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The Load, Capacity and Availability of Quorum Systems
, 1998
"... A quorum system is a collection of sets (quorums) every two of which intersect. Quorum systems have been used for many applications in the area of distributed systems, including mutual exclusion, data replication and dissemination of information Given a strategy to pick quorums, the load L(S) is th ..."
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Cited by 89 (12 self)
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A quorum system is a collection of sets (quorums) every two of which intersect. Quorum systems have been used for many applications in the area of distributed systems, including mutual exclusion, data replication and dissemination of information Given a strategy to pick quorums, the load L(S) is the minimal access probability of the busiest element, minimizing over the strategies. The capacity Cap(S) is the highest quorum accesses rate that S can handle, so Cap(S) = 1=L(S).
Evaluating Quorum Systems over the Internet
, 1996
"... Quorum systems serve as a basic tool providing a uniform and reliable way to achieve coordination in a distributed system. They are useful for distributed and replicated databases, name servers, mutual exclusion, and distributed access control and signatures. Traditionally, two basic methods have be ..."
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Cited by 51 (8 self)
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Quorum systems serve as a basic tool providing a uniform and reliable way to achieve coordination in a distributed system. They are useful for distributed and replicated databases, name servers, mutual exclusion, and distributed access control and signatures. Traditionally, two basic methods have been used to evaluate quorum systems: the analytical approach, and simulation. This paper proposes a third, empirical approach. We collected 6 months' worth of connectivity and operability data of a system consisting of 14 real computers using a wide area group communication protocol. The system spanned two geographic sites and three different Internet segments. We developed a mechanism that merges the local views into a unified history of the events that took place, ordered according to an imaginary global clock. We then developed a tool called the Generic Quorumsystem Evaluator (GQE), which evaluates the behavior of any given quorum system over the unified, reallife history. We compared fo...
Are Quorums an Alternative for Data Replication
 ACM TRANSACTIONS ON DATABASE SYSTEMS
, 2003
"... ... this article, we analyze several quorum types in order to better understand their behavior in practice. The results obtained challenge many of the assumptions behind quorum based replication. Our evaluation indicates that the conventional readone/writeallavailable approach is the best choice ..."
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Cited by 33 (10 self)
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... this article, we analyze several quorum types in order to better understand their behavior in practice. The results obtained challenge many of the assumptions behind quorum based replication. Our evaluation indicates that the conventional readone/writeallavailable approach is the best choice for a large range of applications requiring data replication. We believe this is an important result for anybody developing code for computing clusters as the readone/writeallavailable strategy is much simpler to implement and more flexible than quorumbased approaches. In this article, we show that, in addition, it is also the best choice using a number of other selection criteria
Crumbling Walls: A Class of Practical and Efficient Quorum Systems
, 1996
"... A quorum system is a collection of sets (quorums) every two of which intersect. Quorum systems have been used for many applications in the area of distributed systems, including mutual exclusion, data replication and dissemination of information In this paper we introduce a general class of quorum ..."
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Cited by 32 (8 self)
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A quorum system is a collection of sets (quorums) every two of which intersect. Quorum systems have been used for many applications in the area of distributed systems, including mutual exclusion, data replication and dissemination of information In this paper we introduce a general class of quorum systems called Crumbling Walls and study its properties. The elements (processors) of a wall are logically arranged in rows of varying widths. A quorum in a wall is the union of one full row and a representative from every row below the full row. This class considerably generalizes a number of known quorum system constructions. The best crumbling wall is the CWlog quorum system. It has small quorums, of size O(lg n), and structural simplicity. The CWlog has optimal availability and optimal load among systems with such small quorum size. It manifests its high quality for all universe sizes, so it is a good choice not only for systems with thousands or millions of processors but also for systems with as few as 3 or 5 processors. Moreover, our analysis shows that the availability will increase and the load will decrease at the optimal rates as the system increases in size.
Quorum Systems in Replicated Databases: Science or Fiction?
 BULL. IEEE TECHNICAL COMMITTEE ON DATA ENGINEERING
, 1998
"... A quorum system is a collection of subsets of servers, every two of which intersect. Quorum systems have been suggested as a tool for concurrency control in replicated databases almost twenty years ago. They promised to guarantee strict consistency and to provide high availability and faulttoleranc ..."
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Cited by 24 (1 self)
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A quorum system is a collection of subsets of servers, every two of which intersect. Quorum systems have been suggested as a tool for concurrency control in replicated databases almost twenty years ago. They promised to guarantee strict consistency and to provide high availability and faulttolerance in the face of server crashes and network partitions. Despite these promises, current commercial replicated databases typically do not use quorum systems. Instead they use mechanisms which guarantee much weaker consistency, if any. Moreover, the interest in quorum systems seems to be waning even in the database research community. This paper
Optimal Availability Quorum Systems: Theory and Practice
, 1998
"... Quorum systems serve as a basic tool providing a uniform and reliable way to achieve coordination in a distributed system. They are useful for distributed and replicated databases, name servers, mutual exclusion, and distributed access control and signatures. The unavailability of a quorum system i ..."
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Cited by 24 (3 self)
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Quorum systems serve as a basic tool providing a uniform and reliable way to achieve coordination in a distributed system. They are useful for distributed and replicated databases, name servers, mutual exclusion, and distributed access control and signatures. The unavailability of a quorum system is the probability of the event that no live quorum exists in the system. When such an event occurs the service is completely halted. The unavailability is widely accepted as the measure by which quorum systems are evaluated. In this paper we characterize the optimal availability quorum system in the general case, when the failure probabilities may take any value in the range 0 ! p i ! 1. Then we deal with the practical scenario in which the failure probabilities are unknown, but can be estimated. We give a robust and efficient algorithm that calculates a near optimal quorum system based on the estimated failure probabilities. Keywords: Quorum systems, distributed computing, fault tolerance...
Two Approaches for High Concurrency in MulticastBased Object Replication
, 1994
"... This report presents a replica control protocol for atomic objects. The protocol is derived from an atomic broadcast primitive, and places constraints on the delivery of messages to provide a consistent message order among sites. Several heuristic techniques are proposed to reduce the latency of mes ..."
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Cited by 5 (0 self)
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This report presents a replica control protocol for atomic objects. The protocol is derived from an atomic broadcast primitive, and places constraints on the delivery of messages to provide a consistent message order among sites. Several heuristic techniques are proposed to reduce the latency of message delivery, for two types of orders. Messages are delivered either in the same order for all sites, or in an order semantically equivalent to this unique ordering. The equivalence relation is based on the commutativity property of operations on objects, i.e: two deposit operations commute. The protocol uses a reliable causal multicast primitive, and is fully distributed. The first set of heuristics is based on a voting scheme, and delivers messages in a unique order. Totally ordered atomic multicast can be built on top of a reliable causal multicast by waiting until each processor in the group has multicast a message, inserting them in a causal graph, and then delivering the roots of this...
Optimal decision strategies in Byzantine environments
 In Proceedings of the 11 th Colloquium on Structural Information and Communication Complexity (SIROCCO 2004
, 2004
"... www.elsevier.com/locate/jpdc A Boolean value of given a priori probability distribution is transmitted to a deciding agent by several processes. Each process fails independently with given probability, and faulty processes behave in a Byzantine way. A deciding agent has to make a decision concerning ..."
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Cited by 4 (0 self)
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www.elsevier.com/locate/jpdc A Boolean value of given a priori probability distribution is transmitted to a deciding agent by several processes. Each process fails independently with given probability, and faulty processes behave in a Byzantine way. A deciding agent has to make a decision concerning the transmitted value on the basis of messages obtained by processes. We construct a deterministic decision strategy which has the provably highest probability of correctness. It computes the decision in time linear in the number of processes. Decision optimality may be alternatively approached from a local, rather than global, point of view. Instead of maximizing the total probability of correctness of a decision strategy, we may try to find, for every set of values conveyed by processes, the conditionally most probable original value that could yield this set. We call such a strategy locally optimal, as it locally optimizes the probability of a decision, given a set of relayed values, disregarding the impact of such a choice on the overall probability of correctness. We construct a locally optimal decision strategy which again computes the decision value in time linear in the number of processes. We establish the surprising fact that, in general, local probability maximization may lead to a decision strategy which does not have the highest probability of correctness. However, if the probability distribution of the Boolean value to be conveyed is uniform, and all processes have the same failure probability smaller than 1 2, this anomaly does not occur. We first design and analyze our strategies in the synchronous setting and then show how they should be modified to work in asynchronous systems.
Highly Available Replicated Atomic Data
, 1994
"... 3 1.1 Acknowledgments : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 3 2 Introduction 4 2.1 High available replicated atomic data : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 4 2.2 Contributions : : : : : : : : : : : : : : : : : : : : : : ..."
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3 1.1 Acknowledgments : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 3 2 Introduction 4 2.1 High available replicated atomic data : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 4 2.2 Contributions : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 6 2.3 Organization : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 7 3 Related work 8 3.1 Replica control protocols : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 8 3.2 Process group communication : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 10 3.3 Replicated databases and Atomic data types : : : : : : : : : : : : : : : : : : : : : : : : : : : 12 4 Group communication 14 4.1 System model : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 15 4.2 Reliable communication : : : : : : : : : : : : : : : : : : : :...
The Load, Capacity and Availability of Quorum Systems (Extended Abstract)
, 1994
"... A quorum system is a collection of sets (quorums) every two of which have a nonempty intersection. Quorum systems have been used for a number of applications in the area of distributed systems. We investigate the load, capacity and availability of quorum systems. A protocol that uses a quorum system ..."
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A quorum system is a collection of sets (quorums) every two of which have a nonempty intersection. Quorum systems have been used for a number of applications in the area of distributed systems. We investigate the load, capacity and availability of quorum systems. A protocol that uses a quorum system S needs to access some quorum occasionally, which causes work to be done by the elements (processors) of the chosen quorum. If the protocol uses a strategy w which picks a quorum S j with probability w j , then the load on an element i is the probability that i is accessed. The load L(S) is defined to be the minimal load on the busiest element, minimizing over the strategies. The capacity Cap(S) is defined to be the highest rate of quorum accesses that the system S can handle. We show that for any quorum system, Cap(S) = 1=L(S). Therefore all the information regarding the capacity is captured by L(S), which further motivates our emphasis on low lo...