5355347
An introduction to the renaming problem
The aim of this paper is to provide a brief introduction to the renaming problem for unfamiliar readers. In the renaming problem the processes have to acquire new names from a small bounded space despite possible process crashes and asynchrony. The problem is first introduced. Then two solutions are presented. One considers the shared memory model, while the second considers the message-passing model.
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.
13251232
Tracking immediate predecessors in distributed computations
A distributed computation is usually modeled as a partially ordered set of relevant events (the relevant events are a subset of the primitive events produced by the computation). An important causality-related distributed computing problem, that we call the Immediate Predecessors Tracking (IPT) problem, consists in associating with each relevant event, on the fly and without using additional control messages, the set of relevant events that are its immediate predecessors in the partial order. So, IPT is the on-the-fly computation of the transitive reduction (i.e., Hasse diagram) of the causality relation defined by a distributed computation. This paper addresses the IPT problem: it presents a family of protocols that provides each relevant event with a timestamp that exactly identifies its immediate predecessors. The family is defined by a general condition that allows application messages to piggyback control information whose size can be smaller than $n$ (the number of processes). In that sense, this family defines message size-efficient IPT protocols. According to the way the general condition is implemented, different IPT protocols can be obtained. Two of them are exhibited.
7058731
Approximate real-time clocks for scheduled events
This paper presents a simple protocol that provides application processes with an approximate real-time notion. This time notion is very versatile. At one extreme, it behaves at least as virtual time (being thereby consistent with causality). At the other extreme, it behaves as real-time. More precisely, with respect to approximate real-time all scheduled events are always executed at their scheduled time. If local computations took no time, approximate real-time would always be real-time. The system facilitates the rescheduling of events to bring real-time and approximate time closer together if local computations take too much time. A time-slotted leader election protocol using the proposed approximate time notion is described.
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.
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.
38731127
The Synchronous Condition-Based Consensus Hierarchy
In the context of a system made up of n processes where at most t can crash, the condition-based approach studies restrictions on the inputs of a distributed problem, called conditions, that make it solvable, or easier to solve (in case it is solvable without restricting its inputs). Previous work studied conditions for consensus and other agreement problems, mostly for asynchronous systems. This paper considers the condition-based approach for consensus in synchronous systems, and establishes a bridge between the asynchronous and synchronous models, with a hierarchy \( {\cal S}_t^{[-t]}\subset\cdots\subset {\cal S}_t^{[0]}\subset\cdots\subset {\cal S}_t^{[t]} \) where \({\cal S}_t^{[t]}\) includes all conditions (and in particular the trivial one made up of all possible input vectors). For a condition \(C \in{\cal S}_t^{[d]}\), –t ≤ d ≤ t, we have:
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.
10936330
Uniform agreement despite process omission failures
A process fails by omission if it "forgets" to send or receive messages. Considering omission failures is crucial for distributed systems, as such failures model both crash failures and incorrect behavior of process input/output buffers (such as buffer overflow). So, designing protocols that cope not only with crash failures but also with omission failures is a real challenge as soon as one is interested in obtaining real-time dependable distributed systems. While the consensus problem has received a lot of attention in the crash failure model and in the Byzantine failure model, it has received less attention in the omission failure model. This paper presents a simple uniform consensus protocol for synchronous systems made up of n processes where up to t can commit crash or omission failures. This protocol requires t+1 communication steps. Interestingly, as this bound is tight for crash failures and those are included in omission failures, this shows that t+1 is a tight lower bound for protocols solving uniform consensus in synchronous systems prone to process omission failures. The protocol assumes t<n/2 that is a necessary requirement on the maximum number of faulty processes that can be tolerated by any uniform consensus protocol in presence of omission failures. The proposed protocol is then extended, at no additional cost, to solve the interactive consistency problem.
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.
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.