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23
Unreliable Failure Detectors for Reliable Distributed Systems
 Journal of the ACM
, 1996
"... We introduce the concept of unreliable failure detectors and study how they can be used to solve Consensus in asynchronous systems with crash failures. We characterise unreliable failure detectors in terms of two properties — completeness and accuracy. We show that Consensus can be solved even with ..."
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Cited by 899 (18 self)
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We introduce the concept of unreliable failure detectors and study how they can be used to solve Consensus in asynchronous systems with crash failures. We characterise unreliable failure detectors in terms of two properties — completeness and accuracy. We show that Consensus can be solved even with unreliable failure detectors that make an infinite number of mistakes, and determine which ones can be used to solve Consensus despite any number of crashes, and which ones require a majority of correct processes. We prove that Consensus and Atomic Broadcast are reducible to each other in asynchronous systems with crash failures; thus the above results also apply to Atomic Broadcast. A companion paper shows that one of the failure detectors introduced here is the weakest failure detector for solving Consensus [Chandra et al. 1992].
The Topological Structure of Asynchronous Computability
 JOURNAL OF THE ACM
, 1996
"... We give necessary and sufficient combinatorial conditions characterizing the tasks that can be solved by asynchronous processes, of which all but one can fail, that communicate by reading and writing a shared memory. We introduce a new formalism for tasks, based on notions from classical algebra ..."
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Cited by 114 (11 self)
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We give necessary and sufficient combinatorial conditions characterizing the tasks that can be solved by asynchronous processes, of which all but one can fail, that communicate by reading and writing a shared memory. We introduce a new formalism for tasks, based on notions from classical algebraic and combinatorial topology, in which a task's possible input and output values are each associated with highdimensional geometric structures called simplicial complexes. We characterize computability in terms of the topological properties of these complexes. This characterization has a surprising geometric interpretation: a task is solvable if and only if the complex representing the task's allowable inputs can be mapped to the complex representing the task's allowable outputs by a function satisfying certain simple regularity properties. Our formalism thus replaces the "operational" notion of a waitfree decision task, expressed in terms of interleaved computations unfolding ...
More Choices Allow More Faults: Set Consensus Problems In Totally Asynchronous Systems
 Information and Computation
, 1992
"... We define the kset consensus problem as an extension of the consensus problem, where each processor decides on a single value such that the set of decided values in any run is of size at most k. We require the agreement condition that all values decided upon are initial values of some processor. ..."
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Cited by 99 (3 self)
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We define the kset consensus problem as an extension of the consensus problem, where each processor decides on a single value such that the set of decided values in any run is of size at most k. We require the agreement condition that all values decided upon are initial values of some processor. We show that the problem has a simple (k  1)resilient protocol in a totally asynchronous system. In an attempt to come up with a matching lower bound on the number of failures, we study the uncertainty condition, which requires that there must be some initial configuration from which all possible input values can be decided. We prove using a combinatorial argument that any kresilient protocol for the kset agreement problem would satisfy the uncertainty condition, while this is not true for any (k  1)resilient protocol.
The asynchronous computability theorem for tresilient tasks
 In Proceedings of the 1993 ACM Symposium on Theory of Computing
, 1993
"... We give necessary and sufficient combinatorial conditions characterizing the computational tasks that can be solved by N asynchronous processes, up to t of which can fail by halting. The range of possible input and output values for an asynchronous task can be associated with a highdimensional geom ..."
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Cited by 94 (14 self)
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We give necessary and sufficient combinatorial conditions characterizing the computational tasks that can be solved by N asynchronous processes, up to t of which can fail by halting. The range of possible input and output values for an asynchronous task can be associated with a highdimensional geometric structure called a simplicial complex. Our main theorem characterizes computability y in terms of the topological properties of this complex. Most notably, a given task is computable only if it can be associated with a complex that is simply connected with trivial homology groups. In other words, the complex has “no holes!” Applications of this characterization include the first impossibility results for several longstanding open problems in distributed computing, such as the “renaming ” problem of Attiya et. al., the “kset agreement ” problem of Chaudhuri, and a generalization of the approximate agreement problem. 1
WaitFree Data Structures in the Asynchronous PRAM Model
 In Proceedings of the 2nd Annual Symposium on Parallel Algorithms and Architectures
, 2000
"... In the asynchronous PRAM model, processes communicate by atomically reading and writing shared memory locations. This paper investigates the extent to which asynchronous PRAM permits longlived, highly concurrent data structures. An implementation of a concurrent object is waitfree if every operati ..."
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Cited by 65 (13 self)
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In the asynchronous PRAM model, processes communicate by atomically reading and writing shared memory locations. This paper investigates the extent to which asynchronous PRAM permits longlived, highly concurrent data structures. An implementation of a concurrent object is waitfree if every operation will complete in a finite number of steps, and it is kbounded waitfree, for some k > 0, if every operation will complete within k steps. In the first part of this paper, we show that there are objects with waitfree implementations but no kbounded waitfree implementations for any k, and that there is an infinite hierarchy of objects with implementations that are kbounded waitfree but not Kbounded waitfree for some K > k. In the second part of the paper, we give an algebraic characterization of a large class of objects that do have waitfree implementations in asynchronous PRAM, as well as a general algorithm for implementing them. Our tools include simple iterative algorithms for waitfree approximate agreement and atomic snapshot.
Are WaitFree Algorithms Fast?
, 1991
"... The time complexity of waitfree algorithms in "normal" executions, where no failures occur and processes operate at approximately the same speed, is considered. A lower bound of log n on the time complexity of any waitfree algorithm that achieves approximate agreement among n processes is proved. ..."
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Cited by 42 (12 self)
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The time complexity of waitfree algorithms in "normal" executions, where no failures occur and processes operate at approximately the same speed, is considered. A lower bound of log n on the time complexity of any waitfree algorithm that achieves approximate agreement among n processes is proved. In contrast, there exists a nonwaitfree algorithm that solves this problem in constant time. This implies an (log n) time separation between the waitfree and nonwaitfree computation models. On the positive side, we present an O(log n) time waitfree approximate agreement algorithm; the complexity of this algorithm is within a small constant of the lower bound.
A simple constructive computability theorem for waitfree computation
 In: Proceedings of the 1994 ACM Symposium on Theory of Computing 243–252
, 1994
"... I ..."
A Simple Algorithmically Reasoned Characterization of Waitfree Computations
 In Proceedings of the 16th Annual ACM Symposium on Principles of Distributed Computing
, 1996
"... ) Elizabeth Borowsky (borowsky@hpl.hp.com) HewlettPackard Laboratories PaloAlto, CA 94303 U.S.A. Eli Gafni (eli@cs.ucla.edu) Computer Science Department University of California, Los Angeles Los Angeles, CA 90024 U.S.A. July 1, 1996 Abstract In a sequence of two pioneering papers Herlihy and S ..."
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Cited by 34 (11 self)
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) Elizabeth Borowsky (borowsky@hpl.hp.com) HewlettPackard Laboratories PaloAlto, CA 94303 U.S.A. Eli Gafni (eli@cs.ucla.edu) Computer Science Department University of California, Los Angeles Los Angeles, CA 90024 U.S.A. July 1, 1996 Abstract In a sequence of two pioneering papers Herlihy and Shavit characterized waitfree sharedmemory computations. The derivation of the characterization involves homology for the necessary conditions, and complex geometry arguments for the sufficiency. This paper gives an alternative proof of the conditions using familiar algorithmic arguments. Our only reliance on geometry is the use of a corollary to the simplicial approximation. Furthermore, this paper is the first to present another consequence of the relation between distributed algorithms and topology: that certain theorems in topology are naturally proven by distributed algorithms interpretations. Our techniques can be extended to characterize models that are more complex than the waitfree...
Algebraic spans
, 2000
"... Topological methods have yielded a variety of lower bounds and impossibility results for distributed computing. In this paper, we introduce a new tool for proving impossibility results, which is based on a core theorem of algebraic topology, the acyclic carrier theorem, and unifies, generalizes and ..."
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Cited by 32 (16 self)
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Topological methods have yielded a variety of lower bounds and impossibility results for distributed computing. In this paper, we introduce a new tool for proving impossibility results, which is based on a core theorem of algebraic topology, the acyclic carrier theorem, and unifies, generalizes and extends earlier results.
Geometry and Concurrency: A User's Guide
, 2000
"... Introduction "Geometry and Concurrency" is not yet a wellestablished domain of research, but is rather made of a collection of seemingly related techniques, algorithms and formalizations, coming from different application areas, accumulated over a long period of time. There is currently a certain ..."
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Cited by 29 (7 self)
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Introduction "Geometry and Concurrency" is not yet a wellestablished domain of research, but is rather made of a collection of seemingly related techniques, algorithms and formalizations, coming from different application areas, accumulated over a long period of time. There is currently a certain amount of effort made for unifying these (in particular see the article (Gunawardena, 1994)), following the workshop "New Connections between Computer Science and Mathematics" held at the Newton Institute in Cambridge, England in November 1995 (and sponsored by HP/BRIMS). More recently, the first workshop on the very same subject has been held in Aalborg, Denmark (see http://www.math.auc.dk/~raussen/admin/workshop/workshop.html where the articles of this issue, among others, have been first sketched. But what is "Geometry and Concurrency" composed of then? It is an area of research made of techniques which use geometrical reasoning for describing and solving problems