A Perspective on "CCS Expressions, Finite State Processes, and Three Problems of Equivalence"

With the publication of the Kannellakis-Smolka 1983 PODC paper, Kanellakis and Smolka pioneered the development of efficient algorithms for deciding behavioral equivalence of concurrent and distributed processes, especially bisimulation equivalence. Bisimulation is the cornerstone of the process-algebraic approach to modeling and verifying concurrent and distributed systems. They also presented complexity results that showed certain behavioral equivalences are computationally intractable. Collectively, their results founded the subdiscipline of algorithmic process theory, and established the associated bridges between the European research community, whose focus at the time was on process theory, and that of the US, with a rich tradition in algorithm design and computational complexity, but to whom process theory was largely unknown.

An Abstraction Technique for Verifying Shared-Memory Concurrency

Modern concurrent and distributed software is highly complex. Techniques to reason about the correct behaviour of such software are essential to ensure its reliability. To be able to reason about realistic programs, these techniques must be modular and compositional as well as practical by being supported by automated tools. However, many existing approaches for concurrency verification are theoretical and focus primarily on expressivity and generality. This paper contributes a technique for verifying behavioural properties of concurrent and distributed programs that balances expressivity and usability. The key idea of the approach is that program behaviour is abstractly modelled using process algebra, and analysed separately. The main difficulty is presented by the typical abstraction gap between program implementations and their models. Our approach bridges this gap by providing a deductive technique for formally linking programs with their process-algebraic models. Our verification technique is modular and compositional, is proven sound with Coq, and has been implemented in the automated concurrency verifier VerCors. Moreover, our technique is demonstrated on multiple case studies, including the verification of a leader election protocol.

Exploring Type-Level Bisimilarity towards More Expressive Multiparty Session Types

A key open problem with multiparty session types (MPST) concerns their expressiveness: current MPST have inflexible choice, no existential quantification over participants, and limited parallel composition. This precludes many real protocols to be represented by MPST. To overcome these bottlenecks of MPST, we explore a new technique using weak bisimilarity between global types and endpoint types, which guarantees deadlock-freedom and absence of protocol violations. Based on a process algebraic framework, we present well-formed conditions for global types that guarantee weak bisimilarity between a global type and its endpoint types and prove their check is decidable. Our main practical result, obtained through benchmarks, is that our well-formedness conditions can be checked orders of magnitude faster than directly checking weak bisimilarity using a state-of-the-art model checker.

Integrated Information in Process-Algebraic Compositions

Integrated Information Theory (IIT) is most typically applied to Boolean Nets, a state transition model in which system parts cooperate by sharing state variables. By contrast, in Process Algebra, whose semantics can also be formulated in terms of (labeled) state transitions, system parts—“processes”—cooperate by sharing transitions with matching labels, according to interaction patterns expressed by suitable composition operators. Despite this substantial difference, questioning how much additional information is provided by the integration of the interacting partners above and beyond the sum of their independent contributions appears perfectly legitimate with both types of cooperation. In fact, we collect statistical data about ϕ —integrated information—relative to pairs of boolean nets that cooperate by three alternative mechanisms: shared variables—the standard choice for boolean nets—and two forms of shared transition, inspired by two process algebras. We name these mechanisms α , β and γ . Quantitative characterizations of all of them are obtained by considering three alternative execution modes, namely synchronous, asynchronous and “hybrid”, by exploring the full range of possible coupling degrees in all three cases, and by considering two possible definitions of ϕ based on two alternative notions of distribution distance.

Enhancing Formal Methods Integration with ACP2Petri

The paper deals with the ACP2Petri tool, providing a transformation of process algebraic specification to equivalent Petri net-based specification. Long-term practical experiences with the tool revealed some suggestions for its update and extension. Shortcomings and limitations found are described and proposed solutions provided within the paper. Implemented extensions, simplifying the usage of the tool and providing more options for analysis of particular transformation, are also presented. One of the most evident extensions of the tool is its graphical user interface, which allows for convenient management and detailed control over the process of transformation.