Mila E. Majster-Cederbaum

An Approach to Modelling and Verification of Component Based Systems

We build on a framework for modelling and investigating component-based systems that strictly separates the description of behavior of components from the way they interact. We discuss various properties of system behavior as liveness, local progress, local and global deadlock, and robustness. We present a criterion that ensures liveness and can be tested in polynomial time.

Denotational semantics in the cpo and metric approach

Abstract We investigate the implications of choosing a cpo-framework resp. a complete metric space framework for defining denotational semantics of languages that allow for recursion/iteration, communication and concurrency. We first establish a general framework for the cpo and the metric approach. The existence and uniqueness of meaning functions is studied. In the metric case the existence and uniqueness of a meaning function can be established under some reasonable assumptions. In the cpo-case we obtain the existence of a least meaning function. From these theorems consistency results can be concluded. In the second part we study the impact of the choice between cpo and metric for semantics based on event-structures and for semantics based on pomset classes.

Trace Machines for Observing Continuous-Time Markov Chains

In this paper, we study several linear-time equivalences (Markovian trace equivalence, failure and ready trace equivalence) for continuous-time Markov chains that refer to the probabilities for timed execution paths. Our focus is on testing scenarios by means of push-button experiments with appropriate trace machines and a discussion of the connections between the equivalences. For Markovian trace equivalence, we provide alternative characterizations, including one that abstracts away from the time instances where actions are observed, but just reports on the average sojourn times in the states. This result is used for a reduction of the question whether two finite-state continuous-time Markov chains are Markovian trace equivalent to the probabilistic trace equivalence problem for discrete-time Markov chains (and the latter is known to be solvable in polynomial time).

A Polynomial-Time Checkable Sufficient Condition for Deadlock-Freedom of Component-Based Systems

Interaction systems are a formal model for component-based systems. Combining components via connectors to form more complex systems may give rise to deadlock situations. Deciding the existence of deadlocks is NP-hard as it involves global state analysis. We present here a parametrized polynomial-time algorithm that is able to confirm deadlock-freedom for a certain class of interaction systems. The discussion includes characteristic examples and displays the role of the parameter of the algorithm.

Towards a unified view of bisimulation: a comparative study

Abstract The realm of approaches to operational descriptions and equivalences for concurrent systems in the literature lead to a series of different attempts to give a uniform characterization of what should be considered a bisimulation, mostly in an algebraic and/or categorical framework. Meanwhile the realm of such approaches calls itself for comparison and/or unification. We investigate how different abstract characterizations of bisimulations are related. In particular, we consider the coalgebraic approach of Aczel and Mendler, the observation structures (Kripke structures) of Degano, De Nicola and Montanari, the algebraic approach of Malacaria, the domain theoretic view of Abramsky and the categorical setting of Joyal, Nielsen and Winskel. The framework of Aczel and Mendler turns out to be the most general one in the sense that the other approaches can be translated into it. These translations, where the relation between the categorical setting of Joyal, Nielsen and Winskel with the coalgebraic approach is the most complicated one, enhance the understanding of the different approaches and contribute to a unified view of bisimulation.

Action refinement for true concurrent real time

Action refinement is an essential operation in the design of concurrent systems, real-time or not. In this paper, we develop an action refinement technique in a real-time non-interleaving causality-based setting, a timed extension of bundle event structures that allows for urgent interactions to model time-out. A syntactic action refinement operation is presented in a timed process algebra based on the internationally standardised specification language LOTOS. We show: (1) that the behavior of the refined system can be inferred compositionally from the behavior of the original system; (2) from the behaviors of the processes substituted for actions with explicitly represented start-points, that the timed versions of a linear-time equivalence (pomset trace equivalence) and a branching-time equivalence (history-preserving bisimulation equivalence) are both congruences under our refinement; and (3) that the syntactic and semantic action refinements we developed coincide under these equivalence notions with respect to a metric and a CPO (complete partial order) based denotational semantics. Therefore, our refinement operations behave well. They also meet the commonly expected properties.

The connection between an event structure semantics and an operational semantics TCSP

The relation between an operational interleaving semantics forTSCP based on a transition system and a compositional true concurrency semantics based on event structures is studied. In particular we extend the consistency result of Goltz and Loogan [15] forTCSP processes without recursion to the general case. Thus we obtain for everyTCSP processP that its operational meaningO(P) and the interleaving behaviourO( M∥3P∥3) which is derived from the event structureM∥3P∥3 associated withP are bisimilar.

Ensuring properties of interaction systems

We propose results ensuring properties of a component-based system from properties of its interaction model and of its components.We consider here deadlock-freedom and local progress of subsystems. This is done in the framework of interaction systems, a model for component based modelling described in [9]. An interaction system is the superposition of two models: a behavior model and an interaction model. The behavior model describes the behavior of individual components. The interaction model describes the way the components may interact by introducing connectors that relate actions from different components. We illustrate our concepts and results with examples.

Towards action refinement for true concurrent real time

Abstract.Action refinement is an essential operation in the design of concurrent systems, real-time or not. In this paper we develop a theory of action refinement in a real-time non-interleaving causality based setting, a timed extension of bundle event structures that allows for urgent interactions to model timeout. The syntactic action refinement operation is presented in a timed process algebra as incorporated in the internationally standardised specification language LOTOS. We show that the behaviour of the refined system can be inferred compositionally from the behaviour of the original system and from the behaviour of the processes substituted for actions with explicitly represented start points, that the timed versions of a linear-time equivalence, termed pomset trace equivalence, and a branching-time equivalence, termed history preserving bisimulation equivalence, are both congruences under the refinement, and that the syntactic and semantic action refinements developed coincide under these equivalence relations with respect to a metric denotational semantics. Therefore, our refinement operations behave well. They meet the commonly expected properties.

Bundle event structures: a revised cpo approach

Bundle event structures equipped with a partial order ≤ have been used to give a true concurrency denotational semantics for LOTOS. This model has also been extended by time and stochastic information. Unfortunately it fails to yield a complete partial order (cpo) as we illustrate by an example.We propose a subset of all bundle event structures such that it forms a cpo. This subset is closed under the usual operators on bundle event structures. And as a consequence these operators are continuous. Therefore, this subset can be used to give a denotational semantics of LOTOS.