32842522
A generic framework for indulgent consensus
Consensus is a fundamental distributed agreement problem that has to be solved when one has to design or implement reliable applications. As consensus cannot be solved in pure asynchronous distributed systems, those systems have to be equipped with appropriate oracles to circumvent the impossibility. Several oracles (unreliable failure detector leader capability, random number generator) have been proposed, and consensus protocols based on such ad hoc oracles have been designed This paper presents a generic consensus framework that can be instantiated with any oracle, or combination of oracles, that satisfies a set of properties. This generic framework provides indulgent consensus protocols that are particularly simple and efficient both in well-behaved runs (i.e., when there are no failures), and in stable runs (i.e., when there is no failure during the execution although some processes can be initially crashed). In those runs, the protocols terminate in two communication steps (which is optimal). Indulgence means that the resulting protocol never violates its safety property even when the underlying oracle behaves arbitrarily. Interestingly, the protocol can also allow processes to decide in one communication step in some specific configurations.
14022316
Conditions on input vectors for consensus solvability in asynchronous distributed systems
This paper introduces and explores a new condition based approach to solve the consensus problem in asynchronous systems. The approach consists of identifying sets of input vectors, called conditions, for which it is possible to design a protocol solving consensus despite the occurrence of up to f process crashes.
The first main result is a generic consensus protocol for the shared memory model. It always guarantees agreement, and also termination (at least) when the inputs satisfy the condition with which the protocol has been instantiated, or when there are no crashes. Then, two particular realistic conditions are defined, and proved to be maximal.
The second main result is a characterization of the conditions that allow to solve consensus. The consensus protocol works for any such condition. An efficient version of the protocol is then designed for the message passing model that works when f\geq, and it is shown that no such protocol exists when f\geq n/2. It is also shown how the protocol's safety can be traded for its liveness.
53225819
Conditions on input vectors for consensus solvability in asynchronous distributed systems
This article introduces and explores the condition-based approach to solve the consensus problem in asynchronous systems. The approach studies conditions that identify sets of input vectors for which it is possible to solve consensus despite the occurrence of up to f process crashes. The first main result defines acceptable conditions and shows that these are exactly the conditions for which a consensus protocol exists. Two examples of realistic acceptable conditions are presented, and proved to be maximal, in the sense that they cannot be extended and remain acceptable. The second main result is a generic consensus shared-memory protocol for any acceptable condition. The protocol always guarantees agreement and validity, and terminates (at least) when the inputs satisfy the condition with which the protocol has been instantiated, or when there are no crashes. An efficient version of the protocol is then designed for the message passing model that works when f < n/2, and it is shown that no such protocol exists when f ≥ n/2. It is also shown how the protocol's safety can be traded for its liveness.
34685653
Token-based sequential consistency
52866155
Atomic Broadcast in Asynchronous Crash-Recovery Distributed Systems and Its Use in Quorum-Based Replication
Atomic broadcast is a fundamental problem of distributed systems: It states that messages must be delivered in the same order to their destination processes. This paper describes a solution to this problem in asynchronous distributed systems in which processes can crash and recover. A consensus-based solution to atomic broadcast problem has been designed by Chandra and Toueg for asynchronous distributed systems where crashed processes do not recover. We extend this approach: it transforms any consensus protocol suited to the crash-recovery model into an atomic broadcast protocol suited to the same model. We show that atomic broadcast can be implemented requiring few additional log operations in excess of those required by the consensus. The paper also discusses how additional log operations can improve the protocol in terms of faster recovery and better throughput. To illustrate the use of the protocol, the paper also describes a solution to the replica management problem in asynchronous distributed systems in which processes can crash and recover. The proposed technique makes a bridge between established results on weighted voting and recent results on the consensus problem.
10131958
An optimal atomic broadcast protocol and an implementation framework
Atomic broadcast (where all processes deliver broadcast messages in the same order) is a very useful group communication primitive for building fault-tolerant distributed systems. This paper presents an atomic broadcast protocol that can be claimed to be optimal in terms of failure detection, resilience, and latency. The protocol requires only the weakest of the useful failure detectors for liveness, and permits up to (n-1)/2 processes to crash in a system of n processes; at most two communication steps and n broadcasts are needed in a run during which process crashes and failure-suspicions do not occur. We also introduce the notion of notifying broadcast which can reduce the message overhead further in 'nice' runs in which all processes are operational and communication delays do not exceed the bound assumed. If nice runs persist, the average message overhead is just one broadcast. That is, the protocol extracts no message overhead for providing crash-tolerance if process failures and unanticipated fluctuations in communication delays do not occur. We are currently implementing our protocol as a CORBA (Common Object Request Broker Architecture) component. All known ORBs use IIOP as the standard protocol for inter-process communication, which in turn uses TCP/IP (transmission control protocol/Internet protocol) as the common transport protocol. It turns out that the notifying broadcast is easy to implement on top of the TCP transport layer.
14451158
A Time-free Assumption to Implement Eventual Leadership
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 introduces a novel approach to implement an eventual leader protocol, namely a time-free behavioral assumption on the flow of messages that are exchanged. It presents a very simple leader protocol based on this assumption. It then presents a second leader protocol combining this timeless assumption with eventually timely channels. As it considers several assumptions, the resulting hybrid protocol has the noteworthy feature to provide an increased overall assumption coverage. A probabilistic analysis shows that the time-free assumption is practically always satisfied.
10492741
Sequential consistency as lazy linearizability
This revue shows that, from an implementation point of view, sequential consistency can be considered as a form of lazy linearizability. This claim is supported by a versatile protocol that can be tailored to implement any of them.
15901717
From failure detectors with limited scope accuracy to system-wide leadership
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. /spl diams/S/sub k/ 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 /spl Omega/)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 /spl diams/S/sub t+1/ where t is the maximum number of processes that can crash during a run. The fact that /spl diams/S/sub t+1/ is easier to build than /spl diams/S or /spl Omega/ and the design simplicity of the proposed protocol makes it attractive.
17868570
Condition-Based Protocols for Set Agreement Problems
A condition C is a set of input vectors to a problem. A class of conditions that allow to solve k-set agreement in spite of f crashes in an asynchronous system is identified. A k-set agreement protocol that is always safe is described: it is guaranteed to terminate when the input vector belongs to C and it always decides on at most k different values, even if the input vector does not belong to C. While there are simple solutions when f < k, it is known that the k-set agreement problem has no solution when f ? k. Thus, the paper identifies classes of conditions that allow to solve this problem even when f ? k. The paper gives evidence that these are the only conditions that allow to solve set agreement, by proving the wait-free case. Two natural concrete conditions that belong to such a class are described. Finally, a more efficient k-set agreement protocol with only linear complexity (does not use snapshots), for any C that allows to solve consensus, when k ? f/(n - f) + 1 is presented. This shows how to trade fault-tolerance for agreement precision using the condition based approach.
35647117
Early-Stopping k-Set Agreement in Synchronous Systems Prone to Any Number of Process Crashes
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 presents a surprisingly simple protocol that solves the k-set agreement problem in synchronous systems prone to up to t<n processes can crash (where n is the total number of processes). The proposed protocol is the first early stopping k-set agreement protocol that does not impose a constraint on t. It allows the processes to decide and stop by min $(\lfloor f/k \rfloor +2,\lfloor t/k \rfloor+1)$ rounds where f is the number of actual crashes (0≤ f ≤ t). In addition to its conceptual simplicity, the protocol has an additional noteworthy feature, namely, it is particularly efficient in common case scenarios. This comes from the fact that it is based on a mechanism that allows the processes to take into account the actual pattern of failures and not only their number, thereby allowing the processes to decide in much less than ${\lfloor f/k \rfloor +2}$ coding from an InlineMediaObject here! rounds in a lot of cases.