... 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 read-one/write-all-available 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 read-one/write-all-available strategy is much simpler to implement and more flexible than quorum-based approaches. In this article, we show that, in addition, it is also the best choice using a number of other selection criteria

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 un-availability 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 un-availability 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...

Since its founding, NASA has been dedicated to the advancement of aeronautics and space science. The NASA Scientific and Technical Information (STI) Program Office plays a key part in helping NASA maintain this important role. The NASA STI Program Office is operated by Langley Research Center, the lead center for NASA's scientific and technical information. The NASA STI Program Office provides access to the NASA STI Database, the largest collection of aeronautical and space science STI in the world. The Program Office is also NASA's institutional mechanism for disseminating the results of its research and development activities. These results are published by NASA in the NASA STI Report Series, which includes the following report types: TECHNICAL PUBLICATION. Reports of completed research or a major significant phase of research that present the results of NASA programs and include extensive data or theoretical analysis. Includes compilations of significant scientific and technical data and information deemed to be of continuing reference value. NASA counterpart of peer-reviewed formal professional papers, but having less stringent limitations on manuscript length and extent of graphic presentations. TECHNICAL MEMORANDUM. Scientific and technical findings that are preliminary or of specialized interest, e.g., quick release reports, working papers, and bibliographies that contain minimal annotation. Does not contain extensive analysis.

In safety critical systems such as brake-by-wire, fault tolerance is usually provided by virtue of having redundant sensors and processing hardware. The redundant information provided by such components should be properly fused to achieve a reliable estimate of the safety critical variable that is sensed or processed by the redundant sensors or hardware. Voting methods are well-known solutions for this category of fusion problems. In this paper, a new voting method, using a fuzzy system for decision-making, is presented. The voted output of the proposed scheme is a weighted average of the sensors signals where the weights are calculated based on the antecedents and consequences of some fuzzy rules in a rulebase. In a case study, we have tested the fuzzy voter along with the well-known majority voting method for a by-wire brake pedal that is equipped with a displacement sensor and two force sensors. Our experimental results show that the performance of the proposed voting method is desirable in the presence of short circuits to ground or supply, excessive noise and sensor drifts. Voting error (in terms of mean square error) is reduced by 82 % by the proposed fuzzy voting method, compared to majority voting.

Although replica control has been the subject of intensive research, it has yet to fulfill its promise in practical applications. A major reason for this lack of acceptance is the complexity of the implementation of these protocols. This paper describes a simple and efficient implementational approach for one such protocols, the quorum consensus protocol. 1 Introduction Replicated data is employed in distributed databases to enhance data availability: multiple copies of a critical data item are maintained on different sites, so that the data item can be retrieved even if some copies of the data item cannot be accessed due to system failures. However, this benefit of data availability is only realized at the cost of elaborate algorithms that hide the underlying complexity of maintaining multiple copies of a single data item. Ideally, an application should not be cognizant of the existence of replicated data, so that it is written as if for a non-replicated database. This criterion is ...

Replicated data is employed in distributed databases to enhance data availability. However, the benefit of data availability is only realized at the cost of elaborate algorithms which hide the underlying complexity of maintaining multiple copies of a single data item. The concern about the performance impact of replica control is part of the reasons why replication, although extensively researched, has yet to receive wide acceptance in practice. This paper makes use of a simulation model to explore the performance tradeoffs of data replication. Keywords: replication control, transaction response time, throughput, data availability. 1 INTRODUCTION Replicated data is employed in distributed databases to enhance data availability: multiple copies of a critical data item are maintained, typically on separate sites, so that the data item can be retrieved even if some copies of the data item cannot be accessed due to system failures. However, this benefit of data availability is only reali...

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