To provide high availability for services such as mail or bulletin boards, data must be replicated. One way to guarantee consistency of replicated data is to force service operations to occur in the same order at all sites, but this approach is expensive. For some applications a weaker causal operation order can preserve consistency while providing better performance. This paper describes a new way of implementing causal operations. Our technique also supports two other kinds of operations: operations that are totally ordered with respect to one another, and operations that are totally ordered with respect to all other operations. The method performs well in terms of response time, operation processing capacity, amount of stored state, and number and size of messages; it does better than replication methods based on reliable multicast techniques. This research was supported in part by the National Science Foundation under Grant CCR-8822158 and in part by the Advanced Research Projects ...

To provide high availability for services such as mail or bulletin boards, data must be replicated. One way to guarantee consistency of replicated data is to force service operations to occur in the same order at all sites, but this approach is expensive. In this paper, we propose lazy replication as a way to preserve consistency by exploiting the semantics of the service's operations to relax the constraints on ordering. Three kinds of operations are supported: operations for which the clients define the required order dynamically during the execution, operations for which the service defines the order, and operations that must be globally ordered with respect to both client ordered and service ordered operations. The method performs well in terms of response time, amount of stored state, number of messages, and availability. It is especially well suited to applications in which most operations require only the client-defined order.

Thor is a new object-oriented database management system (OODBMS), intended to be used in heterogeneous distributed systems to allow programs written in different programming languages to share objects in a convenient manner. Thor objects are persistent in spite of failures, are highly likely to be accessible whenever they are needed, and can be structured to reflect the kinds of information of interest to users. Thor combines the advantages of the object-oriented approach with those of database systems: users can store and manipulate objects that capture the semantics of their applications, and can also access objects via queries. Thor is an ongoing project, and this paper is a snapshot: we describe our first design and a partial implementation of that design. This design is primarily concerned with issues related to the implementation of an OODBMS as a distributed system. 1 INTRODUCTION Distributed systems contain different kinds of computers, and users write programs in different...

Thor is a distributed object-oriented database where objects are stored persistently at highly-available servers called object repositories, or ORs. In a large Thor system, performance tuning and system reconfiguration dictate that objects must be able to migrate among ORs. The paper describes two schemes for object references that support object migration, one using location-independent names and the other, location-dependent names. The paper analyzes the performance of the two schemes and concludes that location-dependent names are the right choice for systems like Thor, where we want fast access to objects that have migrated. Categories and Subject Descriptors: H.2.4 [Database Management]: Systems: Distributed systems; C.2.4 [Computer-Communication networks]: Distributed Systems --- Distributed databases; D.4.7 [Operating Systems]: Organization and Design --- Distributed systems; E.2 [Data Storage Representations]: Linked representations; H.2.2 [Database Management]: Physical Desig...

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This paper presents a new asynchronous replication protocol that is especially suitable for wide area and mobile systems, and allows reads and writes to occur at any replica. Updates reach other replicas using a propagation scheme based on nodes organized into a logical hierarchy. The hierarchical structure enables the scheme to scale well for thousands of replicas, while ensuring reliable delivery. A new service interface is proposed that provides different levels of asynchrony, allowing strong consistency and weak consistency to be integrated into the same framework. Further, due to the hierarchical pattern of propagation, the scheme provides the ability to locate replicas that are more up-to-date than others, depending upon the needs of various applications. Also, it allows a selection from a number of reconciliation techniques based on delivery order mechanisms. Restructuring operations are provided to build and reconfigure the hierarchy dynamically without disturbing normal operat...

in multiprocessor systems. Pipes allow a sequence of remote invocations to be performed in order, but asynchronously with respect to the calling thread. Using pipes results in programs that are easier to understand and debug than those with explicit synchronization between asynchronous invocations.