Ad-hoc wireless networks have no wired component, and may have unpredictable mobility pattern. Such networks can get partitioned and reconnected several times. One possible approach for information dissemination in such networks is to replicate information at multiple nodes acting as repositories, and employ quorum based strategies to update and query information. We propose three such strategies that also use local knowledge about the reachability of repositories to judiciously select quorums. The primary goal is high availability of information in the face of network partitioning. We also consider four policies to determine the appropriate time to perform updates. Experimental results indicate that a hybrid information management strategy and an absolute connectivity-based update trigger policy are most suited for partitionable ad-hoc networks.

The mobile wireless computing environment of the future will contain large numbers of low powered palmtop machines. Replication will be an essential technique in this environment, providing data availability to the system. In a mobile environment it is important to have dynamic replicated data management algorithms that allow for instance copies to migrate from one site to another or for new copies to be generated. In this paper we show that such dynamic algorithms can be obtained simply by letting transaction update the directory that specifies sites holding copies. Thus we argue that no fundamentally new algorithms are needed to cope with mobility. However, exisiting algorithms may have to be "tuned" for a mobile environment, and we discuss what this may entail. As an illustration, we present a variation of the primary copy algorithm, Primary By Row, that is well suited for migrating copies 1 . Keywords: Distributed Data Bases, replication, mobility, availability. 1 Intr...

Voting protocols ensure the consistency of replicated data objects by disallowing all read and write requests that cannot collect the appropriate quorum of replicas. Voting protocols require a minimum number of three copies to be of any practical use and often disallow a relatively high number of read and write requests. We present here a voting protocol overcoming this limitation and providing a significant amount of faulttolerance with as little as two replicas. Voting with Bystanders (VWB), as this protocol is named, applies to all networks consisting of LAN segments that are immune to partial failures linked by gateways that might fail. A stochastic analysis of the protocol under general Markovian assumptions is presented showing that VWB provides excellent read availabilities and good write availabilities with as little as two or three replicas. Keywords: file consistency, distributed file systems, replicated files, voting. 1. INTRODUCTION Redundancy has been widely used to in...

Available copy protocols guarantee the consistency of replicated data objects against any combination of non-Byzantine failures that do not result in partial communication failures. While the original available copy protocol assumed instantaneous detection of failures and instantaneous propagation of this information, more realistic protocols that do not rely on these assumptions have been devised. Two such protocols are investigated in this paper: a naive available copy (NAC) protocol that does not maintain any state information, and an optimistic available copy (OAC) protocol that only maintains state information at write and recovery times. Markov models are used to compare the performance of these two protocols with that of the original available copy protocol. These protocols are shown to perform nearly as well as the original available copy protocol, which is shown to perform much better than quorum consensus protocols.

We 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 new 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. Our 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. 1 Introduction In a distributed computing environment, two types of failures may occur: the processor at a given site may fail (referred to as site failure), and communication between two sites may fail (referred to as communication link failure). When a site fails, processing at...

We propose a highly available replication control protocol tailored to environments where network partitions are always the result of a gateway failure. Our protocol divides nodes holding replicas into local nodes that can communicate directly with each other and non-local nodes that communicate with other nodes through one or more gateways. While local nodes are assumed to remain up to date as long as they don'tcrash, non-local nodes are required to maintain a volatile witness on the same network segment as the local nodes and must poll this witness before answering any user request. To speed up recovery from a total failure, each site maintains a list of replicas that were available the last time the data were updated or a replica recovered from a crash. Markov models are used to compare the performance of our protocol with that of the dynamic-linear voting protocol (DLV), which is the best replication control protocol tolerating communication failures. We also observe that volatile ...

We 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 new 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. Our 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 our algorithm shows that it provides improved availability as compared to previously proposed dynamic voting algorithms. 1 Introduction In a distributed computing environment, two types of failures may occur: the processor at a given site may...

Ad-hocwireless networks have no wiredcomponent, and may have unpredictable mobility pattern. Such networks can get partitioned and reconnected several times. One possible approach for information dissemination in such networks is to replicate information at multiple nodes acting as repositories, and employ quorum based strategies to update and query information. We propose three such strategies that also use local knowledge about the reachability of repositories to judiciously select quorums. The primary goal is high availability of information in the face of network partitioning. We also consider four policies to determine the appropriate time to perform updates. Experimental results indicate that a hybrid information management strategy and an absolute connectivity-basedupdate trigger policy are most suited for partitionable ad-hocnetworks.