Papers by Nicolas Braud-Santoni
Fault-tolerant distributed algorithms play an increasingly important role in many applications, a... more Fault-tolerant distributed algorithms play an increasingly important role in many applications, and their correct and efficient implementation is notoriously difficult. We present an automatic approach to synthesise provably correct fault-tolerant distributed algorithms from formal specifications in linear-time temporal logic. The supported system model covers synchronous reactive systems with finite local state, while the failure model includes strong self-stabilisation as well as Byzantine failures. The synthesis approach for a fixed-size network of processes is complete for realisable specifications, and can optimise the solution for small implementations and short stabilisation time. To solve the bounded synthesis problem with Byzantine failures more efficiently, we design an incremental, CEGIS-like loop. Finally, we define two classes of problems for which our synthesis algorithm obtains solutions that are not only correct in fixed-size networks, but in networks of arbitrary size.
Project-Everest/Everest: Icfp 2017 Artifact Evaluation
Project-Everest/Everest: Icfp2017 Evaluation Artifact
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ArXiv, 2016
We introduce a novel generalization of Counterexample-Guided Inductive Synthesis (CEGIS) and inst... more We introduce a novel generalization of Counterexample-Guided Inductive Synthesis (CEGIS) and instantiate it to yield a novel, competitive algorithm for solving Quantified Boolean Formulas (QBF). Current QBF solvers based on counterexample-guided expansion use a recursive approach which scales poorly with the number of quantifier alternations. Our generalization of CEGIS removes the need for this recursive approach, and we instantiate it to yield a simple and efficient algorithm for QBF solving. Lastly, this research is supported by a competitive, though straightforward, implementation of the algorithm, making it possible to study the practical impact of our algorithm design decisions, along with various optimizations.
Formal Methods in System Design, 2021
In recent years, expansion-based techniques have been shown to be very powerful in theory and pra... more In recent years, expansion-based techniques have been shown to be very powerful in theory and practice for solving quantified Boolean formulas (QBF), the extension of propositional formulas with existential and universal quantifiers over Boolean variables. Such approaches partially expand one type of variable (either existential or universal) for obtaining a propositional abstraction of the QBF. If this formula is false, the truth value of the QBF is decided, otherwise further refinement steps are necessary. Classically, expansion-based solvers process the given formula quantifier-block wise and use one SAT solver per quantifier block. In this paper, we present a novel algorithm for expansion-based QBF solving that deals with the whole quantifier prefix at once. Hence recursive applications of the expansion principle are avoided and only two incremental SAT solvers are required. While our algorithm is naturally based on the $$\forall $$ ∀ Exp+Res calculus that is the formal foundati...
2018 Formal Methods in Computer Aided Design (FMCAD), 2018
In recent years, expansion-based techniques have been shown to be very powerful in theory and pra... more In recent years, expansion-based techniques have been shown to be very powerful in theory and practice for solving quantified Boolean formulas (QBF), the extension of propositional formulas with existential and universal quantifiers over Boolean variables. Such approaches partially expand one type of variable (either existential or universal) and pass the obtained formula to a SAT solver for deciding the QBF. State-of-the-art expansionbased solvers process the given formula quantifier-block wise and recursively apply expansion until a solution is found. In this paper, we present a novel algorithm for expansionbased QBF solving that deals with the whole quantifier prefix at once. Hence recursive applications of the expansion principle are avoided. Experiments indicate that the performance of our simple approach is comparable with the state of the art of QBF solving, especially in combination with other solving techniques.
Lecture Notes in Computer Science, 2016
Fault-tolerant distributed algorithms play an increasingly important role in many applications, a... more Fault-tolerant distributed algorithms play an increasingly important role in many applications, and their correct and efficient implementation is notoriously difficult. We present an automatic approach to synthesise provably correct fault-tolerant distributed algorithms from formal specifications in linear-time temporal logic. The supported system model covers synchronous reactive systems with finite local state, while the failure model includes strong self-stabilisation as well as Byzantine failures. The synthesis approach for a fixed-size network of processes is complete for realisable specifications, and can optimise the solution for small implementations and short stabilisation time. To solve the bounded synthesis problem with Byzantine failures more efficiently, we design an incremental, CEGIS-like loop. Finally, we define two classes of problems for which our synthesis algorithm obtains solutions that are not only correct in fixed-size networks, but in networks of arbitrary size.
International Journal of Networking and Computing, 2016
We consider highly dynamic distributed systems modelled by time-varying graphs (TVGs). We first a... more We consider highly dynamic distributed systems modelled by time-varying graphs (TVGs). We first address proof of impossibility results that often use informal arguments about convergence. We provide a general framework that formally proves the convergence of the sequence of executions of any deterministic algorithm over TVGs of any convergent sequence of TVGs. Next, we focus of the weakest class of long-lived TVGs, i.e., the class of TVGs where any node can communicate any other node infinitely often. We illustrate the relevance of our result by showing that no deterministic algorithm is able to compute various distributed covering structure on any TVG of this class. Namely, our impossibility results focus on the eventual footprint, the minimal dominating set and the maximal matching problems. 1 This paper is an extended version of [3] and includes materials from [7]. The title is a tribute to a series of papers including "The Next 700 BFT Protocols" (ACM TOCS, 2015) and "The Next 700 Programming Languages" (CACM, 1966) due to the common point between them: the genericity of proposed results. 2 This work was performed within the Labex SMART, supported by French state funds managed by the ANR within the "Investissements d'Avenir" programme under reference ANR-11-LABX-65.
2015 IEEE International Parallel and Distributed Processing Symposium Workshop, 2015
We address highly dynamic distributed systems modelled by time-varying graphs (TVGs). We are inte... more We address highly dynamic distributed systems modelled by time-varying graphs (TVGs). We are interested in proof of impossibility results that often use informal arguments about convergence. First, we provide a topological distance metric over sets of TVGs to correctly define the convergence of TVG sequences in such sets. Next, we provide a general framework that formally proves the convergence of the sequence of executions of any deterministic algorithm over TVGs of any convergent sequence of TVGs. Finally, we illustrate the relevance of the above result by proving that no deterministic algorithm exists to compute the underlying graph of any connected-overtime TVG, i.e., any TVG of the weakest class of long-lived TVGs.
Proceedings of the 2013 ACM symposium on Principles of distributed computing - PODC '13, 2013
is paper presents the first probabilistic Byzantine Agreement algorithm whose communication and ... more is paper presents the first probabilistic Byzantine Agreement algorithm whose communication and time complexities are poly-logarithmic. So far, the most effective probabilistic Byzantine Agreement algorithm had communication complexityÕ n and time complexityÕ (1). Our algorithm is based on a novel, unbalanced, almost everywhere to everywhere Agreement protocol which is interesting in its own right.
ACM SIGACT News, 2013
The 32nd 1 Symposium on the Principles of Distributed Computing (PODC 2013) was held on July 22-2... more The 32nd 1 Symposium on the Principles of Distributed Computing (PODC 2013) was held on July 22-24, in Montréal, Canada. PODC was co-located with the 25th 2 Symposium on Parallelism in Algorithms and Architectures (SPAA'2013), at the Mont Royal conference center. Moreover, two workshop were hosted on July 21 by Concordia University: • the PODC Social Network Workshop; and • WRAWN'13, the fourth Workshop on Realistic models for Algorithms in Wireless Networks.
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Papers by Nicolas Braud-Santoni