30548621
Interval Consistency of Asynchronous Distributed Computations
An interval of a sequential process is a sequence of consecutive events of this process. The set of intervals defined on a distributed computation defines an abstraction of this distributed computation, and the traditional causality relation on events induces a relation on the set of intervals that we call I-precedence. An important question is then, “Is the interval-based abstraction associated with a distributed computation consistent?” To answer this question, this paper introduces a consistency criterion named interval consistency (IC). Intuitively, this criterion states that an interval-based abstraction of a distributed computation is consistent if its I-precedence relation does not contradict the sequentiality of each process. More formally, IC is defined as a property of a precedence graph. Interestingly, the IC criterion can be operationally characterized in terms of timestamps (whose values belong to a lattice). The paper uses this characterization to design a versatile protocol that, given intervals defined by a daemon whose behavior is unpredictable, breaks them (in a nontrivial manner) in order to produce an abstraction satisfying the IC criterion. Applications to communication-induced checkpointing are suggested.
1921101
An introduction to oracles for asynchronous distributed systems
This paper is an introduction to oracles the aim of which is to help solving distributed computing problems in asynchronous distributed systems prone to process crash failures and fair lossy channels. Actually, the combination of asynchrony and failures makes a lot of problems impossible to solve in unreliable asynchronous distributed systems. Hence, those systems have to be extended with appropriate oracles in order these problems become solvable. Using two such problems (namely, the design of a quiescent uniform reliable broadcast facility, and the consensus problem), this paper presents appropriate oracles allowing to solve these problems. In that sense, the paper is a guided tour to the definition of oracles suited to unreliable asynchronous distributed systems.
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.
32302413
Sequential Consistency as Lazy Linearizability
This paper shows that actually sequential consistency is a form of "lazy" atomic consistency. More precisely, it proposes a new particularly simple sequential consistency protocol that orders the conflicting operations on each object separately, and appropriately invalidates object copies to prevent consistency violation. When compared to invalidation-based protocols that ensure atomic consistency (such as Li-Hudak's protocol), the proposed protocol can be seen as using lazy invalidation. Hence, in addition to a new consistency protocol, the paper provides a new insight into the concepts and mechanisms that underlie consistency protocols: while atomic consistency is based on physical time and requires eager invalidation, sequential consistency is based on logical time and needs only lazy invalidation.
207104353
A Note on the Determination of the Immediate Predecessors in a Distributed Computation
A distributed computation can be modeled as a partially ordered set (poset) of relevant events (the relevant events are the subset of the primitive events that are meaningful for an observer). This short note presents a general protocol that, when superimposed on a distributed computation, provides each relevant event with a timestamp that identifies exactly its immediate predecessors in the poset. This determination is done on the fly and without using additional control messages. So, the proposed protocol provides an on the fly computation of the Hasse diagram (transitive reduction) of the event graph produced by a distributed computation. The protocol is particularly simple and efficient. It is based on a general condition that allows application messages to piggyback control information whose size can be smaller than n (the number of processes). Interestingly, when one is not interested in tracking the immediate predecessors, the proposed protocol can be simplified to get an efficient vector clock protocol that does not require particular channel assumptions.
1238245
A simple protocol offering both atomic consistent read operations and sequentially consistent read operations
A concurrent object is an object that can be concurrently accessed by several processes. Two well-known consistency criteria for such objects are atomic consistency (also called linearizability) and sequential consistency. Both criteria require that all the operations on the concurrent objects can be totally ordered in such a way that each read operation obtains the last value written into the corresponding object. They differ in the meaning of the word "last" that refers to physical time for atomic consistency, and to logical time for sequential consistency. This paper investigates the merging of these consistency criteria in a multiprocess program. The proposed combination offers two read operations to the processes, namely, an atomic read operation and a sequentially consistent read operation. While the first provides a process with the last "physical" value of an object, the second provides it with a value that is approximate with respect to real-time but whose semantics is perfectly well defined. A protocol that implements the combination on top of an asynchronous distributed system is described. The protocol provides a better understanding of the similarities and differences between these consistency criteria. Moreover, the protocol is generic in the sense that it can be tailored to provide only one of these consistency criteria.
14128423
Elastic vector time
In recent years there has been an increasing demand to build "soft" real-time applications on top of asynchronous distributed systems. Designing and implementing such applications is a non-trivial task and application designers are often faced with the need to circumvent impossibility results. In this paper we discuss how to ensure that actions are executed in the correct order even in the face of failures. We propose a novel time base and a new synchronization mechanism for the design of distributed "soft" real-time applications. We demonstrate (1) how this time base can be used to enforce an externally consistent ordering, and (2) how it permits to circumvent impossibility results by sketching how to solve the leader election and perfect failure detection problem.
14362073
The Alpha of Indulgent Consensus
This paper presents a simple framework unifying a family of consensus algorithms that can tolerate process crash failures and asynchronous periods of the network, also called indulgent consensus algorithms. Key to the framework is a new abstraction we introduce here, called Alpha, and which precisely captures consensus safety. Implementations of Alpha in shared memory, storage area network, message passing and active disk systems are presented, leading to directly derived consensus algorithms suited to these communication media. The paper also considers the case where the number of processes is unknown and can be arbitrarily large.
116376
Atomic broadcast in asynchronous crash-recovery distributed systems
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 (1996) for asynchronous distributed systems where crashed processes do nor recover. Although our solution is based on different algorithmic principles, it follows the same 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 without requiring any 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.
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.