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. In this paper we are going to analyze mobile code issues in the perspective of Object Oriented systems in which thread migration is not supported. This means that both objects' code and data can be transmitted from a place to another but not the current execution state (if any) associated to the object. This is the case with the Java language which is often used in the Word Wide Webfor developing applets which are little applications downloaded on the fly and executed in the client machine. While this mechanism is quite useful for enhancing HTML documents with sound and animation, we think that this technology can give its best in the field of distributed-cooperative work, both in the perspective of Internet and Intranet connectivity. Java is indeed a concurrent, multithreaded language, but it offers little help for distributed programming. Thus, we introduce Jada, a coordination toolkit for Java where coordination among either concurrent threads or distributed Java objects is achiev...

A programmable coordination medium is a shared communication device whose behaviour can be defined by means of a suitable programming language according to the global system needs. This notion has indeed an impact over the design of multi-component software system, since (part of) the system computational charge of can be in principle shifted from the communicating components (agents) to the communication device. At the same time, this raises the issue of the computational power of a programmable coordination medium, and of its expressiveness. As a meaningful example, this paper discusses the case of ReSpecT, the first-order logic language used by the ACLT coordination model to define the behaviour of its multiple programmable logic tuple spaces, called tuple centres. Its expressiveness is discussed first by comparing ReSpecT with Petri Nets, taken as a reference model for the specification and design of concurrent and distributed systems, then by presenting some simple cases of coor...

Abstract The presents the TuCSoN coordination model for Internet applications based on network-aware (possibly mobile) agents. The model is based on the notion of tuple centre, an enhanced tuple space whose behaviour can be extended according to the application needs. Every node of a TuCSoN environment provides its local communication space, made up of a multiplicity of independently-programmable tuple centres. This makes it possible both to embed global system properties into the space of components ' interaction, thus enabling flexible cooperation over space and time between agents and permitting to easily face many issues critical to Internet applications, such as heterogeneity and dynamicity of the execution environments.

Shared space coordination models such as Linda are ill-suited for structuring applications composed of erroneous or insecure components. This paper presents theSecure Object Space model. In this model, a data element can be locked with a key and is only visible to a process that presents a matching key to unlock the element. We give a precise semantics for Secure Object Space operations and discuss an implementation in JAVA for a mobile agent system. An implementation of the semantics that employs encryption is also outlined for use in untrusted environments.

Messengers are autonomous objects, each capable of navigating through the underlying network and performing various tasks at each node. Messengers applications are written using navigational commands rather than the send/receive primitives of conventional messagepassing approaches. In this paper we contrast the two programming styles. From a software engineering viewpoint, the navigational style generally results in a smaller semantic gap between abstract algorithm descriptions and their actual implementations, which makes programs easier to construct, understand, and maintain. In terms of performance, Messengers programs are highly competitive with message-passing. We demonstrate these advantages using two concrete applications programmed using Messengers and PVM. 1 Introduction The objective of this paper is to contrast two different styles of programming in distributed systems. The first, which is by far the most prevalent among all distributed programming languages, is based on ...

Logic-based languages have alredy proved to be effective to build individual agents and to enable inter-agent communication in multi-agent systems. Also, logic tuple centres have shown that logic-based languages can be effectively exploited to rule inter-agent communication so as to build social behaviours. In this paper, we formally define the notion of logic tuple centre as well as the operational semantics of the logic-based language ReSpecT for the behaviour specification of logic tuple centres. For this purpose, we exploit a formal framework for asynchronous systems allowing coordination media to be represented in a separate and independent way with respect to the coordinated entities. As a by-product, this shows that a logic-based approach may be effectively exploited for the coordination of heterogeneous agents of different sorts and technologies.

. This work describes an agent interaction model ( , Agent Communicating through Logic Theories) rooted in the concept of logic theory. agents and their behaviour are conceived as inferential as well as procedural activities within a multiple theory space. The communication unit (CU) abstraction is exploited, subsuming traditional communication models (both shared memory and message passing) based on explicit and extensional knowledge, while allowing agents to exploit partial/incomplete knowledge through deduction. Agent synchronization is reconducted to the concept of theory evolution, by allowing agents to wait for theory modification until facts can be deduced from a CU. Agent cooperation/competition is re-interpreted in terms of knowledge generation/consumption. A coherent notion of logic consequence in a time-dependent environment is proposed. As a result, the traditional dichotomy between reactive and symbolic systems is here exploited as a feature ra...

This paper presents a comparative survey of formalisms related to mobile agents. It describes the -calculus and its extensions, the Ambient calculus, Petri nets, Actors, and the family of generative communication languages. Each of these formalisms defines a mathematical framework that can be used to reason about mobile code; they vary greatly in their expressiveness, in the mechanisms they provide to specify mobile code based applications and in their practical usefulness for the validation and the verification of such applications. In this paper we show how these formalisms can be used to represent the mobility and communication aspects of two mobile code environments: Obliq and Messengers. We compare and classify the different formalisms with respect to mobility and discuss some shortcomings and desirable extensions. We also point to other emerging concepts in formalisms for mobile code systems.

assumptions, client-server applications in Prolog, embedded logic engines, natural language analysis, control through speech, multilingual interface \Delta The first and the the third authors thank for support from NSERC (grants OGP0107411 and 611024), the second author is research associate with the Fund for Scientific Research -- Flanders (Belgium). Address correspondence to Paul Tarau, Departement d'Informatique, Universite de Moncton, CANADA E1A-3E9 E-mail: tarau@info.umoncton.ca. y Vakgroep Elektronica en Informatiesystemen, Universiteit Gent, E-mail: kdb@elis.rug.ac.be. z Logic and Functional Programming Group, Department of Computing Sciences, Simon Fraser University, E-mail: fveronica,srochefog@cs.sfu.ca. THE JOURNAL OF LOGIC PROGRAMMING c flElsevier Science Publishing Co., Inc., 1993 655 Avenue of the Americas, New