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ABSTRACT: Due to their capability to hide the complexity generated by the messages exchanged between processes, shared objects are one of the main abstractions provided to developers of distributed applications. Implementations of such objects, in modern distributed systems, have to take into account the fact that almost all services, implemented on top of distributed infrastructures, are no longer fully managed due to either their size or their maintenance cost. Therefore, these infrastructures exhibit several autonomic behaviors in order to, for example, tolerate failures and continuous arrival and departure of nodes (churn phenomenon). Among all the shared objects, the register object is a fundamental one. Several protocols have been proposed to build fault resilient registers on top of message-passing system, but, unfortunately, failures are not the only challenge in modern distributed systems and new issues arise in the presence of churn. This paper addresses the construction of a multiwriter/multireader regular register in an eventually synchronous distributed system affected by the continuous arrival/departure of participants. In particular, a general protocol implementing a regular register is proposed and feasibility conditions associated with the arrival and departure of the processes are given. The protocol is proved correct under the assumption that a constraint on the churn is satisfied.
IEEE Transactions on Parallel and Distributed Systems 02/2012; · 1.40 Impact Factor
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ABSTRACT: Understanding the relative computability power of tasks, in the presence of asynchrony and failures, is a central concern of distributed computing theory. In the wait-free case, where the system consists of n processes and any of them can fail by crashing, substantial attention has been devoted to understanding the relative power of the subconsensus family of tasks, which are too weak to solve consensus for two processes. The first major results showed that set agreement and renaming (except for some particular values of n) cannot be solved wait-free in read/write memory. Then it was proved that renaming is strictly weaker than set agreement (when n is odd). This paper considers a natural family of subconsensus tasks that includes set agreement, renaming and other generalized symmetry breaking (GSB) tasks. It extends previous results, and proves various new results about when there is a reduction and when not, among these tasks. Among other results, the paper shows that there are incomparable subconsensus tasks. Enrichir la carte de réduction des tâches sous-consensus Résumé : Ce rapport enrichit la carte de réduction des tâches sous-consensus Mots clés : Tâche de décision, Calculabilité distribuée, Hiérarchie deprobì emes, Renommage, k-Accord ensembliste, Cassage de symétrie, Sans-attente.
04/2011;
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ABSTRACT: Providing distributed processes with concurrent objects is a fundamental service that has to be offered by any distributed system. The classical shared read/write register is one of the most basic ones. Several protocols have been proposed that build an atomic register on top of an asynchronous message-passing system prone to process crashes. In the same spirit, this paper addresses the implementation of a regular register (a weakened form of an atomic register) in an asynchronous dynamic message-passing system. The aim is here to cope with the net effect of the adversaries that are asynchrony and dynamicity (the fact that processes can enter and leave the system). The paper focuses on the class of dynamic systems the churn rate c of which is constant. It presents two protocols, one applicable to synchronous dynamic message passing systems, the other one to eventually synchronous dynamic systems. Both protocols rely on an appropriate broadcast communication service (similar to a reliable broadcast). Each requires a specific constraint on the churn rate c. Both protocols are first presented in an as intuitive as possible way, and are then proved correct.
Distributed Computing Systems, 2009. ICDCS '09. 29th IEEE International Conference on; 07/2009
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ABSTRACT: Eventual leadership has been identified as a basic building block to solve synchronization or coordination problems in distributed computing systems. However, it is a challenging task to implement the eventual leadership facility, especially in dynamic distributed systems, where the global system structure is unknown to the processes and can vary over time. This paper studies the implementation of a leadership facility in infrastructured mobile networks, where an unbounded set of mobile hosts arbitrarily move in the area covered by fixed mobile support stations. Mobile hosts can crash and suffer from disconnections. We develop an eventual leadership protocol based on a time-free approach. The mobile support stations exchange queries and responses on behalf of mobile hosts. With assumptions on the message exchange flow, a correct mobile host is eventually elected as the unique leader. Since no time property is assumed on the communication channels, the proposed protocol is especially effective and efficient in mobile environments, where time-based properties are difficult to satisfy due to the dynamics of the network.
Dependable Computing, 2007. PRDC 2007. 13th Pacific Rim International Symposium on; 01/2008
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ABSTRACT: Designing protocols for solving the consensus problem faces new challenges in mobile computing environments. Among others, how we can achieve message efficiency for saving resource consumption has been the focus of research. In this paper, we present the HC protocol, a message efficient consensus protocol for MANETs. We consider the widely used system model where the hosts fail by crashes and the system is equipped with Chandra-Toueg's unreliable failure detectors. Unlike existing consensus protocols, the HC protocol uses a two-layer hierarchy based on clusters to achieve message efficiency. The messages from and to the hosts in the same cluster are merged so as to reduce the message cost. However, adding such a hierarchy is not trivial. Due to host movements and failures, the hierarchy changes from time to time and this may cause message loss. In designing HC, we also propose methods to handle such message losses. Extensive simulations have been carried out to evaluate and compare the performance of the HC protocol and similar protocols in a MANET environment. Simulation results show that, in most cases, our protocol can significantly reduce both the message cost and time cost. With increases in the system scale or the percentage of faulty hosts, the advantage of our protocol becomes more obvious.
IEEE Transactions on Computers 09/2007; 56(8):1055-1070. · 1.10 Impact Factor
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ABSTRACT: The condition-based approach identifies sets of input vectors, called conditions, for which it is possible to design an asynchronous protocol solving a distributed problem despite process crashes. This paper establishes a direct correlation between distributed agreement problems and error-correcting codes. In particular, crash failures in distributed agreement problems correspond to erasure failures in error-correcting codes and Byzantine and value domain faults correspond to corruption errors. This correlation is exemplified by concentrating on two well-known agreement problems, namely, consensus and interactive consistency, in the context of the condition-based approach. Specifically, the paper presents the following results: first, it shows that the conditions that allow interactive consistency to be solved despite f<sub>c</sub> crashes and f<sub>c</sub> value domain faults correspond exactly to the set of error-correcting codes capable of recovering from f<sub>c</sub> erasures and f<sub>c</sub> corruptions. Second, the paper proves that consensus can be solved despite f<sub>c</sub> crash failures if the condition corresponds to a code whose Hamming distance is f<sub>c</sub> + 1 and Byzantine consensus can be solved despite f<sub>b</sub> Byzantine faults if the Hamming distance of the code is 2 f<sub>b</sub> + 1. Finally, the paper uses the above relations to establish several results in distributed agreement that are derived from known results in error-correcting codes and vice versa.
IEEE Transactions on Computers 08/2007; 56(7):865-875. · 1.10 Impact Factor
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04/2007;
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ABSTRACT: Leader-based protocols rest on a primitive able to provide the processes with the same unique leader. Such protocols are very common in distributed computing to solve synchronization or coordination problems. Unfortunately, providing such a primitive is far from being trivial in asynchronous distributed systems prone to process crashes. (It is even impossible in fault-prone purely asynchronous systems.) To circumvent this difficulty, several protocols have been proposed that build a leader facility on top of an asynchronous distributed system enriched with additional assumptions. The protocols proposed so far consider either additional assumptions based on synchrony or additional assumptions on the pattern of the messages that are exchanged. Considering systems with n processes and up to f process crashes, 1≤f<n, this paper investigates the combination of a time-free assumption on the message pattern with a synchrony assumption on process speed and message delay. It shows that both types of assumptions can be combined to obtain a hybrid eventual leader protocol benefiting from the best of both worlds. This combined assumption considers a star communication structure involving f+1 processes. Its noteworthy feature lies in the level of combination of both types of assumption that is "as fine as possible" in the sense that each of the f channels of the star has to satisfy a property independently of the property satisfied by each of the f-1 other channels (the f channels do not have to satisfy the same assumption). More precisely, this combined assumption is the following: There is a correct process p (center of the star) and a set Q of f processes q (p∉Q) such that, eventually, either 1) each time it broadcasts a query, q receives a response from p among the (n-f) first responses to that query, or 2) the channel from p to q is timely. (The processes in the set Q can crash.) A surprisingly simple eventual leader protocol based on this fine grain hybrid assumption is proposed and proved correct. An improvement is also presented.
IEEE Transactions on Parallel and Distributed Systems 08/2006; 17(7):656- 666. · 1.40 Impact Factor
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ABSTRACT: A failure detector is a device that provides the processes with information on failures. The accuracy property of a failure detector defines the type of mistakes it is not allowed to make. The limited scope of the accuracy property restricts it to only a part of the system. ◊ S<sub>k</sub> is a class of unreliable failure detectors with a limited scope accuracy. Eventually each process that crashes is suspected by every correct process, and there is a time after which some correct process is never suspected by only k processes. An eventual leader facility (usually denoted Ω)is a device that eventually provides all the processes with the identity of one of them that is correct. Such a facility is used as a basic service in a lot of fault-tolerant distributed protocols (e.g., asynchronous consensus protocols). This paper proposes a protocol that builds an eventual leader service from any unreliable failure detector of the class ◊ S<sub>t+1</sub> where t is the maximum number of processes that can crash during a run. The fact that ◊ S<sub>t+1</sub> is easier to build than ◊ S or Ω and the design simplicity of the proposed protocol makes it attractive.
Advanced Information Networking and Applications, 2006. AINA 2006. 20th International Conference on; 05/2006
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ABSTRACT: Mobile ad hoc networks (MANETs) raise new challenges in designing protocols for solving the consensus problem. Among the others, how to design message efficient protocols so as to save resource consumption, has been the focus of research. In this paper, we present the design of such an efficient consensus protocol. We consider the system model for MANETs with host crashes, but equipped with Chandra-Toueg's unreliable failure detectors of class ◊ P. At most f hosts can crash where f < n/2 (n is the total number of the hosts). The protocol adopts the coordinator rotation paradigm to achieve consensus. Unlike existing consensus protocols, the proposed protocol is based on a two-layer hierarchy with hosts associated with proxies. At least f + 1 hosts act as proxies and each host is associated with one proxy host. The messages from and/or to the local hosts of the same proxy are merged so as to reduce the message cost. Moreover, the hierarchical approach can improve the scalability of the consensus protocol. Performance analysis shows that the proposed protocol can significantly save cost compared existing protocols.
Parallel, Distributed, and Network-Based Processing, 2006. PDP 2006. 14th Euromicro International Conference on; 03/2006
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ABSTRACT: Defining good abstractions is a central issue when one wants to understand the deep structure and basic principles that underlie computing mechanisms. This paper introduces a basic and particularly simple distributed computing abstraction suited to asynchronous distributed agreement problems. This abstraction, called intersecting sets, requires each process to deposit a value and allows each non-faulty process to obtain a subset of these values such that any two such sets have a non-empty intersection. This simple abstraction captures an essential part of distributed agreement problems. After having introduced and motivated this abstraction, the paper investigates its properties, its power and its benefit when solving distributed agreement problems.
Dependable Computing, 2005. Proceedings. 11th Pacific Rim International Symposium on; 01/2006
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ABSTRACT: This short paper characterizes a round-based synchronous (timely) computing model that is equivalent to the popular crash prone round-based asynchronous (time-free) distributed computing model. Equivalence means here that any problem that can be solved by a protocol in one model can be solved by the same protocol in the other model. The style of this note is voluntarily informal. Its aim is mainly pedagogical. Its ambition is to help better understand relations linking synchronous and asynchronous distributed computing systems, and the nature of failures that make them difficult to master.
Dependable Computing, 2005. Proceedings. 11th Pacific Rim International Symposium on; 01/2006
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ABSTRACT: The k-set agreement problem is a generalization of the consensus problem: each process proposes a value, and each non-faulty process has to decide a value such that a decided value is a proposed value, and no more than k different values are decided. This paper focuses on the k-set agreement problem in the context of synchronous systems where up to t < n processes can experience crash or send omission failures (n being the total number of processes). The paper presents a k-set agreement protocol for this failure model (the first to our knowledge) which has two main outstanding features. (1) It provides the following early deciding and stopping property: no process decides or halts after the round min( └ f/k ┘ + 2, └ t/k ┘ + 1) where f is the number of actual crashes (0 ≤ f ≤ t). (2) It is decision-optimal. This new optimality criterion, suited to the omission failure model, concerns the number of processes that decide, namely, the protocol forces all the processes that do not crash to decide (regardless of whether they commit omission faults or not). It is noteworthy that each of these properties (early deciding/stopping vs decision-optimality) is not obtained at the detriment of the other. Last but not least, the protocol enjoys another first-class property, namely, simplicity.
Dependable Computing, 2005. Proceedings. 11th Pacific Rim International Symposium on; 01/2006
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ABSTRACT: A noteworthy advance in distributed computing is due to the recent development of peer-to-peer systems. These systems are essentially dynamic in the sense that no process can get a global knowledge on the system structure. They mainly allow processes to look up for data that can be dynamically added/suppressed in a permanently evolving set of nodes. Although protocols have been developed for such dynamic systems, to our knowledge, up to date no computation model for dynamic systems has been proposed. Nevertheless, there is a strong demand for the definition of such models as soon as one wants to develop provably correct protocols suited to dynamic systems. This paper proposes a model for (a class of) dynamic systems. That dynamic model is defined by (1) a parameter (an integer denoted a) and (2) two basic communication abstractions (query-response and persistent reliable broadcast). The new parameter is a threshold value introduced to capture the liveness part of the system (it is the counterpart of the minimal number of processes that do not crash in a static system). To show the relevance of the model, the paper adapts an eventual leader protocol designed for the static model, and proves that the resulting protocol is correct within the proposed dynamic model. In that sense, the paper has also a methodological flavor, as it shows that simple modifications to existing protocols can allow them to work in dynamic systems.
Reliable Distributed Systems, 2005. SRDS 2005. 24th IEEE Symposium on; 11/2005
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ABSTRACT: Heterogeneous users usually have different requirements as far as consistency of shared data is concerned. This paper proposes and investigates a mixed consistency model to meet this heterogeneity challenge in large scale distributed systems that support shared objects. This model allows combining strong (sequential) consistency and weak (causal) consistency. The paper defines the model, motivates it and proposes a protocol implementing it
Distributed Computing Systems, 2005. ICDCS 2005. Proceedings. 25th IEEE International Conference on; 07/2005
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ABSTRACT: This paper is on the consensus problem in asynchronous distributed systems where (up to f) processes (among n) can exhibit a Byzantine behavior, i.e., can deviate arbitrarily from their specification. One way to solve the consensus problem in such a context consists of enriching the system with additional oracles that are powerful enough to cope with the uncertainty and unpredictability created by the combined effect of Byzantine behavior and asynchrony. This paper presents two kinds of Byzantine asynchronous consensus protocols using two types of oracles, namely, a common coin that provides processes with random values and a failure detector oracle. Both allow the processes to decide in one communication step in favorable circumstances. The first is a randomized protocol for an oblivious scheduler model that assumes n > 6f. The second one is a failure detector-based protocol that assumes n > tif. These protocols are designed to be particularly simple and efficient in terms of communication steps, the number of messages they generate in each step, and the size of messages. So, although they are not optimal in the number of Byzantine processes that can be tolerated, they are particularly efficient when we consider the number of communication steps they require to decide and the number and size of the messages they use. In that sense, they are practically appealing.
IEEE Transactions on Dependable and Secure Computing 02/2005; 2(1):46- 56. · 1.14 Impact Factor
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ABSTRACT: Leader-based protocols rest on a primitive able to provide the processes with the same unique leader. Such protocols are very common in distributed computing to solve synchronization or coordination problems. Unfortunately, providing such a primitive is far from being trivial in asynchronous distributed systems prone to process crashes. (It is even impossible in fault-prone purely asynchronous systems.) To circumvent this difficulty, several protocols have been proposed that build a leader facility on top of an asynchronous distributed system enriched with synchrony assumptions. This paper consider another approach to build a leader facility, namely, it considers a behavioral property on the flow of messages that are exchanged. This property has the noteworthy feature not to involve timing assumptions. Two protocols based on this time-free property that implement a leader primitive are described. The first one uses potentially unbounded counters, while the second one (which is a little more involved) requires only finite memory. These protocols rely on simple design principles that make them attractive, easy to understand and provably correct.
Reliable Distributed Systems, 2004. Proceedings of the 23rd IEEE International Symposium on; 11/2004
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ABSTRACT: Unreliable failure detectors introduced by Chandra and Toueg are abstract mechanisms that provide information about process crashes. On the one hand, failure detectors allow a statement of the minimal requirements on process failures that allow solutions to problems that cannot otherwise be solved in purely asynchronous systems. However, on the other hand, they cannot be implemented in such systems: their implementation requires that the underlying distributed system be enriched with additional assumptions. Classic failure detector implementations rely on additional synchrony assumptions such as partial synchrony. More recently, a new approach for implementing failure detectors has been proposed: it relies on behavioral properties on the flow of messages exchanged. This shows that these approaches are not antagonistic and can be advantageously combined. A hybrid protocol (the first to our knowledge) implementing failure detectors with eventual accuracy properties is presented. Interestingly, this protocol benefits from the best of both worlds in the sense that it converges (i.e., provides the required failure detector) as soon as either the system behaves synchronously or the required message exchange pattern is satisfied. This shows that, to expedite convergence, it can be interesting to consider that the underlying system can satisfy several alternative assumptions.
Dependable Computing, 2004. Proceedings. 10th IEEE Pacific Rim International Symposium on; 04/2004
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ABSTRACT: Vector clocks are the appropriate mechanism used to track causality among the events produced by a distributed computation. Traditional implementations of vector clocks require application messages to piggyback a vector of n integers (where n is the number of processes). This paper investigates the tracking of the causality relation on a subset of events (namely, the events that are defined as "relevant" from the application point of view) in a context where communication channels are not required to be FIFO, and where there is no a priori information on the connectivity of the communication graph or the communication pattern. More specifically, the paper proposes a suite of simple and efficient implementations of vector clocks that address the reduction of the size of message timestamps, i.e., they do their best to have message timestamps whose size is less than n. The relevance of such a suite of protocols is twofold. From a practical side, it constitutes the core of an adaptive timestamping software layer that can used by underlying applications. From a theoretical side, it provides a comprehensive view that helps better understand distributed causality-tracking mechanisms.
IEEE Transactions on Knowledge and Data Engineering 10/2003; · 1.66 Impact Factor
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Dependable Systems and Networks, 2003. Proceedings. 2003 International Conference on; 07/2003
