GSTE is partitioned model checking

Verifying whether an ω-regular property is satisfied by a finite-state system is a core problem in model checking. Standard techniques build an automaton with the complementary language, compute its product with the system, and then check for emptiness. Generalized symbolic trajectory evaluation (GSTE) has been recently proposed as an alternative approach, extending the computationally efficient symbolic trajectory evaluation (STE) to general ω-regular properties. In this paper, we show that the GSTE algorithms are essentially a partitioned version of standard symbolic model-checking (SMC) algorithms, where the partitioning is driven by the property under verification. We export this technique of property-driven partitioning to SMC and show that it typically does speed up SMC algorithm

Automata-theoretic and bounded model checking for linear temporal logic

In this work we study methods for model checking the temporal logic LTL. The focus is on the automata-theoretic approach to model checking and bounded model checking. We begin by examining automata-theoretic methods to model check LTL safety properties. The model checking problem can be reduced to checking whether the language of a finite state automaton on finite words is empty. We describe an efficient algorithm for generating small finite state automata for so called non-pathological safety properties. The presented implementation is the first tool able to decide whether a formula is non-pathological. The experimental results show that treating safety properties can benefit model checking at very little cost. In addition, we find supporting evidence for the view that minimising the automaton representing the property does not always lead to a small product state space. A deterministic property automaton can result in a smaller product state space even though it might have a larger number states. Next we investigate modular analysis. Modular analysis is a state space reduction method for modular Petri nets. The method can be used to construct a reduced state space called the synchronisation graph. We devise an on-the-fly automata-theoretic method for model checking the behaviour of a modular Petri net from the synchronisation graph. The solution is based on reducing the model checking problem to an instance of verification with testers. We analyse the tester verification problem and present an efficient on-the-fly algorithm, the first complete solution to tester verification problem, based on generalised nested depth-first search. We have also studied propositional encodings for bounded model checking LTL. A new simple linear sized encoding is developed and experimentally evaluated. The implementation in the NuSMV2 model checker is competitive with previously presented encodings. We show how to generalise the LTL encoding to a more succint logic: LTL with past operators. The generalised encoding compares favourably with previous encodings for LTL with past operators. Links between bounded model checking and the automata-theoretic approach are also explored.reviewe

Proceedings of SUMo and CompoNet 2011

International audienc

Alternative Automata-based Approaches to Probabilistic Model Checking

In this thesis we focus on new methods for probabilistic model checking (PMC) with linear temporal logic (LTL). The standard approach translates an LTL formula into a deterministic ω-automaton with a double-exponential blow up. There are approaches for Markov chain analysis against LTL with exponential runtime, which motivates the search for non-deterministic automata with restricted forms of non-determinism that make them suitable for PMC. For MDPs, the approach via deterministic automata matches the double-exponential lower bound, but a practical application might benefit from approaches via non-deterministic automata. We first investigate good-for-games (GFG) automata. In GFG automata one can resolve the non-determinism for a finite prefix without knowing the infinite suffix and still obtain an accepting run for an accepted word. We explain that GFG automata are well-suited for MDP analysis on a theoretic level, but our experiments show that GFG automata cannot compete with deterministic automata. We have also researched another form of pseudo-determinism, namely unambiguity, where for every accepted word there is exactly one accepting run. We present a polynomial-time approach for PMC of Markov chains against specifications given by an unambiguous Büchi automaton (UBA). Its two key elements are the identification whether the induced probability is positive, and if so, the identification of a state set inducing probability 1. Additionally, we examine the new symbolic Muller acceptance described in the Hanoi Omega Automata Format, which we call Emerson-Lei acceptance. It is a positive Boolean formula over unconditional fairness constraints. We present a construction of small deterministic automata using Emerson-Lei acceptance. Deciding, whether an MDP has a positive maximal probability to satisfy an Emerson-Lei acceptance, is NP-complete. This fact has triggered a DPLL-based algorithm for deciding positiveness

Is there a best Büchi automaton for explicit model checking?

LTL to Büchi automata (BA) translators are traditionally optimized to produce automata with a small number of states or a small number of non-deterministic states. In this paper, we search for properties of Büchi automata that really influence the performance of explicit model checkers. We do that by manual analysis of several automata and by experiments with common LTL-to-BA translators and realistic verification tasks. As a result of these experiences, we gain a better insight into the characteristics of automata that work well with Spin.Překladače LTL na Büchiho automaty jsou obvykle optimalizovány tak, aby produkovaly automaty s co nejmenším počtem stavů, či s co nejmenším počtem nedeterministických stavů. V této publikaci hledáme vlastnosti Büchiho automatů, které skutečně ovlivňují výkon nástrojů pro explicitní metodu ověřování modelu (model checking). A to pomocí manuální analýzy několika automatů a experimenty s běžnými překladače LTL na automaty a realistickými verifikačními úlohami. Výsledkem těchto experimentů je lepší porozumění charakteristik automatů, které jsou dobré pro model checker Spin

Robust execution of workflows in a distributed environment

In many business applications, workflows are used to describe business processes. Employees and machines get instructions from a plan (the workflow) to be guided or controlled. The workflows make it easier to create and manage business processes. Therefore, using workflows is the standard procedure in the business area today. The distributed execution of workflows plays an important role as almost all nodes are connected to a network today. The importance even increases with the emerging of pervasive environments. Because these systems are prone to failures, it is important to develop reliability methods that ensure that the system works properly even if failures occur. When the robustness of a system in a distributed environment shall be increased, the service that has to be executed is usually replicated and executed by two or more nodes. This means that the exact same behavior is executed by multiple nodes and thereby increases the reliability of the system by being able to cope with node failures. Changing the order of the activities or using alternative activities to increase the robustness is promising because when each node receives a different workflow that achieves the same goal, the possibility of failures should be further reduced by decoupling the replicas in respect of time and hardware dependencies. We developed a robustness metric that evaluates the robustness of a set of workflow replicas. We also developed methods and algorithms that generate workflows with different orders and alternative tasks within reasonable time. Our evaluations show that our proposed methods work significantly better than deploying a brute-force method to achieve the same behavior

Compositional synthesis of reactive systems

Synthesis is the task of automatically deriving correct-by-construction implementations from formal specifications. While it is a promising path toward developing verified programs, it is infamous for being hard to solve. Compositionality is recognized as a key technique for reducing the complexity of synthesis. So far, compositional approaches require extensive manual effort. In this thesis, we introduce algorithms that automate these steps. In the first part, we develop compositional synthesis techniques for distributed systems. Providing assumptions on other processes' behavior is fundamental in this setting due to inter-process dependencies. We establish delay-dominance, a new requirement for implementations that allows for implicitly assuming that other processes will not maliciously violate the shared goal. Furthermore, we present an algorithm that computes explicit assumptions on process behavior to address more complex dependencies. In the second part, we transfer the concept of compositionality from distributed to single-process systems. We present a preprocessing technique for synthesis that identifies independently synthesizable system components. We extend this approach to an incremental synthesis algorithm, resulting in more fine-grained decompositions. Our experimental evaluation shows that our techniques automate the required manual efforts, resulting in fully automated compositional synthesis algorithms for both distributed and single-process systems.Synthese ist die Aufgabe korrekte Implementierungen aus formalen Spezifikation abzuleiten. Sie ist zwar ein vielversprechender Weg für die Entwicklung verifizierter Programme, aber auch dafür bekannt schwer zu lösen zu sein. Kompositionalität gilt als eine Schlüsseltechnik zur Verringerung der Komplexität der Synthese. Bislang erfordern kompositionale Ansätze einen hohen manuellen Aufwand. In dieser Dissertation stellen wir Algorithmen vor, die diese Schritte automatisieren. Im ersten Teil entwickeln wir kompositionale Synthesetechniken für verteilte Systeme. Aufgrund der Abhängigkeiten zwischen den Prozessen ist es in diesem Kontext von grundlegender Bedeutung, Annahmen über das Verhalten der anderen Prozesse zu treffen. Wir etablieren Delay-Dominance, eine neue Anforderung für Implementierungen, die es ermöglicht, implizit anzunehmen, dass andere Prozesse das gemeinsame Ziel nicht böswillig verletzen. Darüber hinaus stellen wir einen Algorithmus vor, der explizite Annahmen über das Verhalten anderer Prozesse ableitet, um komplexere Abhängigkeiten zu berücksichtigen. Im zweiten Teil übertragen wir das Konzept der Kompositionalität von verteilten auf Einzelprozesssysteme. Wir präsentieren eine Vorverarbeitungmethode für die Synthese, die unabhängig synthetisierbare Systemkomponenten identifiziert. Wir erweitern diesen Ansatz zu einem inkrementellen Synthesealgorithmus, der zu feineren Dekompositionen führt. Unsere experimentelle Auswertung zeigt, dass unsere Techniken den erforderlichen manuellen Aufwand automatisieren und so zu vollautomatischen Algorithmen für die kompositionale Synthese sowohl für verteilte als auch für Einzelprozesssysteme führen

Symbolic reactive synthesis

In this thesis, we develop symbolic algorithms for the synthesis of reactive systems. Synthesis, that is the task of deriving correct-by-construction implementations from formal specifications, has the potential to eliminate the need for the manual—and error-prone—programming task. The synthesis problem can be formulated as an infinite two-player game, where the system player has the objective to satisfy the specification against all possible actions of the environment player. The standard synthesis algorithms represent the underlying synthesis game explicitly and, thus, they scale poorly with respect to the size of the specification. We provide an algorithmic framework to solve the synthesis problem symbolically. In contrast to the standard approaches, we use a succinct representation of the synthesis game which leads to improved scalability in terms of the symbolically represented parameters. Our algorithm reduces the synthesis game to the satisfiability problem of quantified Boolean formulas (QBF) and dependency quantified Boolean formulas (DQBF). In the encodings, we use propositional quantification to succinctly represent different parts of the implementation, such as the state space and the transition function. We develop highly optimized satisfiability algorithms for QBF and DQBF. Based on a counterexample-guided abstraction refinement (CEGAR) loop, our algorithms avoid an exponential blow-up by using the structure of the underlying symbolic encodings. Further, we extend the solving algorithms to extract certificates in the form of Boolean functions, from which we construct implementations for the synthesis problem. Our empirical evaluation shows that our symbolic approach significantly outperforms previous explicit synthesis algorithms with respect to scalability and solution quality.In dieser Dissertation werden symbolische Algorithmen für die Synthese von reaktiven Systemen entwickelt. Synthese, d.h. die Aufgabe, aus formalen Spezifikationen korrekte Implementierungen abzuleiten, hat das Potenzial, die manuelle und fehleranfällige Programmierung überflüssig zu machen. Das Syntheseproblem kann als unendliches Zweispielerspiel verstanden werden, bei dem der Systemspieler das Ziel hat, die Spezifikation gegen alle möglichen Handlungen des Umgebungsspielers zu erfüllen. Die Standardsynthesealgorithmen stellen das zugrunde liegende Synthesespiel explizit dar und skalieren daher schlecht in Bezug auf die Größe der Spezifikation. Diese Arbeit präsentiert einen algorithmischen Ansatz, der das Syntheseproblem symbolisch löst. Im Gegensatz zu den Standardansätzen wird eine kompakte Darstellung des Synthesespiels verwendet, die zu einer verbesserten Skalierbarkeit der symbolisch dargestellten Parameter führt. Der Algorithmus reduziert das Synthesespiel auf das Erfüllbarkeitsproblem von quantifizierten booleschen Formeln (QBF) und abhängigkeitsquantifizierten booleschen Formeln (DQBF). In den Kodierungen verwenden wir propositionale Quantifizierung, um verschiedene Teile der Implementierung, wie den Zustandsraum und die Übergangsfunktion, kompakt darzustellen. Wir entwickeln hochoptimierte Erfüllbarkeitsalgorithmen für QBF und DQBF. Basierend auf einer gegenbeispielgeführten Abstraktionsverfeinerungsschleife (CEGAR) vermeiden diese Algorithmen ein exponentielles Blow-up, indem sie die Struktur der zugrunde liegenden symbolischen Kodierungen verwenden. Weiterhin werden die Lösungsalgorithmen um Zertifikate in Form von booleschen Funktionen erweitert, aus denen Implementierungen für das Syntheseproblem abgeleitet werden. Unsere empirische Auswertung zeigt, dass unser symbolischer Ansatz die bisherigen expliziten Synthesealgorithmen in Bezug auf Skalierbarkeit und Lösungsqualität deutlich übertrifft

Model Checking and Model-Based Testing : Improving Their Feasibility by Lazy Techniques, Parallelization, and Other Optimizations

This thesis focuses on the lightweight formal method of model-based testing for checking safety properties, and derives a new and more feasible approach. For liveness properties, dynamic testing is impossible, so feasibility is increased by specializing on an important class of properties, livelock freedom, and deriving a more feasible model checking algorithm for it. All mentioned improvements are substantiated by experiments