746 research outputs found
Parameterized synthesis of self-stabilizing protocols in symmetric networks
Self-stabilization in distributed systems is a technique to guarantee convergence to a set of legitimate states without external intervention when a transient fault or bad initialization occurs. Recently, there has been a surge of efforts in designing techniques for automated synthesis of self-stabilizing algorithms that are correct by construction. Most of these techniques, however, are not parameterized, meaning that they can only synthesize a solution for a fixed and predetermined number of processes. In this paper, we report a breakthrough in parameterized synthesis of self-stabilizing algorithms in symmetric networks, including ring, line, mesh, and torus. First, we develop cutoffs that guarantee (1) closure in legitimate states, and (2) deadlock-freedom outside the legitimate states. We also develop a sufficient condition for convergence in self-stabilizing systems. Since some of our cutoffs grow with the size of the local state space of processes, scalability of the synthesis procedure is still a problem. We address this problem by introducing a novel SMT-based technique for counterexample-guided synthesis of self-stabilizing algorithms in symmetric networks. We have fully implemented our technique and successfully synthesized solutions to maximal matching, three coloring, and maximal independent set problems for ring and line topologies
Automated Synthesis of Distributed Self-Stabilizing Protocols
In this paper, we introduce an SMT-based method that automatically
synthesizes a distributed self-stabilizing protocol from a given high-level
specification and network topology. Unlike existing approaches, where synthesis
algorithms require the explicit description of the set of legitimate states,
our technique only needs the temporal behavior of the protocol. We extend our
approach to synthesize ideal-stabilizing protocols, where every state is
legitimate. We also extend our technique to synthesize monotonic-stabilizing
protocols, where during recovery, each process can execute an most once one
action. Our proposed methods are fully implemented and we report successful
synthesis of well-known protocols such as Dijkstra's token ring, a
self-stabilizing version of Raymond's mutual exclusion algorithm,
ideal-stabilizing leader election and local mutual exclusion, as well as
monotonic-stabilizing maximal independent set and distributed Grundy coloring
Parameterized Synthesis of Self-Stabilizing Protocols in Symmetric Rings
Self-stabilization in distributed systems is a technique to guarantee convergence to a set of legitimate states without external intervention when a transient fault or bad initialization occurs. Recently, there has been a surge of efforts in designing techniques for automated synthesis of self-stabilizing algorithms that are correct by construction. Most of these techniques, however, are not parameterized, meaning that they can only synthesize a solution for a fixed and predetermined number of processes. In this paper, we report a breakthrough in parameterized synthesis of self-stabilizing algorithms in symmetric rings. First, we develop tight cutoffs that guarantee (1) closure in legitimate states, and (2) deadlock-freedom outside the legitimates states. We also develop a sufficient condition for convergence in silent self-stabilizing systems. Since some of our cutoffs grow with the size of local state space of processes, we also present an automated technique that significantly increases the scalability of synthesis in symmetric networks. Our technique is based on SMT-solving and incorporates a loop of synthesis and verification guided by counterexamples. We have fully implemented our technique and successfully synthesized solutions to maximal matching, three coloring, and maximal independent set problems
Verification and Synthesis of Symmetric Uni-Rings for Leads-To Properties
This paper investigates the verification and synthesis of parameterized
protocols that satisfy leadsto properties on symmetric
unidirectional rings (a.k.a. uni-rings) of deterministic and constant-space
processes under no fairness and interleaving semantics, where and are
global state predicates. First, we show that verifying for
parameterized protocols on symmetric uni-rings is undecidable, even for
deterministic and constant-space processes, and conjunctive state predicates.
Then, we show that surprisingly synthesizing symmetric uni-ring protocols that
satisfy is actually decidable. We identify necessary and
sufficient conditions for the decidability of synthesis based on which we
devise a sound and complete polynomial-time algorithm that takes the predicates
and , and automatically generates a parameterized protocol that
satisfies for unbounded (but finite) ring sizes. Moreover, we
present some decidability results for cases where leadsto is required from
multiple distinct predicates to different predicates. To demonstrate
the practicality of our synthesis method, we synthesize some parameterized
protocols, including agreement and parity protocols
Synthesis of a simple self-stabilizing system
With the increasing importance of distributed systems as a computing
paradigm, a systematic approach to their design is needed. Although the area of
formal verification has made enormous advances towards this goal, the resulting
functionalities are limited to detecting problems in a particular design. By
means of a classical example, we illustrate a simple template-based approach to
computer-aided design of distributed systems based on leveraging the well-known
technique of bounded model checking to the synthesis setting.Comment: In Proceedings SYNT 2014, arXiv:1407.493
Automated Synthesis of Timed and Distributed Fault-Tolerant Systems
This dissertation concentrates on the problem of automated synthesis and repair of fault-tolerant systems. In particular, given the required specification of the system, our goal is to synthesize a fault-tolerant system, or repair an existing one. We study this problem for two classes of timed and distributed systems.
In the context of timed systems, we focus on efficient synthesis of fault-tolerant timed models from their fault-intolerant version. Although the complexity of the synthesis problem is known to be polynomial time in the size of the time-abstract bisimulation of the input model, the state of the art lacked synthesis
algorithms that can be efficiently implemented. This is in part due to the fact that synthesis is in general a
challenging problem and its complexity is significantly magnified in the context of timed systems. We
propose an algorithm that takes a timed automaton, a set of fault actions, and a set of safety and bounded-time response properties as input, and utilizes a space-efficient symbolic representation of the timed
automaton (called the zone graph) to synthesize a fault-tolerant timed automaton as output. The output
automaton satisfies strict phased recovery, where it is guaranteed that the output model behaves similarly
to the input model in the absence of faults and in the presence of faults, fault recovery is achieved in two
phases, each satisfying certain safety and timing constraints.
In the context of distributed systems, we study the problem of synthesizing fault-tolerant systems from their
intolerant versions, when the number of processes is unknown. To synthesize a distributed fault-tolerant
protocol that works for systems with any number of processes, we use counter abstraction. Using this
abstraction, we deal with a finite-state abstract model to do the synthesis. Applying our proposed algorithm,
we successfully synthesized a fault-tolerant distributed agreement protocol in the presence of Byzantine fault. Although the synthesis problem is known to be NP-complete in the state space of the input
protocol (due to partial observability of processes) in the non-parameterized setting, our parameterized
algorithm manages to synthesize a solution for a complex problem such as Byzantine agreement within less than two minutes.
A system may reach a bad state due to wrong initialization or fault occurrence. One of the well-known
types of distributed fault-tolerant systems are self-stabilizing systems. These are the systems that converge
to their legitimate states starting from any state, and if no fault occurs, stay in legitimate states thereafter.
We propose an automated sound and complete method to synthesize self-stabilizing systems starting from
the desired topology and type of the system. Our proposed method is based on SMT-solving, where the
desired specification of the system is formulated as SMT constraints. We used the Alloy solver to
implement our method, and successfully synthesized some of the well-known self-stabilizing algorithms.
We extend our method to support a type of stabilizing algorithm called ideal-stabilization, and also the case
when the set of legitimate states is not explicitly known.
Quantitative metrics such as recovery time are crucial in self-stabilizing systems when used in practice
(such as in networking applications). One of these metrics is the average recovery time. Our automated
method for synthesizing self-stabilizing systems generate some solution that respects the desired system
specification, but it does not take into account any quantitative metrics. We study the problem of repairing
self-stabilizing systems (where only removal of transitions is allowed) to satisfy quantitative limitations.
The metric under study is average recovery time, which characterizes the performance of stabilizing
programs. We show that the repair problem is NP-complete in the state space of the given system
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