We investigate the issue of designing a kernel programming language for Mobile Computing and describe Klaim, a language that supports a programming paradigm where processes, like data, can be moved from one computing environment to another. The language consists of a core Linda with multiple tuple spaces and of a set of operators for building processes. Klaim naturally supports programming with explicit localities. Localities are first-class data (they can be manipulated like any other data), but the language provides coordination mechanisms to control the interaction protocols among located processes. The formal operational semantics is useful for discussing the design of the language and provides guidelines for implementations. Klaim is equipped with a type system that statically checks access rights violations of mobile agents. Types are used to describe the intentions (read, write, execute, etc.) of processes in relation to the various localities. The type system is used...

This chapter surveys the semantic rami cations of extending traditional process algebras with notions of priority that allow for some transitions to be given precedence over others. The need for these enriched formalisms arises when one wishes to model system features such asinterrupts, prioritized choice, orreal-time behavior. Approaches to priority in process algebras can be classi ed according to whether the induced notion of pre-emption on transitions is global or local and whether priorities are static or dynamic. Early work in the area concentrated on global preemption and static priorities and led to formalisms for modeling interrupts and aspects of real-time, such as maximal progress, in centralized computing environments. More recent research has investigated localized notions of pre-emption in which the distribution of systems is taken into account, as well as dynamic priority approaches, i.e., those where priority values may change as systems evolve. The latter allows one to model behavioral phenomena such as scheduling algorithms and also enables the e cient encoding of real-time semantics. Technically, this chapter studies the di erent models of priorities by presenting extensions of Milner's Calculus of Communicating Systems (CCS) with static and dynamic priority as well as with notions of global and local pre-emption. In each case the operational semantics of CCS is modi ed appropriately, behavioral theories based on strong and weak bisimulation are given, and related approaches for di erent process-algebraic settings are discussed.

We present a simple extension of the ß-calculus with located actions and channels and with location names as first-class data, which models the notion of locality and failure present in the higher-order, distributed programming language Facile. The interaction between localities and failures distinguishes our approach from previous ones where the notion of locality is considered in isolation. We argue that the combination of these two features leads, at least from the distributed programming viewpoint, to a more natural semantics. We then discuss the translation of this calculus into a standard simply-sorted ß-calculus and show its adequacy with respect to a barbed bisimulation based semantics. In the translation each location is represented by a special process which interacts, by means of a simple protocol, with any process of the original program that wants to access resources depending on that location. We also employ our translation in the verification of a very simple fault-toler...

In a similar way as 2-categories can be regarded as a special case of double categories, rewriting logic (in the unconditional case) can be embedded into the more general tile logic, where also side-effects and rewriting synchronization are considered. Since rewriting logic is the semantic basis of several language implementation efforts, it is useful to map tile logic back into rewriting logic in a conservative way, to obtain executable specifications of tile systems. We extend the results of earlier work by two of the authors, focusing on some interesting cases where the mathematical structures representing configurations (i.e., states) and effects (i.e., observable actions) are very similar, in the sense that they have in common some auxiliary structure (e.g., for tupling, projecting, etc.). In particular, we give in full detail the descriptions of two such cases where (net) process-like and usual term structures are employed. Corresponding to these two cases, we introduce two ca...

In this paper we present history-dependent automata (HD-automata in brief). They are an extension of ordinary automata that overcomes their limitations in dealing with history-dependent formalisms. In a history-dependent formalism the actions that a system can perform carry information generated in the past history of the system. The most interesting example is -calculus: channel names can be created by some actions and they can then be referenced by successive actions. Other examples are CCS with localities and the history-preserving semantics of Petri nets. Ordinary

Process algebra semantics can be categorised into non-interleaving semantics, where parallel composition is considered a primitive operator, and interleaving semantics, where concurrency is reduced to sequentiality plus nondeterminism. The former have an appealing intuitive justification, but the latter are mathematically more tractable. This paper addresses the study of non-interleaving semantics in the framework of process algebras for mobile systems, like -calculus [MPW92, Mil91]. We focus on location bisimulation ( ` ), in our opinion one of the most convincing non-interleaving equivalences, which aims to describe the spatial dependencies on processes. We introduce ` in -calculus following the definition for CCS given in [BCHK91b]. Our main contribution is to show that in -calculus ` can be expressed, or implemented, within the ordinary interleaving observation equivalence [Mil89, MPW92] by means of a fairly simple and fully abstract encoding. Thus, we can take advantage of the...

Abstract. One obtains in this paper a process algebra RCCS, in the style of CCS, where processes can backtrack. Backtrack, just as plain forward computation, is seen as a synchronization and incurs no additional cost on the communication structure. It is shown that, given a past, a computation step can be taken back if and only if it leads to a causally equivalent past. 1

Tile logic extends rewriting logic, taking into account rewriting with side-effects and rewriting synchronization. Since rewriting logic is the semantic basis of several language implementation efforts, it is interesting to map tile logic back into rewriting logic in a conservative way, to obtain executable specifications of tile systems. The resulting implementation requires a meta-layer to control the rewritings, so that only tile proofs are accepted. However, by exploiting the reflective capabilities of the Maude language, such meta-layer can be specified as a kernel of internal strategies. It turns out that the required strategies are very general and can be reformulated in terms of search algorithms for non-confluent systems equipped with a notion of success. We formalize such strategies, giving their detailed description in Maude, and showing their application to modeling uniform tile systems. 1 Introduction The evolution of a process in a concurrent system often depends on the ...

The distributed structure of CCS processes can be made explicit by assigning different locations to their parallel components. The assignment of locations may be done statically, or dynamically as the execution proceeds. The dynamic approach was developed first, by Boudol et al., as it appeared more convenient for defining notions of location equivalence and preorder. Extending previous work by L. Aceto we study here the static approach, which is more natural from an intuitive point of view, and more manageable for verification purposes. We define static notions of location equivalence and preorder, and show that they coincide with the dynamic ones. To establish the equivalence of the two location semantics, we introduce an intermediate transition system called occurrence system, which incorporates both notions of locality. This system supports a definition of local history preserving bisimulation for CCS, which is a third formulation of location equivalence.

In this paper we present history-dependent automata (HD-automata in brief). They are an extension of ordinary automata that overcomes their limitations in dealing with history-dependent formalisms. In a history-dependent formalism the actions that a system can perform carry information generated in the past history of the system. The most interesting example is -calculus: channel names can be created by some actions and they can then be referenced by successive actions. Other examples are CCS with localities and the history-preserving semantics of Petri nets. Ordinary automata are an unsatisfactory operational model for these formalisms: infinite automata are obtained for all the systems with in nite computations, even for very simple ones; moreover, the ordinary definition of bisimulation does not apply in these cases, thus preventing the reusage of standard theories and algorithms. In this paper we show that HD-automata are an adequate model for the history-dependent formalisms. We pr...