Voting Algorithms Research Articles

Regeneration with virtual copies for distributed computing systems

The authors consider the consistency control problem for replicated data in a distributed computing system (DCS) and propose a new algorithm to dynamically regenerate copies of data objects in response to node failures and network partitioning in the system. The DCS is assumed to have strict consistency constraints for data object copies. The algorithm combines the advantages of voting-based algorithms and regeneration mechanisms to maintain mutual consistency of replicated data objects in the case of node failures and network partitioning. The algorithm extends the feasibility of regeneration to DCS on wide area networks and is able to satisfy user queries as long as there is one current partition in the system. A stochastic availability analysis of the algorithm shows that it provides improved availability as compared to previously proposed dynamic voting algorithms. >

Architectural support for designing fault-tolerant open distributed systems

An overview of the main techniques for designing fault-tolerant software and hardware systems is provided. The important features of the building blocks (computers, memories, buses, etc.) that can support an efficient implementation of fault-tolerant open distributed systems (FTODSs) are identified. Taking into account the features of these building blocks, an organization for FTODS is proposed. A distributed voting algorithm and a two-level hierarchy for permanent memory are key elements in this scheme. The algorithms needed for transferring files and synchronizing the concurrent activities of the computing modules and for recovery-are ISO standard protocols. Low-level voting and recovery algorithms that can run as a layer of software above the operating system make the open distributed system an attractive environment for applying fault-tolerant techniques.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dynamic voting algorithms for maintaining the consistency of a replicated database

There are several replica control algorithms for managing replicated files in the face of network partitioning due to site or communication link failures. Pessimistic algorithms ensure consistency at the price of reduced availability; they permit at most one (distinguished) partition to process updates at any given time. The best known pessimistic algorithm, voting , is a “static” algorithm, meaning that all potential distinguished partitions can be listed in advance. We present a dynamic extension of voting called dynamic voting . This algorithm permits updates in a partition provided it contains more than half of the up-to-date copies of the replicated file. We also present an extension of dynamic voting called dynamic voting with linearly ordered copies (abbreviated as dynamic-linear ). These algorithms are dynamic because the order in which past distinguished partitions were created plays a role in the selection of the next distinguished partition. Our algorithms have all the virtues of ordinary voting, including its simplicity, and provide improved availability as well. We provide two stochastic models to support the latter claim. In the first (site) model, sites may fail but communication links are infallible; in the second (link) model the reverse is true. We prove that under the site model, dynamic-linear has greater availability than any static algorithm, including weighted voting, if there are four or more sites in the network. In the link model, we consider all biconnected five-site networks and a wide variety of failure and repair rates. In all cases considered, dynamic-linear had greater availability than any static algorithm.

Probabilistic language analysis of weighted voting algorithms

We present a method of analyzing the performance of weighted voting algorithms in a fault-tolerant distributed system. In many distributed systems, some processors send messages more frequently than others and all processors share a common communication medium, such as an Ethernet. Typical fault-tolerant voting algorithms require that a certain minimum number of votes be collected from different processors. System performance is significantly affected by the time required to collect those votes. We formulate the problem of weighted voting in terms of probabilistic languages and then use the calculus of generating functions to compute the expected delay to collect that number of votes. An application of the method to a particular voting algorithm, the Total protocol, is given.

Constructing replicated systems using processors with point-to-point communication links

Replicated processing with majority voting is a well known method of achieving fault tolerance. We consider the problem of constructing a distributed system composed of an arbitrarily large number of N-modular redundant (NMR) nodes, where each node itself is composed of N, N = 2m + 1 and m ≥ 1, processing and voting elements. Advanced microprocessors, such as Inmos Transputers, provide fast serial communication links for inter-processor communication, making it possible to construct large networks of processors. We describe how replicated processing with majority voting can be achieved for such processor networks. This paper will present the overall systems architecture, including voting and NMR synchronization algorithms specially developed to exploit fast point to point communication facilities.

A Dynamic majority determination algorithm for reconfiguration of network partitions

We present a conservative consistency and recovery control algorithm for replicated files in the presence of network partitioning due to communication link failures. This algorithm supports partial replication, provides nonblocking operations by allowing update access to a file in that file's majority partition, and brings all copies up to date on all sites whenever the communication links among them are repaired. This algorithm belongs to the class of dynamic voting algorithms proposed in the recent literature. When the communication link among some partitions is reestablished, the algorithms proposed so far do not always allow the merge (reconciliation) of these partitions to form a single partition. A merge condition has to be satisfied to avoid possible inconsistencies. This is undesirable because in a system with more than one replicated file, two or more partitions cannot be integrated to form a single partition if any one of the replicated fields in these partitions does not satisfy the merge condition. This restriction might cause the system to remain partitioned for a long time even if communication links are repaired. (In the previous papers, such a problem is not addressed, since a system with only one replicated file is assumed.) The algorithm proposed in this paper avoids any such merge condition and integrates the partitions whenever a communication link failure is repaired, thus providing a higher degree of availability. This work formalizes the presentation of algorithms and data structures for implementation.

The development of multiple redundant flight control systems for high integrity applications

Multiple redundant flight control systems have been the subject of continuous development since the 1950s. Early examples include the automatic landing systems of the BAC 1–11 and the VC10; these were followed by the full flight envelope control systems fitted to the Concorde and Tornado. These examples all use analogue computing techniques, with redundancy levels appropriate to the individual fail-passive or fail-operational requirements of each system.However, recent advances in digital technology have enabled highly sophisticated flight control systems to be implemented which would be wholly impractical using analogue computing methods. Furthermore, digital computing is amenable to self-monitoring techniques which can be used not only to improve system maintainability, but also to augment the cross comparison or majority voting algorithms which have been developed on analogue systems.