CAP - Concurrent Action and Planning: Using PVM-Prolog to Implement Vivid Agents

ion it ranges between PVM-Prolog which is a pure paradigm extendedby message passing and a high-level multi-agent language such as vivid agents.7.3 ICEICE [Amt95, AB96] de nes a model to support distributed AI applications overhybrid languages and heterogeneous, distributed platforms. It is available fromProlog, Lisp, C, C++ and Tcl/Tk and is, similar to PVM-Prolog, implementedon top of PVM.Its communication model is point to point through named channels but italso supports broadcasting of messages to all components. Components maybe created dynamically but their communication topology must be de ned ina con guration le. Con guration of components and their communicationchannels is done through a con guration le by a special process, the licenseserver.PVM-Prolog, as ICE, also supports communication with tasks written indi erent languages. ICE's communication model is de ned at a higher levelthan PVM-Prolog's. However it is necessary to specify the topology of thecommunication before running the system this is a clear limitation in contrastwith PVM dynamic con guration, communication and process control features.7.4 Other Distributed Logic Programming LanguagesThe rst distributed logic programming language to appear in the literature isDelta-Prolog[LMP84, CCMP92]. Delta Prolog supports a declarative distrib-uted logic programming model which requires distributed backtracking mech-anisms which are di cult to implement. Other distributed logic program-ming systems such as PMS-Prolog[WJH92] and Shared-Prolog[BC91] elude theneed for distributed backtracking either by using message passing extra-logicalpredicates in the former or preventing it in the language syntax in the lat-ter. Shared-Prolog relies on the LINDA shared space communication modelwhile PMS-Prolog and PVM-Prolog use distributed space communication mod-els. Neither Shared-Prolog or PMS-Prolog support multi-threading of a Prologprocess. Delta-Prolog and PMS-Prolog have been implemented in transputermulti-computers Delta-Prolog has also been implemented in UNIX systems con-nected through local area networks.8 ConclusionTo conclude let us compare which of the requirements mentioned in the intro-duction are met in the current implementation of vivid agents and how theunderlying PVM-Prolog contributes.Reactive and Pro-active Behaviour The agents' reactive and pro-activebehaviour can be implemented by using PVM-Prolog's thread concept to run a17 perception-reaction-cycle concurrently to a planning facility. Such light-weightconcurrency has the advantage of fast communication between strongly con-nected parts of a single agent.Formal Semantics The formal semantics of the CAP architecture helps un-derstanding and allows program veri cation. The formal semantics is indepen-dent of PVM-Prolog as implementaiton language, but it was in uenced by theconcepts provided by PVM-Prolog and therefore easily implementable in PVM-Prolog.Executable Speci cations The idea of Prolog executing speci cations islifted by the vivid agent concept and its implementation in PVM-Prolog to ahigher and more declarative level of multi-agent speci cations.Hardware Independence PVM is hardware independent and available fora range of architectures including PCs, Workstations and Multi-computer plat-forms. PVM's and PVM-Prolog's hardware independence also makes vividagents more exible.Openness PVM is best suited for local area networks. A more appropriateglobal network interface is not yet implemented. Currently we are experiment-ing with Email. We included a primitive to send mails into PVM-Prolog andtranslate incoming emails addressed to the MAS to PVM messages.Heterogeneity PVM has been widely used up to now, so many tools anddi erent language interfaces are available. The PVM distribution supports Cand Fortran; interfaces for C++, Ada, Lisp, Perl, Tcl, and Prolog have subse-quently been developed. First results show that heterogeneous agents can runtogehter in a net but that the MAS con guration has to be public to ensurethat agents can use the proper formats to communicate. A possible solution is apublic yellow pages service, which might be implemented itsel n PVM-Prolog.AcknowledgementsWe would like to thankWolfgang Nejdl and Lu s Moniz Pereira for their support.Financially, the work was supported by the German BMBF, the PortugueseJNICT, and the PRAXIS PROLOPPE project.References[AB96]Jan W. Amtrup and Jorg Benra. Communication in large distrib-uted ai systems for natural language processing. 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