We classify practical replica control (RC) mechanisms according to two main criteria: (1) the abstract RC protocol, e.g., how many physical copies are written for a logical update, and (2) the kind of integration between the RC mechanism and system services such as atomic broadcast, commit protocols, and concurrency control. We describe about a dozen representative RC mechanisms (most of them implemented) classified by their integration with communications, concurrency control, or both. This classification based on a few essential components of RC mechanisms show that practical replicated systems overwhelmingly adopt the read-one/write-one RC protocol (read/write one physical copy for each logical read/write), due to its low overhead and high availability, despite some inherent inconsistency. Furthermore, this trend points to the need for the integration of weak consistency methods (e.g., epsilon serializability) with read-one/write-one RC protocols if replicated databases are to be re...

Voting protocols ensure the consistency of replicated objects by requiring all read and write requests to collect an appropriate quorum of replicas. We propose to replace some of these replicas by volatile witnesses that have no data and require no stable storage, and to regenerate them instead of waiting for recovery. The small size of volatile witnesses allows them to be regenerated much easier than full replicas. Regeneration attempts are also much more likely to succeed since volatile witnesses can be stored on diskless sites. We show that under standard Markovian assumptions two full replicas and one regenerable volatile witness managed by a two-tier dynamic voting protocol provide a higher data availability than three full replicas managed by majority consensus voting or optimistic dynamic voting provided site failures can be detected significantly faster than they can be repaired. Keywords: distributed file systems, replicated data, voting, witnesses. 1. INTRODUCTION Fault-tol...

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 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...

This paper proposes a protocol suite for dynamic replication and migration of Internet objects. It consists of an algorithm for deciding on the number and location of object replicas and an algorithm for distributing requests among currently available object replicas. Our approach attempts to place replicas in the vicinity of a majority of requests while ensuring at the same time that no servers be overloaded. The request distribution algorithm uses the same simple mechanism to take into account both server proximity and load, without actually knowing the latter. The replica placement algorithm executes autonomously on each node, without the knowledge of other object replicas in the system. The proposed algorithms rely on the information available in databases maintained by Internet routers. A simulation study using synthetic workloads and the network backbone of UUNET, one of the largest Internet service providers, shows that the proposed protocol is effective in eliminating hot spots and achieves a significant reduction of backbone traffic and server response time at the expense of creating only a small number of extra replicas. Dynamic Replication on the Internet 2 1 Introduction

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