Hagen Völzer

Probabilistic event structures and domains

This paper studies how to adjoin probability to event structures, leading to the model of probabilistic event structures. In their simplest form, probabilistic choice is localised to cells, where conflict arises; in which case probabilistic independence coincides with causal independence. An event structure is associated with a domain--that of its configurations ordered by inclusion. In domain theory, probabilistic processes are denoted by continuous valuations on a domain. A key result of this paper is a representation theorem showing how continuous valuations on the domain of a confusion-free event structure correspond to the probabilistic event structures it supports. We explore how to extend probability to event structures which are not confusion-free via two notions of probabilistic runs of a general event structure. Finally, we show how probabilistic correlation and probabilistic event structures with confusion can arise from event structures which are originally confusion-free by using morphisms to rename and hide events.

Petri net based verification of distributed algorithms: An example

A technique to model and to verify distributed algorithms is suggested. This technique (based on Petri nets) reduces the modelling and analysis effort to a reasonable level. The paper outlines the technique using the example of a typical network algorithm, theecho algorithm.

Temporal Logics and Model Checking for Fairly Correct Systems

We motivate and study a generic relaxation of correctness of reactive and concurrent systems with respect to a temporal specification. We define a system to be fairly correct if there exists a fairness assumption under which it satisfies its specification. Equivalently, a system is fairly correct if the set of runs satisfying the specification is large from a topological point of view, i.e., it is a co-meager set. We compare topological largeness with its more popular sibling, probabilistic largeness, where a specification is probabilistically large if the set of runs satisfying the specification has probability 1. We show that topological and probabilistic largeness of omega-regular specifications coincide for bounded Borel measures on finite-state systems. As a corollary, we show that, for specifications expressed in LTL or by Buchi automata, checking that a finite-state system is fairly correct has the same complexity as checking that it is correct. Finally we study variants of the logics CTL and CTL*, where the for all runs quantifier is replaced by a for a large set of runs quantifier. We show that the model checking complexity for these variants is the same as for the original logics

Randomized Non-sequential Processes

A non-sequential, i.e. true concurrency, semantics for randomized distributed algorithms is suggested. It is based on Petri nets and their branching processes. We introduce randomized Petri nets and their semantics, probabilistic branching processes. As a main result, we show that each probabilistic branching process defines a unique canonical probability space. Finally, we show that the non-sequential semantics differs from the classical sequential semantics, modelling a new adversary, called the distributed adversary.

Flexibility in Algebraic Nets

Algebraic Petri nets as defined by Reisig [17] lack a feature for modelling distributed network algorithms, viz. flexible arcs. In this paper we equip algebraic Petri nets with flexible arcs and we call the resulting extension algebraic system nets. We demonstrate that algebraic system nets are better suited for modelling distributed algorithms.

A tool for subsystem configuration management

This paper describes a tool that manages a hierarchical, is a subsystem of-structure on a set of software development artefacts and that provides configuration management (CM) for subsystems by interacting with an existing CM tool. The tool is based on a recently proposed framework for subsystem-based configuration management. The tool demonstrates the feasibility of the framework and develops it further The design of the framework and the tool was developed in collaboration with Invensys SCADA Development and it is discussed in relation to their current software development process.

On conspiracies and hyperfairness in distributed computing

We study the phenomenon of conspiracies, a certain class of livelocks, in distributed computations. This elementary phenomenon occurs in systems with shared variables, shared actions as well as in message-passing systems. We propose a new and simple characterization via a new notion of hyperfairness, which postulates the absence of conspiracies. We argue that hyperfairness is a useful tool for understanding some impossibility results, in particular results involving crash-tolerance. As a main result, we show that a large subclass of hyperfairness can be implemented through partial synchrony and randomization.

Refinement-Robust Fairness

We motivate and study the robustness of fairness notions under refinement of transitions and places in Petri nets. We show that the classical notions of weak and strong fairness are not robust and we propose a hierarchy of increasingly strong, refinement-robust fairness notions. That hierarchy is based on the conflict structure of transitions, which characterizes the interplay between choice and synchronization in a fairness notion. Our fairness notions are defined on non-sequential runs, but we show that the most important notions can be easily expressed on sequential runs as well. The hierarchy is further motivated by a brief discussion on the computational power of the fairness notions.

SubCM: a tool for improved visibility of software change in an industrial setting

Software configuration management is the discipline of managing large collections of software development artefacts from which software products are built. Software configuration management tools typically deal with artefacts at fine levels of granularity - such as individual source code files - and assist with coordination of changes to such artefacts. This paper describes a lightweight tool, designed to be used on top of a traditional file-based configuration management system. The add-on tool support enables users to flexibly define new hierarchical views of product structure, independent of the underlying artefact-repository structure. The tool extracts configuration and change data with respect to the user-defined hierarchy, leading to improved visibility of how individual subsystems have changed. The approach yields a range of new capabilities for build managers, and verification and validation teams. The paper includes a description of our experience using the tool in an organization that builds large embedded software systems.

Verifying fault tolerance of distributed algorithms formally-an example

It is shown how fault-tolerant distributed algorithms can be formally verified using the Petri net based verification framework DAWN. A complete example study, the verification of a distributed self diagnosis algorithm, is presented to demonstrate the benefits of the proposed approach.It is shown how fault-tolerant distributed algorithms can be formally verified using the Petri net based verification framework DAWN. A complete example study, the verification of a distributed self diagnosis algorithm, is presented to demonstrate the benefits of the proposed approach.