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.
34685653
Token-based sequential consistency
20443923
The Renaming Problem as an Introduction to Structures for Wait-Free Computing
The aim of this introductory survey paper is twofold: to be an introduction to wait-free computing and present the renaming problem. “Wait-free” means that the progress of a process depends only on it, regardless of the other processes (that can progress slowly or even crash). It is shown that the design of wait-free algorithms rests on the definition and the use of appropriate data/control structures. To illustrate such structures, the paper considers the renaming problem where the processes have to acquire new names from a small bounded space despite possible process crashes. Two renaming algorithms are presented. The first is a protocol due to Moir and Anderson; it is based on a grid of splitters. The second is due to Attiya and Fouren; it is based on a network of reflectors. It appears that splitters and reflectors are basic data/control structures that permit to define switching networks well-suited to wait-free computing.
39476661
The Committee Decision Problem
We introduce the (b,n)-Committee Decision Problem (CD) – a generalization of the consensus problem. While set agreement generalizes consensus in terms of the number of decisions allowed, the CD problem generalizes consensus in the sense of considering many instances of consensus and requiring a processor to decide in at least one instance. In more detail, in the CD problem each one of a set of n processes has a (possibly distinct) value to propose to each one of a set of b consensus problems, which we call committees. Yet a process has to decide a value for at least one of these committees, such that all processes deciding for the same committee decide the same value. We study the CD problem in the context of a wait-free distributed system and analyze it using a combination of distributed algorithmic and topological techniques, introducing a novel reduction technique. 
 
We use the reduction technique to obtain the following results. We show that the (2,3)-CD problem is equivalent to the musical benches problem introduced by Gafni and Rajsbaum in [10], and both are equivalent to (2,3)-set agreement, closing an open question left there. Thus, all three problems are wait-free unsolvable in a read/write shared memory system, and they are all solvable if the system is enriched with objects capable of solving (2,3)-set agreement. While the previous proof of the impossibility of musical benches was based on the Borsuk-Ulam (BU) Theorem, it now relies on Sperner's Lemma, opening intriguing questions about the relation between BU and distributed computing tasks.
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.
6896229
The Power and Limit of Adding Synchronization Messages for Synchronous Agreement
This paper investigates the use of additional synchronization messages in round-based message-passing synchronous systems. It first presents a synchronous computation model allowing a process to send such messages. The difference with respect to the traditional round-based synchronous model lies in the sending phase, where a process can first send a data message to each other process, and then, without a break, a synchronization message (their sendings can be pipelined). This model is suited to the class of local area networks where communication channels are reliable. (It is not for networks where unreliable communication requires message retransmission.) To illustrate the model, the paper presents a uniform consensus algorithm suited to this model. This algorithm, based on the rotating coordinator paradigm, allows the processes to decide in at most f + 1 rounds where f is the actual number of processes that crash in the corresponding run. (This improves the f + 2 lower bound of the traditional synchronous model.) In addition to its efficiency, the algorithm enjoys another first class property, namely, design simplicity. The paper focuses also on lower bound results, and shows that any uniform consensus algorithm designed for the proposed model, requires at least f + 1 rounds in the worst case. The proposed algorithm is consequently optimal. In that sense the paper has to be seen as an investigation of both the power and the limit of adding synchronization messages to synchronous systems built on top of local networks with reliable communication