Eventually Strong Failure Detector with Unknown Membership

The distributed computing scenario is rapidly evolving for integrating self-organizing and dynamic wireless networks. Unreliable failure detectors are classical mechanisms which provide information about process failures and can help systems to cope with the high dynamism of these networks. A number of failure detection algorithms has been proposed so far; nonetheless, most of them assume a global knowledge about the membership as well as a fully communication connectivity; additionally, they are timer-based, requiring that eventually some bound on the message transmission will hold. These assumptions are no longer appropriate to the new scenario. This paper presents a new failure detector protocol which implements a new class of detectors, namely \(\diamondsuit S^{\cal M}\), which adapts the properties of the \(\diamondsuit S\) class to a dynamic network with an unknown membership. It has the interesting feature to be time-free, so that it does not rely on timers to detect failures; moreover, it tolerates mobility of nodes and message losses.KeywordsUnreliable failure detectordynamic distributed systemswireless mobile networksasynchronous systems

A necessary and sufficient condition for transforming limited accuracy failure detectors

This paper is about the weakest failure detector to solve the Election problem in asynchronous distributed systems. We first discuss the relationship between the Election problem and the Consensus problem in asynchronous distributed systems with unreliable failure detectors. Chandra and Toueg have stated that Consensus is solvable in asynchronous systems with unreliable failure detectors. But, in contrast to the Consensus problem, the Election problem is impossible to solve with unreliable failure detectors even with a single crash failure. More precisely, the weakest failure detector that is needed to solve this problem is a Perfect Failure Detector, which is strictly stronger than the weakest failure detector that is needed to solve Consensus.

With the rapid growth of wireless communication technologies, the demands for large-scale wireless networks have been emerged. One promising solution is the hierarchical architecture that consists of global and local wireless networks. The service discovery is a key issue to meet a vast amount of service demands from mobile nodes. However, under dynamic wireless networks, node failures severely affect the performance of service discovery protocols. Therefore, in this paper, we propose the directory hopping (DH) protocol for efficient service discoveries in dynamic large-scale networks. Basically, DH utilizes the directory for low-overhead service discoveries and periodically moves the directory to other operating nodes. In addition, it provides a fast and efficient recovery mechanism for node failures in dynamic network environments. Through extensive simulations, we show that the proposed DH protocol outperforms existing service discovery schemes.

Model checking is a systematic way of checking the absence of errors in a distributed system, i.e., assessing the functional requirements in a distributed system. However, there are certain challenges in this field, e.g., developing true abstract models and on their basis generalizing/guranteeing results, limited capacity of model checking tools and computational resources, identification of all requirements and their accurate specifications, etc. To understand and face such challenges, it is necessary to apply the prominent model checking techniques to different distributed systems designed for different communication models. In this thesis this challenge is accepted and resultantly encountered issues are discussed/addressed. The results reported are sufficient for advocating the need for applying model checking techniques as debugging. Therefore, we report bugs and the propose fixes but for ambiguous algorithms, we reconstruct them. We model check both fixed and reconstructed algorithms. We assess the following protocols: • Accelerated heartbeat protocols, • Consensus protocols in asynchronous distributed systems, • Group membership protocols and • Efficient algorithms to implement failure detectors in partially synchronous systems. We found that the accelerated heartbeat protocols proposed in [M.G. Gouda and T.M. McGuire, Accelerated Heartbeat Protocols, Proc. Of ICDCS’98], violated some natural and essential properties. We proved the results by giving counterexamples and developed the techniques to address the time-triggered events in mCRL2 and investigated the correct time bounds for all the protocols. Regarding consensus problem, we proved the correctness of the proposed algorithms where the failure detectors are unreliable (i.e., failure detectors may make mistakes). These algorithms are proposed in [T. Deepak Chandra and S. Toueg, Unreliable Failure Detectors for Reliable Distributed Systems, J. ACM, 1996 ]. For the group membership protocols proposed in [Y. Amir, D. Dolev, S. Kramer and D. Malki, Membership Algorithms for Multicast Communication Groups, Springer-Verlag, 1992], we found that the original specifications and the text explaining the protocols can be interpreted in different ways and even some natural interpretations contradict each other. Our formalization with respect to different interpretations showed the violation of claimed properties. So to resolve the ambiguities, we reconstructed the protocols and model-checked them. For analyzing the algorithms proposed in [M. Larrea, S. Arevalo and A.Fernndez, Efficient Algorithms to Implement Unreliable Failure Detectors in Partially Synchronous Systems, Proc. of DISC’99 ], we applied symmetry reduction techniques. We found that every algorithm encounters a deadlock if there is a bounded (yet arbitrarily large) buffer in the communication channel between a pair of nodes. We propose fixes for deadlock avoidance and model check the proposed algorithm in UPPAAL, FDR2 and MCRL2. We also present a comparison of these three tools for model checking one of the given four protocols.

This paper presents a new unreliable failure detector, called the Impact failure detector (FD) that, contrarily to the majority of traditional FDs, outputs a trust level value which expresses the degree of confidence in the system. An impact factor is assigned to each process and the trust level is equal to the sum of the impact factors of the processes not suspected of failure. Moreover, a threshold parameter defines a lower bound value for the trust level, over which the confidence in the system is ensured. In particular, we defined a f l exi bi l i t y property that denotes the capacity of the Impact FD to tolerate a certain margin of failures or false suspicions, i.e., its capacity of considering different sets of responses that lead the system to trusted states. The Impact FD is suitable for systems that present node redundancy, heterogeneity of nodes, clustering feature, and allow a margin of failures which does not degrade the confidence in the system. The paper also includes a timer-based distributed algorithm which implements an Impact FD, as well as its proof of correctness, for systems whose links are lossy asynchronous or for those whose all (or some) links are eventually timely. Performance evaluation results, based on PlanetLab [1] traces, confirm the degree of flexible applicability of our failure detector and that, due to the accepted margin of failure, both failures and false suspicions are more tolerated when compared to traditional unreliable failure detectors.

This paper defines the Non-Blocking Atomic Commitment problem in a message-passing asynchronous system and determines a failure detector to solve the problem. This failure detector, which we call the modal failure detector star, and which we denote by M*, is strictly weaker than the perfect failure detector P but strictly stronger than the eventually perfect failure detector ◇P. The paper shows that at any environment, the problem is solvable with M*.

We study the quality of service (QoS) of failure detectors. By QoS, we mean a specification that quantifies: (1) how fast the failure detector detects actual failures and (2) how well it avoids false detections. We first propose a set of QoS metrics to specify failure detectors for systems with probabilistic behaviors, i.e., for systems where message delays and message losses follow some probability distributions. We then give a new failure detector algorithm and analyze its QoS in terms of the proposed metrics. We show that, among a large class of failure detectors, the new algorithm is optimal with respect to some of these QoS metrics. Given a set of failure detector QoS requirements, we show how to compute the parameters of our algorithm so that it satisfies these requirements and we show how this can be done even if the probabilistic behavior of the system is not known. We then present some simulation results that show that the new failure detector algorithm provides a better QoS than an algorithm that is commonly used in practice. Finally, we suggest some ways to make our failure detector adaptive to changes in the probabilistic behavior of the network.

While lots of consensus algorithms have been proposed for crash-prone asynchronous message-passing systems enriched with a failure detector of the class Ω (the class of eventual leader failure detectors), very few algorithms have been proposed for systems enriched with a failure detector of the class P (the class of perfect failure detectors). Moreover, (to the best of our knowledge) the early decision and stopping notion has not been investigated in such systems. This paper presents an early-deciding/stopping P-based consensus algorithm. A process that does not crash decides (and stops) in at most min(f+2, t+1) rounds, where t is the maximum number of processes that may crash, and f the actual number of crashes (0≤ f≤ t). Differently from what occurs in a synchronous system, a perfect failure detector notifies failures asynchronously. This makes the design of an early deciding (and stopping) algorithm not trivial. Interestingly enough, the proposed algorithm meets the lower bound on the number of rounds for early decision in synchronous systems. In that sense, it is optimal. The paper then presents an original algorithm that implements a perfect failure detector in the Theta model, an interesting model that achieves some form of synchrony without relying on physical clocks. Hence, the stacking of these algorithms provides an algorithm that solves consensus in the Theta model in min(f+2, t+1) communication rounds, i.e., in two rounds when there are no failures, which is clearly optimal.

The knowledge about failures needed to solve distributed agreement problems can be expressed in terms of completeness and accuracy properties of failure detectors introduced by Chandra and Toueg. The accuracy properties they have considered restrict the false suspicions that can be made by all the processes in the system. In this paper, we define “Γ-accurate” failure detectors, whose accuracy properties (only) restrict the false suspicions that can be made by a subset Γ of the processes. We discuss the relations between the classes of Γ-accurate failure detectors, and the classes of failure detectors defined by Chandra and Toueg. Then we point out the impact of these relations on the solvability of agreement problems.

Unreliable failure detectors are abstract devices that, when added to asynchronous distributed systems, allow to solve distributed computing problems (e.g., Consensus) that otherwise would be impossible to solve in these systems. This paper focuses on two classes of failure detectors defined by Chandra and Toueg, namely, the classes denoted \(\Diamond {\cal P}\) (eventually perfect) and \(\Diamond {\cal S}\) (eventually strong). Both classes include failure detectors that eventually detect permanently all process crashes, but while the failure detectors of \(\Diamond {\cal P}\) eventually make no erroneous suspicions, the failure detectors of \(\Diamond {\cal S}\) are only required to eventually not suspect a single correct process.In such a context, this paper addresses the following question related to the comparative power of these classes, namely: “Are there one-shot agreement problems that can be solved in asynchronous distributed systems with reliable links but prone to process crash failures augmented with \(\Diamond {\cal P}\), but cannot be solved when those systems are augmented with \(\Diamond {\cal S}\)?” Surprisingly, the paper shows that the answer to this question is “no”. An important consequence of this result is that \(\Diamond {\cal P}\) cannot be the weakest class of failure detectors that enables solving one-shot agreement problems in unreliable asynchronous distributed systems. These results are then extended to the case of more severe failure modes.KeywordsAgreement ProblemAsynchronous Distributed SystemConsensusComputational PowerInput VectorOne-Shot ProblemProcess CrashUnreliable Failure Detector \(\Diamond {\cal P}\) \(\Diamond {\cal S}\) Veto Number

Failure detectors in homonymous distributed systems (with an application to consensus)

We study the time complexity of single and multi token broadcast in adversarial dynamic radio networks. Initially, k tokens (which are k pieces of information) are distributed among the n nodes of a network and all the tokens need to be disseminated to all the nodes in the network. We first consider the single-token broadcast problem (i.e., the case k=1). By presenting upper and lower bounds, we show that the time complexity of single-token broadcast depends on the amount of stability and connectivity of the dynamic network topology and on the adaptiveness of the adversary providing the dynamic topology. Then, we give two generic algorithms which allow to transform generalized forms of single-token broadcast algorithms into multi-token broadcast (k-token broadcast) algorithms. Based on these generic algorithms, we obtain k-token broadcast algorithms for a number of different dynamic network settings. For one of the modeling assumptions, our algorithm is complemented by a lower bound which shows that the upper bound is close to optimal.

Due to the multiplicity of loci of control, a main issue distributed systems have to cope with lies in the uncertainty on the system state created by the adversaries that are asynchrony, failures, dynamicity, mobility, etc. Considering message-passing systems, this paper considers the uncertainty created by the net effect of asynchrony and process crash failures in systems where the processes are anonymous (i.e., processes have no identity and locally execute the same algorithm). Trivially, agreement problems such as consensus, that cannot be solved in non-anonymous asynchronous systems prone to process failures, cannot be solved either if the system is anonymous. The paper investigates failure detectors that allow processes to circumvent this impossibility. It has several contributions. It first presents four failure detectors (denoted AP, $${\overline{AP}}$$ , AΩ, and AΣ) and show that they are the “identity-free” counterparts of perfect failure detectors, eventual leader failure detectors, and quorum failure detectors, respectively. AΣ is new and showing that AΣ and Σ have the same computability power in a non-anonymous system is not trivial. The paper also shows that the notion of failure detector reduction is related to the computation model. Then, the paper presents and proves correct a uniform anonymous consensus algorithm based on the failure detector pair (AΩ, AΣ) (“uniform” means here that not only processes have no identity, but no process is aware of the total number of processes). This new algorithm is not a simple “straightforward extension” of an algorithm designed for non-anonymous systems. To benefit from AΣ, it uses a novel broadcast facility which encapsulates an AΣ-based message exchange pattern that provides the processes with an interesting intersection property on the set of messages they have exchanged. Finally, the paper discusses the notions of failure detector hierarchy, weakest failure detector for anonymous consensus, and the implementation of identity-free failure detectors in anonymous systems.

In a transactional system, an atomic commitment protocol ensures that for any transaction, all data manager processes agree on the same outcome (commit or abort). A non-blocking atomic commitment protocol enables an outcome to be decided at every correct process despite the failure of others. In this paper we apply, for the first time, the fundamental result of T. Chandra and S. Toueg (1991) on solving the abstract consensus problem, to non-blocking atomic commitment. More precisely, we present a non-blocking atomic commitment protocol in an asynchronous system augmented with an unreliable failure detector that can make an infinity of false failure suspicions. If no process is suspected to have failed, then our protocol is similar to a three phase commit protocol. In the case where processes are suspected, our protocol does not require any additional termination protocol: failure scenarios are handled within our regular protocol and are thus much simpler to manage.