Gilbert, D

ACM Computing Surveys

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Can computational logic provide a paradigm for both thespecication and implementation of concurrent systems?yDavid Gilbert, City University, London UKdrg@cs.city.ac.ukMost computational systems in use today comprise a strong element of concurrency,often within a framework of distribution over networks. It is for this reason that I believethat one of the signicant challenges facing the computational logic community is todemonstrate that logic can provide a paradigm for both the specication and ecientimplementation of such systems, and to convince system designers and developers of theusefulness of this methodology. In this article I propose that a serious eort is made toaccept this challenge and suggest that the concurrent constraint paradigm is now matureenough to be used in this role; moreover one way to achieve this goal is to develop tools forautomated reasoning about concurrent systems, based on the semantics of this paradigm.For the purposes of this discussion I shall take the denition of concurrency to be thatgiven by Milner [6].The parallel logic programming languages [9] which were developed a decade ago lieuncomfortably between the categories of ecient implementation languages and languagesused for the formal description of concurrent systems. Historically they evolved by mod-ifying sequential programming languages and were mainly used as experimental systemprogramming languages [4, 8]. Had they been intended to be used for formal descriptionsthen their design would have been shaped by the need for clear and unambiguous seman-tics powerful enough to permit both the description and reasoning the about the dynamicbehaviour of systems.The generalisation of the logic programming paradigm to encompass computing con-straints over various domains [10] and the adaption of this paradigm to concurrent con-straint logic programming (CCLP) [5, 7] is, I believe, the key to progress. There are nowmany researchers, too numerous to cite here, who are developing concurrent constraintbased languages and associated theories of semantics; their work is evidence that thisexciting eld is rapidly expanding.In the CCLP paradigm [3] a simple constraint c is a token which may be added toa store or set of such constraints , but may never be removed from the store | i.e.In the June 1996 Special Issue of ACM Computing Surveys, Volume 28, Number 2, ISSN 0360{0300,pp 303{305: Symposium on Models of Programming Languages and Computation. C. Hankin and HRNielson, Eds.yACM Copyright NoticeCopyrightcACM 1996 0360-0300/96/0600-0303 by the Association for Computing Machinery, Inc.Permission to make digital or hard copies of part or all of this work for personal or classroom use is grantedwithout fee provided that copies are not made or distributed for prot or commercial advantage and thatcopies bear this notice and the full citation on the rst page. Copyrights for components of this workowned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise,to republish, to post on servers, or to redistribute to lists, requires prior specic permission and/or a fee.Request permissions from Publications Dept, ACM Inc., fax +1 (212) 869-0481, or permissions@acm.org.1updates are monotonic. This update operation is called \tell" and has no eect on thestore if it already contains the told constraint. The state of the store can be queriedby an \ask" operation which succeeds if the store entails the asked constraint. We canconceptualise the store as a blackboard (potentially distributed), parts of which can bemade private to designated processes, ask operations as reads and tells as writes on data.In classical concurrency terminology, a process which queries the state of the store with anask is a consumer while one which updates the store with a tell is a producer ; producersand consumers communicate via the store of constraints. The communication engenderedis inherently multi-party, since there is no restriction on the number of concurrent askoperations that may be made on one constraint, nor on the number of concurrent tells onthat constraint, as long as they are consistent with the store. Moreover the paradigm isclassically asynchronous since an ask operation blocks if the information in the store isnot complete enough to entail the asked constraints, whereas the tell operation is eagerand does not block. CCLP languages have inherited the committed choice \don't care"operator from CLP, have a parallel operator with interleaving semantics, and a sequencingoperator over processes.I am exploring the synchronous paradigm together with my colleagues Lubos Brim andMojmr Kretnsky of Masaryk University in the Czech Republic and Jean-Marie Jacquetof the University of Namur in Belgium [1, 2]. The communication primitives act on a givenstore in the following way: as usual, given a constraint c, the process ask(c) succeeds ifc is entailed by the store, otherwise it is suspended until it can succeed. In our languagesynchronisation is achieved by forcing a tell(c) operation to suspend if the constraint cis not entailed by the store; it can be resumed synchronously with an ask(d) operationprovided that the conjunction of the store and c entails d, in which case the store isupdated with c. A tell(c) operation is only eager if the constraint c is entailed by thestore; however the behaviour of ask(c) is classical in that it succeeds if c is entailed bythe store, otherwise it suspends until it can succeed. The scheme is generalised to permitthe synchronisation of more than two partners, and also by the introduction of a variantof the tell primitive which leaves the store unchanged. The latter primitive enables us todescribe the re-use of resources in a succinct manner by avoiding the need to explicitlyprevent the re-use of messages about the state of these resources.The inherently multi-party form of communication, and the natural way in whichcommunication and data passing are integrated in one coherent theory makes the CCLPparadigm an excellent candidate for the formal description of concurrent systems. Todate, most of these languages employ asynchronous producers (\eager tells") for reasonsof ecient implementation. Very little attention has been paid by designers of concurrentconstraint languages to synchronous communication; however, specications of the use oflimited resources are often best made using a synchronous model of communication inorder to facilitate the task of reasoning about the descriptions, for example the bakeryalgorithm. Other work on synchronous communication within the CCLP paradigm isbased on the concept of extending clauses to permit multiple heads and allowing thesimultaneous unication of concurrent atoms with these heads, for example [7]. In contrast,our approach permits synchronisation on data rather than on processes, and in this respectis more akin to algebraic theories of concurrency [6].Our intention is that this language should be used to specify and reason about con-current systems, and that tools based on its semantics can be constructed to permitmechanised reasoning about the behaviour of programs. We are planning the constructionof an integrated workbench which will enable the specier of concurrent constraint based2systems to reason about the behaviour of programs and explore their potential computa-tions. The user of the workbench should be able to dene semantics for his own languageand then to reason mechanically about the behaviour of programs that he has written.We intend to extend the functionality of the workbench to be able to model the executionof asynchronous constraint programs, and hope that the designers of other concurrentconstraint languages will see the building of such specication or program constructionenvironments as a useful goal.Perhaps the real challenge, however, lies in the design and construction of concurrentconstraint languages which permit ecient execution in a distributed environment, andto couple these implementation platforms with tools for automated (or semi-automated)reasoning about programs. Additionally, program transformation and synthesis tools willbe required; ideally these should be designed within the same paradigm of concurrentconstraint programming. All of this should help towards the ultimate goal of getting theadvantages of declarative programming accepted by the wider community. If we cannotachieve this then the danger is that the concurrent logic programming paradigm willremain a curiosity for the majority of system designers and implementers, and the hopesexpressed in the 1980's that logic programming would be widely taken up by industry forthe design and construction of large concurrent systems will nally be dashed.References[1] L. Brim, J-M. Jacquet, D. Gilbert, and M. Kretnsky. New Versions of Ask andTell for Synchronous Communication in Concurrent Constraint Programming. Tech-nical Report TCU/CS/1995/10, Department of Computer Science, City University,London, U.K., 1995. submitted to MFCS96.[2] L. Brim, J-M. Jacquet, D. Gilbert, and M. Kretnsky. Synchronisation in Scc. InJohn Lloyd, editor, ILPS'95: Proceedings of the International Logic ProgrammingSymposium. MIT Press, 1995.[3] Frank S de Boer and Catuscia Palamidessi. A Fully Abstract Model for ConcurrentConstraint Programming. In TAPSOFT'91, 1991.[4] Ian Foster. Systems Programming in Parallel Logic Languages. Prentice Hall, 1990.[5] Michael J. Maher. Logic Semantics for a class of Committed-Choice Programs. InJean-Louis Lassez, editor, Logic Programming, Proceedings of the Fourth Interna-tional Conference, volume 2, pages 858{876, Cambridge, Mass, USA, 1987. MIT.[6] Robin Milner. Communication and Concurrency. Prentice Hall, 1989.[7] Vijay Saraswat. Concurrent Constraint Programming. MIT Press, 1993.[8] H. Sato, T. Chikayama, E. Sugino, and K. Taki. Outlines of PIMOS. In Proceedingsof the 34th Annual Convention IPS Japan, Tokyo, Japan, March 18, 1987. ICOT.[9] Akikazu Takeuchi and Koichi Furukawa. Parallel Logic Programming Languages. InEhud Shapiro, editor, Concurrent Prolog, volume 1, pages 188{201. MIT Press, 1987.[10] Pascal Van Hentenryck. Constraint Satisfaction in Logic Programming. MIT Press,Cambridge, MA, 1989.3

Can computational logic provide a paradigm for both the specification and implementation of concurrent systems?

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