Advancement of Pure Research (ZWO)     

A program specialiser for meta-level compositions of logic programs

Metal-level compositions of object logic programs are naturally implemented by means of meta-programming techniques. Metainterpreters defining program compositions however suffer from a computational overhead that is due partly to the interpretation layer present in all meta-programs, and partly to the specific interpretation layer needed to deal with program compositions. We show that meta-interpreters implementing compositions of object programs can be fruitfully specialised w.r.t. meta-level queries of the form Demo (E, G), where E denotes a program expression and G denotes a (partially instantiated) object level query. More precisely, we describe the design and implementation of declarative program specialiser that suitably transforms such meta-interpreters so as to sensibly reduce — if not to completely remove — the overhead due to the handling of program compositions. In many cases the specialiser succeeds in eliminating also the overhead due to meta-interpretation. Antonio Brogi, Ph.D.: He is currently assistant professor in the Department of Computer Science at the University of Pisa, Italy. He received his Laurea Degree in Computer Science (1987) and his Ph. D. in Computer Science (1993) from the University of Pisa. His research interests include programming language design and semantics, logic programming, deductive databases, and software coordination. Simone Contiero: He is currently a Ph. D. student at the Department of Computer Science, University of Pisa (Italy). He received his Laurea Degree in Computer Science from the University of Pisa in 1994. His research interests are in high-level programming languages, metaprogramming and logic-based coordination of software.     

Adaptive solutions to the mutual exclusion problem

Summary Algorithms for mutual exclusion that adapt to the current degree of contention are developed. Afilter and a leader election algorithm form the basic building blocks. The algorithms achieve system response times that are independent of the total number of processes and governed instead by the current degree of contention. The final algorithm achieves a constant amortized system response time. Manhoi Choy was born in 1967 in Hong Kong. He received his B.Sc. in Electrical and Electronic Engineerings from the University of Hong Kong in 1989, and his M.Sc. in Computer Science from the University of California at Santa Barbara in 1991. Currently, he is working on his Ph.D. in Computer Science at the University of California at Santa Barbara. His research interests are in the areas of parallel and distributed systems, and distributed algorithms. Ambuj K. Singh is an Assistant Professor in the Department of Computer Science at the University of California, Santa Barbara. He received a Ph.D. in Computer Science from the University of Texas at Austin in 1989, an M.S. in Computer Science from Iowa State University in 1984, and a B.Tech. from the Indian Institute of Technology at Kharagpur in 1982. His research interests are in the areas of adaptive resource allocation, concurrent program development, and distributed shared memory.A preliminary version of the paper appeared in the 12th Annual ACM Symposium on Principles of Distributed ComputingWork supported in part by NSF grants CCR-9008628 and CCR-9223094     

Constructive negation and constraint logic programming with sets

The aim of this paper is to extend theConstructive Negation technique to the case ofCLP(SεT), a Constraint Logic Programming (CLP) language based on hereditarily (and hybrid) finite sets. The challenging aspects of the problem originate from the fact that the structure on whichCLP(SεT) is based is notadmissible closed, and this does not allow to reuse the results presented in the literature concerning the relationships betweenCLP and constructive negation. We propose a new constraint satisfaction algorithm, capable of correctly handling constructive negation for large classes ofCLP(SεT) programs; we also provide a syntactic characterization of such classes of programs. The resulting algorithm provides a novel constraint simplification procedure to handle constructive negation, suitable to theories where unification admits multiple most general unifiers. We also show, using a general result, that it is impossible to construct an interpreter forCLP(SεT) with constructive negation which is guaranteed to work for any arbitrary program; we identify classes of programs for which the implementation of the constructive negation technique is feasible. Agostino Dovier, Ph.D.: He is a researcher in the Department of Science and Technology at the University of Verona, Italy. He obtained his master degree in Computer Science from the University of Udine, Italy, in 1991 and his Ph.D. in Computer Science from the University of Pisa, Italy, in 1996. His research interests are in Programming Languages and Constraints over complex domains, such as Sets and Multisets. He has published over 20 research papers in International Journals and Conferences. He is teaching a course entitled “Special Languages and Techniques for Programming” at the University of Verona. Enrico Pontelli, Ph.D.: He is an Assistant Professor in the Department of Computer Science at the New Mexico State University. He obtained his Laurea degree from the University of Udine (Italy) in 1991, his Master degree from the University of Houston in 1992, and his Ph.D. degree from New Mexico State University in 1997. His research interests are in Programming Languages, Parallel Processing, and Constraint Programming. He has published over 50 papers and served on the program committees of different conferences. He is presently the Associate Director of the Laboratory for Logic, Databases, and Advanced Programming. Gianfranco Rossi, Ph.D.: He received his degree in Computer Science from the University of Pisa in 1979. From 1981 to 1983 he was employed at Intecs Co. System House in Pisa. From November 1983 to February 1989 he was a researcher at the Dipartimento di Informatica of the University of Turin. Since March 1989 he is an Associate Professor of Computer Science, currently with the University of Parma. He is the author of several papers dealing mainly with programming languages, in particular logic programming languages and Prolog, and extended unification algorithms. His current research interests are (logic) programming languages with sets and set unification algorithms.     

PAN: A portable,parallel Prolog: Its design,realisation and performance

PAN is a general purpose, portable environment for executing logic programs in parallel. It combines a flexible, distributed architecture which is resilient to software and platform evolution with facilities for automatically extracting and exploiting AND and OR parallelism in ordinary Prolog programs. PAN incorporates a range of compile-time and run-time techniques to deliver the performance benefits of parallel execution while rertaining sequential execution semantics. Several examples illustrate the efficiency of the controls that facilitate the execution of logic programs in a distributed manner and identify the class of applications that benefit from distributed platforms like PAN. George Xirogiannis, Ph.D.: He received his B.S. in Mathematics from the University of Ioannina, Greece in 1993, his M.S in Artificial Intelligence from the University of Bristol in 1994 and his Ph.D. in Computer Science from Heriot-Watt University, Edinburgh in 1998. His Ph.D. thesis concerns the automated execution of Prolog on distributed heterogeneous multi-processors. His research interests have progressed from knowledge-based systems to distributed logic programming and data mining. Currently, he is working as a senior IT consultant at Pricewaterhouse Coopers. He is also a Research Associate at the National Technical University of Athens, researching in knowledge and data mining. Hamish Taylor, Ph.D.: He is a lecturer in Computer Science in the Computing and Electrical Engineering Department of Heriot-Watt University in Edinburgh. He received M.A. and MLitt degrees in philosophy from Cambridge University and an M.S. and a Ph.D. degree in computer science from Heriot-Watt University, Scotland. Since 1985 he has worked on research projects concerned with implementing concurrent logic programming languages, developing formal models for automated reasoning, performance modelling parallel relational database systems, and visualisizing resources in shared web caches. His current research interests are in applications of collaborative virtual environments, parallel logic programming and networked computing technologies.     

Preference queries in deductive databases

Traditional database query languages such as datalog and SQL allow the user to specify only mandatory requirements on the data to be retrieved from a database. In many applications, it may be natural to express not only mandatory requirements but also preferences on the data to be retrieved. Lacroix and Lavency¹⁰⁾ extended SQL with a notion of preference and showed how the resulting query language could still be translated into the domain relational calculus. We explore the use of preference in databases in the setting of datalog. We introduce the formalism of preference datalog programs (PDPs) as preference logic programs without uninterpreted function symbols for this purpose. PDPs extend datalog not only with constructs to specify which predicate is to be optimized and the criterion for optimization but also with constructs to specify which predicate to be relaxed and the criterion to be used for relaxation. We can show that all of the soft requirements in Reference¹⁰) can be directly encoded in PDP. We first develop anaively-pruned bottom-up evaluation procedure that is sound and complete for computing answers to normal and relaxation queries when the PDPs are stratified, we then show how the evaluation scheme can be extended to the case when the programs are not necessarily stratified, and finally we develop an extension of themagic templates method for datalog¹⁴) that constructs an equivalent but more efficient program for bottom-up evaluation. Kannan Govindarajan, Ph.D.: He obtained his bachelors degree in Computer Science and Engineering from the Indian Institute of Technology, Madras, and he completed his Ph.D. degree in Computer Science from the State University of New York at Buffalo. His dissertation research was on optimization and relaxation techniques for logic languages. His interests lie in the areas of programming languages, databases, and distributed systems. He currently leads the trading community effort in the E-speak Operation in Hewlett Packard Company. Prior to that, he was a member of the Java Products Group in Oracle Corporation. Bharat Jayaraman, Ph.D.: He is a Professor in the Department of Computer Science at the State University of New York at Buffalo. He obtained his bachelors degree in Electronics from the Indian Institute of Technology, Madras (1975), and his Ph.D. from the University of Utah (1981). His research interests are in programming languages and declarative modeling of complex systems. Dr. Jayaraman has published over 50 papers in refereed conferences and journals. He has served on the program committees of several conferences in the area of programming languages, and he is presently on the Editorial Board of the Journal of Functional and Logic Programming. Surya Mantha, Ph.D.: He is a manager in the Communications and Software Services Group of Pittiglio Rabin Todd & McGrath (PRTM), a management consulting firm serving high technology industries. He obtained a bachelors degree in Computer Science and Engineering from the Indian Institute of Technology, Kanpur, an MBA in Finance and Competitive Strategy from the University of Rochester, and a Ph.D. in Computer Science from the University of Utah (1991). His research interests are in the modeling of complex business processes, inter-enterprise application integration, and business strategy. Dr. Mantha has two US patents, and has published over 10 research papers. Prior to joining PRTM, he was a researcher and manager in the Architecture and Document Services Technology Center at Xerox Corporation in Rochester, New York.     

A formal approach to designing delay-insensitive circuits

Summary A method for designing delay-insensitive circuits is presented based on a simple formalism. The communication behavior of a circuit with its environment is specified by a regular expression-like program. Based on formal manipulations this program is then transformed into a delay-insensitive network of basic elements realizing the specified circuit. The notion of delay-insensitivity is concisely formalized. Jo C. Ebergen received his Master's degree in Mathematics from Eindhoven University of Technology in 1983. From 1983 until 1987 he has been working as a researcher at the Centre for Mathematics and Computer Science in Amsterdam in the area of VLSI design. In 1987, he received his Ph.D. degree from Eindhoven University of Technology. Currently, he is assistant professor at the University of Waterloo. His main research interests are programming methodology, parallel computations, and delay-insensitive circuit design. Dr. Ebergen is a member of ACM and EATCS.The research reported in this article was carried out while the author was working at CWI (Centre for Mathematics and Computer Science) in Amsterdam     

Programming languages for distributed applications

Much progress has been made in distributed computing in the areas of distribution structure, open computing, fault tolerance, and security. Yet, writing distributed applications remains difficult because the programmer has to manage models of these areas explicitly. A major challenge is to integrate the four models into a coherent development platform. Such a platform should make it possible to cleanly separate an application’s functionality from the other four concerns. Concurrent constraint programming, an evolution of concurrent logic programming, has both the expressiveness and the formal foundation needed to attempt this integration. As a first step, we have designed and built a platform that separates an application’s functionality from its distribution structure. We have prototyped several collaborative tools with this platform, including a shared graphic editor whose design is presented in detail. The platform efficiently implements Distributed Oz, which extends the Oz language with constructs to express the distribution structure and with basic primitives for open computing, failure detection and handling, and resource control. Oz appears to the programmer as a concurrent object-oriented language with dataflow synchronization. Oz is based on a higher-order, state-aware, concurrent constraint computation model. Seif Haridi, Ph.D.: He received his Ph.D. in computer science in 1981 from the Royal Institute of Technology, Sweden. After spending 18 months at IBM T. J. Watson Research Center, he moved to the Swedish Institute of Computer Science (SICS) to form a research lab on logic programming and parallel systems. Dr. Haridi is currently the research director of the Swedish Institute of Computer Science. He has been an active researcher in the area of logic and constraint programming and parallel processing since the beginning of the eighties. His earlier work includes contributions to the design of SICStus Prolog, various parallel Prolog systems and a class of scalable cache-coherent multiprocessors known as Cache-Only Memory Architecture (COMA). During the nineties most of his work focused on the design of multiparadigm programming systems based on Concurrent Constraint Programming (CCP). Currently, he is interested in programming systems and software methodology for distributed and agent-based applications. Peter Van Roy, Ph.D.: He obtained an engineering degree from the Vrije Universiteit Brussel (1983), Masters and Ph.D. degrees from the University of California at Berkeley (1984, 1990), and the Habilitation à Diriger des Recherches from Paris VII Denis Diderot (1996). He has made major contributions to logic language implementation. His research showed for the first time that Prolog can be implemented with the same execution efficiency as C. He was principal developer or codeveloper of Aquarius Prolog, Wild_Life, Logical State Threads, and FractaSketch. He joined the Oz project in 1994 and is currently working on Distributed Oz. His research interests are motivated by the desire to provide increased expressivity and efficiency to application developers. Per Brand: He is a researcher at the Swedish Institute of Computer Science. He has previously worked on the design and implementation of OR-parallel Prolog (the Aurora project) and optimized compilation techniques for Concurrent Constraint Programming Languages (in particular, AKL). He has been a member of the Distributed Oz design team since the project began. His research interests are focused on techniques, languages, and methodology for distributed programming. Christian Schulte: He studied computer science at the University of Karlsruhe, Germany, from 1987 to 1992 where he received his diploma. Since 1992 he has been a member of the Programming Systems Lab at DFKI. He is one of the principal designers of Oz. His research interests include design, implementation, and application of concurrent and distributed programming languages as well as constraint programming.     

A self-stabilizing algorithm for coloring planar graphs

Summary This paper describes an algorithm for coloring the nodes of a planar graph with no more than six colors using a self-stabilizing approach. The first part illustrates the coloring algorithm on a directed acyclic version of the given planar graph. The second part describes a selfstabilizing algorithm for generating the directed acyclic version of the planar graph, and combines the two algorithms into one. Sukumar Ghosh received his Ph.D. degree in Computer Science from Calcutta University in 1971. From 1969 to 1984, he taught at Jadavpur University, Calcutta. During 1976–77, he was a Fellow of the Alexander von Humboldt Foundation at the University of Dortmund, Germany. Since 1984, he is with the Department of Computer Science of the University of Iowa. His current research interests are in the areas of Distributed Systems, Petri Nets and Self-Stabilizating Systems. Mehmet Hakan Karaata received the Sc. B. degree in Computer Science and Engineering from Hacettepe University in Turkey in 1987, and the M.S. degree in Computer Science from the University of Iowa in 1990. He is currently studying towards his Ph.D. at the same university. His research interests are in the areas of Distributed Systems, Self-Stabilizing Systems and Database Systems.This research was supported in part by the National Science Foundation under grant CCR-9109078, and the Old Gold Summer Fellowship of the University of Iowa. An abstract of this paper was presented at the 29th Allerton Conference on Control, Communication & Computing in October 1991.