Mario Bravetti

CONCUR 2009 - Concurrency Theory

Invited Papers.- Perspectives on Transactional Memory.- The Effect of Tossing Coins in Omega-Automata.- Bigraphical Categories.- BlenX Static and Dynamic Semantics.- Contributed Papers.- Flow Policy Awareness for Distributed Mobile Code.- Volume and Entropy of Regular Timed Languages: Discretization Approach.- A Logical Interpretation of the ?-Calculus into the ?-Calculus, Preserving Spine Reduction and Types.- Encoding Asynchronous Interactions Using Open Petri Nets.- Distributed Asynchronous Automata.- Algebra for Infinite Forests with an Application to the Temporal Logic EF.- Deriving Syntax and Axioms for Quantitative Regular Behaviours.- Weighted Bisimulation in Linear Algebraic Form.- A Logic-Based Framework for Reasoning about Composite Data Structures.- Measuring Permissivity in Finite Games.- Contracts for Mobile Processes.- Power of Randomization in Automata on Infinite Strings.- Probabilistic Weighted Automata.- Partially-Commutative Context-Free Processes.- Testing Finitary Probabilistic Processes.- A Demonic Approach to Information in Probabilistic Systems.- HYPE: A Process Algebra for Compositional Flows and Emergent Behaviour.- Variable Binding, Symmetric Monoidal Closed Theories, and Bigraphs.- Partial Order Reduction for Probabilistic Systems: A Revision for Distributed Schedulers.- Model-Checking Games for Fixpoint Logics with Partial Order Models.- Reachability in Succinct and Parametric One-Counter Automata.- Winning Regions of Pushdown Parity Games: A Saturation Method.- Concurrent Kleene Algebra.- Concavely-Priced Probabilistic Timed Automata.- Compositional Control Synthesis for Partially Observable Systems.- Howes Method for Calculi with Passivation.- On the Relationship between ?-Calculus and Finite Place/Transition Petri Nets.- Modeling Radio Networks.- Time-Bounded Verification.- Secure Enforcement for Global Process Specifications.- On Convergence of Concurrent Systems under Regular Interactions.- Computing Stuttering Simulations.- Weak Time Petri Nets Strike Back!.- A General Testability Theory.- Counterexamples in Probabilistic LTL Model Checking for Markov Chains.- Query-Based Model Checking of Ad Hoc Network Protocols.- Strict Divergence for Probabilistic Timed Automata.

The theory of interactive generalized semi-Markov processes

In this paper we introduce the calculus of interactive generalized semi-Markov processes (IGSMPs), a stochastic process algebra which can express probabilistic timed delays with general distributions and synchronizable actions with zero duration, and where choices may be probabilistic, non-deterministic and prioritized. IGSMP is equipped with a structural operational semantics which generates semantic models in the form of generalized semi-Markov processes (GSMPs), i.e. probabilistic systems with generally distributed time, extended with action transitions representing interaction among system components. This is obtained by expressing the concurrent execution of delays through a variant of ST semantics which is based on dynamic names. The fact that names for delays are generated dynamically by the semantics makes it possible to define a notion of observational congruence for IGSMP (that abstracts from internal actions with zero duration) simply as a combination of standard observational congruence and probabilistic bisimulation. We also present a complete axiomatization for observational congruence over IGSMP. Finally, we show how to derive a GSMP from a given IGSMP specification in order to evaluate the system performance and we present a case study.

Contract based multi-party service composition

In the context of Service Oriented Computing, contracts are descriptions of the observable behaviour of services. Contracts have been already successfully exploited to solve the problem of client/service composition. In this paper we consider services where the choice to perform an output may not depend on the environment and we revisit the theory of contracts in order to tackle the problem of composition of multiple services (not only one client with one service). Moreover, we relate our theory of contracts with the theory of testing preorder (interpreted as a subcontract relation) and we show that a compliant group of contracts is still compliant if every contract is replaced by one of its subcontract.

Performance measure sensitive congruences for Markovian process algebras

The modeling and analysis experience with process algebras has shown the necessity of extending them with priority, probabilistic internal/external choice, and time while preserving compositionality. The purpose of this paper is to make a further step by introducing a way to express performance measures, in order to allow the modeler to capture the QoS metrics of interest. We show that the standard technique of expressing stationary and transient performance measures as weighted sums of state probabilities and transition frequencies can be imported in the process algebra framework. Technically speaking, if we denote by n ∈ N the number of performance measures of interest, in this paper we define a family of extended Markovian process algebras with generative master-reactive slaves synchronization mechanism called EMPAgrn including probabilities, priorities, exponentially distributed durations, and sequences of rewards of length n. Then we show that the Markovian bisimulation equivalence ∼MBn is a congruence for EMPAgrn which preserves the specified performance measures and we give a sound and complete axiomatization for finite EMPAgrn terms. Finally, we present a case study conducted with the software tool Two Towers in which we contrast the average performance of a selection of distributed algorithms for mutual exclusion modeled with EMPAgrn.

A process-algebraic approach for the analysis of probabilistic noninterference

We define several security properties for the analysis of probabilistic noninterference as a conservative extension of a classical, nondeterministic, process-algebraic approach to information flow theory. We show that probabilistic covert channels (that are not observable in the nondeterministic setting) may be revealed through our approach and that probabilistic information can be exploited to give an estimate of the amount of confidential information flowing to unauthorized users. Finally, we present a case study showing that the expressiveness of the calculus we adopt makes it possible to model and analyze real concurrent systems.

Towards Performance Evaluation with General Distributions in Process Algebras

We present a process algebra for the performance modeling and evaluation of concurrent systems whose activity durations are expressed through general probability distributions. We first determine the class of generalized semi-Markov processes (GSMPs) as being the class of stochastic processes on which we must rely for performance evaluation to be possible. Then we argue that in this context the right semantics for algebraic terms is a variant of the ST semantics which accounts for both functional and performance aspects. The GSMP based process algebra we propose is introduced together with its formal semantics, an example of performance evaluation, and a notion of probabilistic bisimulation based equivalence accounting for action durations which is shown to be a congruence.

Contract Compliance and Choreography Conformance in the Presence of Message Queues

Choreography conformance and contract compliance have been widely studied in the context of synchronous communication. In this paper we approach a more realistic scenario in which the messages containing the invocations are queued in the called service. More precisely, we study the foundational aspects of contract compliance in a language independent way by just taking contracts to be finite labeled transition systems. Then, we relate the proposed theory of contract compliance with choreography specifications a la WS-CDL where activities are interpreted as pairs of send and receive events. An interesting consequence of adopting a language independent representation of contracts is that choreography projection can be defined in structured operational semantics.

A theory for strong service compliance

We investigate, in a process algebraic setting, a new notion of compliance that we call strong service compliance: composed services are strong compliant if their composition is both deadlock and livelock free (this is the traditional notion of compliance) and whenever a message can be sent to invoke a service, this service is ensured to be ready to serve the invocation. We define also a new notion of refinement, called strong subcontract pre-order, suitable for strong compliance: given a composition of strong compliant services each one executing according to some specific contracts, we can replace the services with other services executing corresponding strong sub contracts preserving strong compliance. Finally, we present a characterization of the strong subcontract preorder resorting to the theory of (should) testing pre-order.

Tutte le Algebre Insieme: Concepts, Discussions and Relations of Stochastic Process Algebras with General Distributions

We report on the state of the art in the formal specification and analysis of concurrent systems whose activity duration depends on general probability distributions. First of all the basic notions and results introduced in the literature are explained and, on this basis, a conceptual classification of the different approaches is presented. We observe that most of the approaches agree on the fact that the specification of systems with general distributions has a three level structure: the process algebra level, the level of symbolic semantics and the level of concrete semantics. Based on such observations, a new very expressive model is introduced for representing timed systems with general distributions. We show that many of the approaches in the literature can be mapped into this model establishing therefore a formal framework to compare these approaches.

A theory of contracts for strong service compliance

We investigate, in a process algebraic setting, a new notion of correctness for service compositions, which we call strong service compliance: composed services are strong compliant if their composition is both deadlock and livelock free (this is the traditional notion of compliance), and whenever a message can be sent to invoke a service, it is guranteed to be ready to serve the invocation. We also define a new notion of refinement, called strong subcontract pre-order, suitable for strong compliance: given a composition of strong compliant services, we can replace any service with any other service in subcontract relation while preserving the overall strong compliance. Finally, we present a characterisation of the strong subcontract pre-order by resorting to the theory of a (should) testing pre-order.