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Management of Replicated Data, 1992., Second Workshop on the

Date 12-13 Nov. 1992

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  • Second Workshop on the Management of Replicated Data (Cat. No.92TH0489-5)

    Publication Year: 1992
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    Freely Available from IEEE
  • Strong, weak and hybrid group membership

    Publication Year: 1992 , Page(s): 34 - 38
    Cited by:  Papers (5)
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    Discusses the group membership problem in distributed systems. The paper describes a set of group membership protocols that are developed as part of a toolkit for building distributed parallel applications on a cluster of workstations. The group membership service is the lowest layer in the toolkit, and it is the glue which unifies all the other layers. The group membership layer supports three protocols: weak, strong, and hybrid membership. These protocols differ significantly in the level of consistency and the number of messages exchanged in reaching agreement. This paper briefly describes each protocol with a focus on what is new about them and where they might be used View full abstract»

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  • Using volatile witnesses to extend the applicability of available copy protocols

    Publication Year: 1992 , Page(s): 30 - 33
    Cited by:  Papers (4)
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    This paper presents a new replication control protocol tailored to environments where network partitions can only occur at a few well-defined partition points and are always the results of a gateway failure. The protocol implements the same `write all read-one' rule as the available copy (AC) protocol. Unlike the AC protocol, this protocol divides servers holding copies into local servers that can communicate directly with each other and non-local servers that communicate with other servers through one or more gateways. While replicas stored on local servers are assumed to remain up to date as long as their server remains operational, replicas stored on non-local servers are required to maintain one or more volatile witnesses on the same LAN segment as the local servers and need to interrogate one of these witnesses before answering any user request View full abstract»

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  • I/O performance of fully-replicated disk systems

    Publication Year: 1992 , Page(s): 68 - 71
    Cited by:  Patents (2)
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    Mirrored disk storage is an accepted technique to enhance fault-tolerance of data through complete replication. Recent research suggested that in addition to higher reliability, mirrored disks can offer better I/O performance by either allowing parallel reads or by reducing the seek time in cases where shortest seek distance algorithms can be used. Accurate analysis of mirrored disk systems shows that there is a significant correlation between the distributions of the disk heads positions, which makes them both interdependent and non-uniform. Furthermore, after each write all disk heads move to the same position to form what the authors call a `bundle'. Such phenomenon may seriously deteriorate system performance. Subsequent reads gradually break up the bundles. Finally, under low rates of request arrivals, the authors provide analytical expressions for optimal anticipation points for two and three mirrored disks under general read/write combinations. Their work makes the following contributions: (1) new heuristics for treating the `bundling phenomenon' adaptively, for applications with unknown read/write ratios; (2) a new technique that improves the expected seek time and workload balancing by using `shifted mirroring' View full abstract»

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  • Protocol modularity in systems for managing replicated data

    Publication Year: 1992 , Page(s): 78 - 81
    Cited by:  Papers (2)
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    The authors describe their experience in attempting to modularize Consul, a fault-tolerant system for managing replicated data. Their experience is that, while modularity is needed to simplify the design and implementation of such systems, protocol dependencies, both direct and indirect, impact the way in which protocols are designed and implemented. They also identify certain operating system features that simplify such a design. Based on this experience, work is in progress to develop a new model for fault-tolerant protocols that will facilitate modularization View full abstract»

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  • Some consequences of excess load on the Echo replicated file system

    Publication Year: 1992 , Page(s): 92 - 95
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    Understanding the workload is crucial for the success of a replicated file system. The system must continue to provide service in the presence of excess load, otherwise, its availability will be compromised. This paper presents some shortcomings of the Echo replicated file system in handling excess load. The authors discuss the consequences of excess load on primary secondary design, on their communication software, on election algorithm, on memory usage in their file servers, and on their distributed client caching algorithm. They speculate on possible solutions View full abstract»

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  • Replication in an information filtering system

    Publication Year: 1992 , Page(s): 66 - 67
    Cited by:  Patents (6)
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    In the Tapestry system developed at Xerox PARC, users provide queries to filter incoming streams of documents. These queries run continuously over a growing database of electronic mail messages, news articles, and other textual documents. In the current implementation, all filter queries for all users run on a single database server. Those documents that match a user's filter query (or queries) are queued up for the user and can be retrieved directly via an RPC interface or, as is typically the case, can be sent to the user via electronic mail. This paper shows that the design of a distributed information filtering service involves challenges not faced in other distributed applications. Replication is needed, not for fault-tolerance or performance but simply for scalability. Of course, once replication is provided, it can be used to increase the fault-tolerance of the system (with some additional work). A new technique called filter-based replication is proposed for deciding what to replicate and where View full abstract»

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  • Efficient recovery in Harp [replicated Unix file system]

    Publication Year: 1992 , Page(s): 104 - 106
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    Harp is a replicated Unix file system accessible via the VFS interface. It provides highly available and reliable storage for files and guarantees that file operations are executed atomically in spite of concurrency and failures. Replication enables Harp to safely trade disk accesses for network communication and thus to provide good performance both during normal operation and during recovery. The authors focus on the techniques Harp uses to achieve efficient recovery View full abstract»

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  • High availability is not enough [distributed systems]

    Publication Year: 1992 , Page(s): 40 - 43
    Cited by:  Papers (3)
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    The author mainly concentrates on transactional distributed systems. Most previous research on replication in such environments has concentrated in employing replication to achieve high availability. The position is that high availability along is not enough. First, it is important to consider the cost of providing high availability through replication. Second, one must exploit the potential of replication as a means of improving performance. Then performance issues (in addition to availability) in which one is mostly interested are: transaction latency, bottlenecks and throughput, and scalability (in particular as it affects the former issues). The author briefly outlines his related research efforts which can be classified in the following areas: replication-control protocols, recovery strategies, and studying availability in large-scale distributed systems View full abstract»

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  • An algorithm for dynamic data distribution

    Publication Year: 1992 , Page(s): 62 - 65
    Cited by:  Papers (9)
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    The replication scheme of a distributed database determines how many replicas of each object are created, and to which processors these replicas are allocated. This scheme critically affects the performance of a distributed system, since reading an object locally is less costly than reading it from a remote processor. Therefore in a read-intensive network a widely distributed replication is mandated. On the other hand, an update of an object is usually written to all, or a majority of the replicas, and therefore in a write-intensive network a narrowly distributed replication is mandated. In other words, the optimal replication scheme depends on the read-write pattern for each object. The authors propose a practical algorithm, called dynamic-data-allocation (DDA), that changes the replication scheme of an object (i.e. the processors which store a replica of the object) dynamically as the read-write pattern of the object changes in the network. They assume that the changes in the read-write pattern are not known a priori View full abstract»

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  • An interface to support lazy replicated file service

    Publication Year: 1992 , Page(s): 6 - 8
    Cited by:  Papers (2)  |  Patents (1)
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    The authors argue that, if they are going to assume low-sharing workloads, then it is possible to build more efficient implementations that provide the same (or stronger) semantics by making fullest use of the assumption. Accordingly, they take an extreme point on the spreading/searching tradeoff: the searcher is made responsible for all the work. Thus, spreading can be made asynchronous and hence very fast from the caller's viewpoint. They employ an important optimization to eliminate most of the searching work when there is little sharing. Their technique carries other advantages as well: simple and flexible recovery algorithms, and suitability for mobile computing View full abstract»

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  • Highly-available services using the primary-backup approach

    Publication Year: 1992 , Page(s): 47 - 50
    Cited by:  Papers (37)
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    The authors derive lower bounds and the corresponding optimal protocols for three parameters for synchronous primary-backup systems. They compare their results with similar results for active replication in order to determine whether the common folklore on the virtues of the two approaches can be shown formally. They also extend some of their results to asynchronous primary-backup systems. They implement an important subclass of primary-backup protocols that they call 0-blocking. These protocols are interesting because they introduce no additional protocol related delay into a failure-free service request. Through implementing these protocols the authors hope to determine the appropriateness of their theoretical system model and uncover other practical advantages or limitations of the primary-backup approach View full abstract»

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  • Replicating the procedure call abstraction

    Publication Year: 1992 , Page(s): 86 - 89
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    The authors recommend replicating the procedure call abstraction as a method for constructing highly available distributed programs. In order to make highly available systems much more widespread than they are today, one must make it easier for application developers to incorporate replication into one's systems. Given that remote procedure call has proven to be a useful abstraction in building distributed programs, replicated procedure call seems to be an appropriate method for introducing high availability while hiding the complexities of replication. The authors argue that the simplicity and familiarity of the procedure call mechanism makes it an excellent model for introducing replication, and they discuss different choices one can make in designing a replicated procedure call system View full abstract»

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  • An integrated approach to fault tolerance

    Publication Year: 1992 , Page(s): 82 - 85
    Cited by:  Papers (1)
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    Describes Manetho, an experimental protocol system, whose goal is to explore the extent to which transparent fault tolerance can be added to long-running distributed applications. Transparent techniques are attractive because they can automatically add fault tolerance to existing applications that were written without consideration for reliability. Previous techniques for providing transparent fault-tolerance relied on rollback-recovery. However, rollback recovery is not appropriate for server processes where the lack of service during rollback is intolerable. Furthermore, rollback-recovery assumes that a process can be restarted on any available host. As a result, extended downtime cannot be tolerated for example in file servers, which have to run on the host where the disks reside. Manetho solves these problems with an integrated approach by using process replication for server processes and rollback-recovery for client processes View full abstract»

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  • A replicated monitoring tool

    Publication Year: 1992 , Page(s): 96 - 99
    Cited by:  Papers (3)
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    Modeling the reliability of distributed systems requires a good understanding of the reliability of the components. Careful modeling allows highly fault-tolerant distributed applications to be constructed at the least cost. Realistic estimates can be found by measuring the performance of actual systems. An enormous amount of information about system performance can be acquired with no special privileges via the Internet. A distributed monitoring tool called a tattler is described. The system is composed of a group of tattler processes that monitor a set of selected hosts. The tattlers cooperate to provide a fault-tolerant distributed data base of information about the hosts they monitor. They use weak-consistency replication techniques to ensure their own fault-tolerance and the eventual consistency of the data base that they maintain View full abstract»

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  • Computing replica placement in distributed systems

    Publication Year: 1992 , Page(s): 58 - 61
    Cited by:  Papers (6)  |  Patents (31)
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    The authors investigate the design of a replica management system (RMS) which allows a programmer to specify the quality of service required for individual replicated objects in terms of availability and performance. From the quality of service specification, information about the replication protocol to be used, and data about the characteristics of the underlying distributed system, the RMS computes an initial placement and replication level for the object. As machines and communications systems are detected to have failed or recovered, the RMS can be reinvoked to compute an updated mapping of replicas which preserves the desired quality of service. Preliminary simulation of the authors RMS shows that its placement algorithm gives consistent improvements in the availability of a replicated service than simply placing replicas on nodes at random, as is done in most distributed systems View full abstract»

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  • Bounds on the effects of replication on availability

    Publication Year: 1992 , Page(s): 44 - 46
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    The author discusses some theoretical limitations on the potential benefits of replication. In particular, he investigates two fundamental questions: (1) does placing copies of data around a network increase the probability that the data will be available, and (2) does such a technique decrease the mean duration of unavailability of the data. Given that many applications require mutually exclusive access to the data, he shows that the potential benefits of replication are rather low with respect to both of these metrics. Although these results are not necessarily surprising, it is interesting that the proofs are protocol independent and, in the case of the availability measure, topology independent. They are useful, therefore, in focusing attention on the replica consistency model and performance measures and away from any particular protocol or network configuration View full abstract»

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  • Primarily disconnected operation: experiences with Ficus

    Publication Year: 1992 , Page(s): 2 - 5
    Cited by:  Papers (21)  |  Patents (9)
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    Ficus is a flexible replication facility with optimistic concurrency control designed to span a wide range of scales and network environments. Support for partitioned operation is fundamental to the Ficus design but was not widely exercised in early Ficus use. This paper reports recent experiences using Ficus in settings where some replicas are only occasionally connected to a network, and hence partitioned operation is the rule rather than the exception. The authors conclude that with some tuning, Ficus adapted quite well to primarily disconnected operation View full abstract»

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  • An object-oriented approach for replication management

    Publication Year: 1992 , Page(s): 74 - 77
    Cited by:  Papers (1)
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    One important bottleneck in research on replication management is the time needed for implementing algorithms in order to validate them. The author's approach, based on the fragmented object model, encourages re-usability of distributed abstractions for many replication management algorithms. He presents high-level building blocks for various replication protocols, ensuring different policies of consistency, replicated data management and failure handling View full abstract»

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  • Replication and mobility

    Publication Year: 1992 , Page(s): 9 - 12
    Cited by:  Papers (21)
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    Mobility of users and services and its impact on data replication and migration will be one of the main technical problems to be resolved. In fact, the authors view mobility as probably the most challenging new issue facing distributed systems of the future. The distinct new element which mobility brings into the issues of data replication is system's uncertainty about its own state (meaning primarily location). Thus the cost of communication between two mobile users is dependent not only on the distance between them but also on the cost of search necessary to determine the exact value of location. The authors address some of the questions, by proposing an adaptive replication scheme for mobile data View full abstract»

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  • Implementation and performance of cluster-based file replication in large-scale distributed systems

    Publication Year: 1992 , Page(s): 100 - 103
    Cited by:  Papers (1)  |  Patents (3)
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    The authors describe the implementation and an initial performance study of Frolic, a cluster-based dynamic file replication system. This system specifically targets large scale commercial environments with extensive file sharing, although it is expected to also perform well in systems with limited file sharing as well. Instead of keeping copies of a widely shared file at each client workstation, these files are dynamically replicated onto the cluster file servers, so that they become locally available. The costs of maintaining consistency among the limited number of server copies should be much less than that among a large number of clients. The problems of network and server congestion as well as that of high access latency may be alleviated. Simulation studies using a statistical workload have shown that cluster-based file replication can significantly reduce file access times for a wide range of workload parameters View full abstract»

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  • Weak consistency group communication for wide-area systems

    Publication Year: 1992 , Page(s): 13 - 16
    Cited by:  Papers (4)  |  Patents (2)
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    Replicated services can be implemented as process groups. Member processes use group communication protocols to communicate amongst themselves and group membership protocols to determine what processes are in the group. These protocols can provide various levels of consistency between members. The author investigates weak consistency protocols that guarantee that messages are delivered to all members, but do not guarantee when. He reports on a new family of communication protocols, an associated group membership mechanism, and current progress in evaluating their efficiency and utility for real applications View full abstract»

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  • The case for independent updates

    Publication Year: 1992 , Page(s): 17 - 19
    Cited by:  Patents (5)
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    The authors present the case for allowing independent updates on replicated databases. In autonomous, heterogeneous, or large scale systems, using two-phase commit for updates may be infeasible. Instead, the authors propose that a site may perform updates independently. Sites that are available can receive these updates immediately. But sites that are unavailable, or otherwise do not participate in the update transaction receive these updates later through propagation, rather than preventing the execution of the update transaction until sufficient sites can participate. Two or more sites come to agreement using a reconciliation procedure that uses reception vectors to determine how much of the history log should be transferred from one site to another. They also consider what events can initiate a reconciliation procedure View full abstract»

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  • An analysis of replica control

    Publication Year: 1992 , Page(s): 22 - 25
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    Early replica control work focused on maintaining the availability of an object despite the unavailability of its copies. More recent work has focused on improving the response time, autonomy, and scalability of the earlier work while preserving their availability characteristics. The authors analyze the several ways the new algorithms make such improvements View full abstract»

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  • Dynamic logical structures: a position statement for managing replicated data

    Publication Year: 1992 , Page(s): 26 - 29
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    The authors discuss extensions to the grid protocol to improve the fault-tolerance of write operations by using the notions of structured read and write grid quorums. As is the case in the standard quorum protocols, the increased fault-tolerance for write operations is at the increased cost of executing read operations. In order to let users continue using the analogues of the read-one-write-all protocol in the context of a logical structure, they develop reconfiguration protocols for dynamically adapting to failures and recovery. This results in the following dichotomy. Users accesses are through the simple analogues of the read-one-write-all protocol with respect to a logical structure and therefore have low communication cost for read operations. On the other hand, the reconfiguration protocol uses the notion of quorums in the context of a logical structure to ensure high data availability. A similar approach can be applied to the tree protocol View full abstract»

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