Frédéric Peschanski

Architectural design of component-based agents: a behavior-based approach

This paper relates an experience in using a component model to design and construct agents. After discussing various rationales and architectural styles for decomposing an agent architecture, we describe a model of component for agents, named MALEVA. In this model, components encapsulate various units of agent behaviors (e.g., follow gradient, flee, reproduce). It provides an explicit notion of control flow between components (reified through specific control ports, connexions and components), for a fine grain control of activation and scheduling. Moreover, a notion of composite component allows complex behaviors to be constructed from simpler ones. Two examples, in the domain of multi-agent based simulation, are presented in this paper. They illustrate the ability of the model to facilitate both bottom-up and top-down approaches for agent design and construction and also to help at different types of potential reuse.

A stackless runtime environment for a Pi-calculus

The Pi-calculus is a formalism to model and reason about highly concurrent and dynamic systems. Most of the expressive power of the language comes from the ability to pass communication channels among concurrent processes, as any other value. We present in this paper the CubeVM, an interpreter architecture for an applied variant of the Pi-calculus, focusing on its operational semantics. The main characteristic of the CubeVM comes from its stackless architecture. We show, in a formal way, that the resource management model inside the VM may be greatly simplified without the need for nested stack frames. This is particularly true for the garbage collection of processes and channels. The proposed GC, based on a reference counting scheme, is highly concurrent and, most interestingly, does automatically detect and reclaim cycles of disabled processes. We also address the main performance issues raised by the fine-grained concurrency model of the Pi-calculus. We introduce the reactive variant of the semantics that allows, when applicable, to increase the performance drastically by bypassing the scheduler. We define the language subset of processes in so called chain-reaction forms for which the sequential semantics can be proved statically. We illustrate the expressive power and performance gains of such chain-reactions with examples of functional, dataflow and object-oriented systems. Encodings for the pure Pi-calculus are also demonstrated.

The Combinatorics of Non-determinism

A deep connection exists between the interleaving semantics of concurrent processes and increasingly labelled combinatorial structures. In this paper we further explore this connection by studying the rich combinatorics of partially increasing structures underlying the operator of non-deterministic choice. Following the symbolic method of analytic combinatorics, we study the size of the computation trees induced by typical non-deterministic processes, providing a precise quantitative measure of the so-called combinatorial explosion phenomenon. Alternatively, we can see non-deterministic choice as encoding a family of tree-like partial orders. Measuring the (rather large) size of this family on average offers a key witness to the expressiveness of the choice operator. As a practical outcome of our quantitative study, we describe an efficient algorithm for generating computation paths uniformly at random.

Coordinating mobile agents in interaction spaces

We present a kernel coordination language for mobile agent systems that considers as first-class citizens both the agents and the channels they use to interact with each other. Channels implement distributed, asynchronous communications with FIFO ordering and multicast routing. Features related to mobility include agent migration as well as remote cloning. Most importantly, a virtual form of channel mobility inspired by the @p-calculus is also supported. This expressive feature allows mobile agents to adapt dynamically to their changing environment. The language semantics, presented formally, is based on a geometrical model named the Interaction Spaces. This provides an intuitive interpretation of the agent features and capabilities in terms of combined spatial projections and transformations. Through spatial composition, we show that standard labeled transition systems and bisimulation-based semantics may be defined above the geometry, enabling reasoning and formal verification. Finally, we describe prototype implementations of the proposed model and language.

A Petri Net Interpretation of Open Reconfigurable Systems

We present a Petri net interpretation of the pi-graphs-a graphical variant of the picalculus where recursion and replication are replaced by iteration. The concise and syntax-driven translation can be used to reason in Petri net terms about open reconfigurable systems. We demonstrate that the pi-graphs and their translated high-level Petri nets agree at the semantic level. In consequence, existing results on pi-graphs naturally extend to the translated Petri nets, most notably a guarantee of finiteness by construction.

Parallel computing with the Pi-calculus

To tackle the multi-core programming challenge, we investigate the design and implementation of concurrency-oriented programming languages. Our approach mimics the evolution from lambda-calculi to functional programming languages, but with the pi-calculus as a starting point. To fill the gap between the abstract calculus and its implementations, we introduce the pi-threads: an intermediate language and its abstract machine.The stackless architecture of the abstract machine makes the underlying algorithms both simple and naturally concurrent. The scheduling, for instance, can be operated in a completely decentralized way. Another remarkable feature of the abstract machine is its garbage collector. We adopt a reference counting scheme that can be characterized formally using only two semantic rules. Moreover, it provides original solutions to the usual shortcomings of reference counting: the overhead caused by the maintenance of the reference counts - we only track global references - and the complex issue of collecting cyclic structures - reinterpreted as the (in our case, much simpler) problem of detecting partial terminations.

Entropic Uniform Sampling of Linear Extensions in Series-Parallel Posets

In this paper, we introduce a uniform random sampler for linear extensions of Series-Parallel posets. The algorithms we present ensure an essential property of random generation algorithms: entropy. They are in a sense optimal in their consumption of random bits.

A Decidable Characterization of a Graphical Pi-calculus with Iterators

This paper presents the Pi-graphs, a visual paradigm for the modelling and verification of mobile systems. The language is a graphical variant of the Pi-calculus with iterators to express non-terminating behaviors. The operational semantics of Pi-graphs use ground notions of labelled transition and bisimulation, which means standard verification techniques can be applied. We show that bisimilarity is decidable for the proposed semantics, a result obtained thanks to an original notion of causal clock as well as the automatic garbage collection of unused names.

Pattern Matching in Link Streams: A Token-Based Approach

Link streams model the dynamics of interactions in complex distributed systems as sequences of links (interactions) occurring at a given time. Detecting patterns in such sequences is crucial for many applications but it raises several challenges. In particular, there is no generic approach for the specification and detection of link stream patterns in a way similar to regular expressions and automata for text patterns. To address this, we propose a novel automata framework integrating both timed constraints and finite memory together with a recognition algorithm. The algorithm uses structures similar to tokens in high-level Petri nets and includes non-determinism and concurrency. We illustrate the use of our framework in real-world cases and evaluate its practical performances.

Pattern Matching in Link Streams: Timed-Automata with Finite Memory

Link streams model the dynamics of interactions in complex distributed systems as sequences of links (interactions) occurring at a given time. Detecting patterns in such sequences is crucial for many applications but it raises several challenges. In particular, there is no generic approach for the specification and detection of link stream patterns in a way similar to regular expressions and automata for text patterns. To address this, we propose a novel automata framework integrating both timed constraints and finite memory together with a recognition algorithm. The algorithm uses structures similar to tokens in high-level Petri nets and includes non-determinism and concurrency. We illustrate the use of our framework in real-world cases and evaluate its practical performances.