Bayou is a replicated, weakly consistent storage system designed for a mobile computing environment that includes portable machines with less than ideal network connectivity. To maximize availability, users can read and write any accessible replica. Bayou's design has focused on supporting apphcation-specific mechanisms to detect and resolve the update conflicts that naturally arise in such a system, ensuring that replicas move towards eventual consistency, and defining a protocol by which the resolution of update conflicts stabilizes. It includes novel methods for conflict detection, called dependency checks, and per-write conflict resolution based on client-provided merge procedures. To guarantee eventual consistency, Bayou servers must be able to rollback the effects of previously executed writes and redo them according to a global senalization order. Furthermore, Bayou permits clients to observe the results of all writes received by a server, Including tentative writes whose conflicts have not been ultimately resolved. This paper presents the motivation for and design of these mechanisms and describes the experiences gained with an initial implementation of the system.

this paper, we describe a new information management service called Astrolabe. Astrolabe monitors the dynamically changing state of a collection of distributed resources, reporting summaries of this information to its users. Like DNS, Astrolabe organizes the resources into a hierarchy of domains, which we call zones to avoid confusion, and associates attributes with each zone. Unlike DNS, zones are not bound to specific servers, the attributes may be highly dynamic, and updates propagate quickly; typically, in tens of seconds

Pangaea is a wide-area file system that supports data sharing among a community of widely distributed users. It is built on a symmetrically decentralized infrastructure that consists of commodity computers provided by the end users. Computers act autonomously to serve data to their local users. When possible, they exchange data with nearby peers to improve the system's overall performance, availability, and network economy. This approach is realized by aggressively creating a replica of a file whenever and wherever it is accessed. This paper presents

Asynchronous collaboration is characterized by the degree of independence collaborators have from one another. In particular, collaborators working asynchronously typically have little need for frequent and fine-grained coordination with one another, and typically do not need to be notified immediately of changes made by others to any shared artifacts they are working with. We present an infrastructure, called Bayou, designed to support the construction of asynchronous collaborative applications. Bayou provides a replicated, weakly-consistent, data storage engine to application writers. The system supports a number of mechanisms for leveraging application semantics; using these mechanisms, applications can implement complex conflict detection and resolution policies, and choose the level of consistency and stability they will see in their databases. We present a number of applications we have built or are building using the Bayou system, and examine how these take advantage of the Bayo...

The number of users connected to the Internet has been growing at an exponential rate, resulting in similar increases in network traffic and Internet server load. Advances in microprocessors and network technologies have kept up with growth so far, but we are reaching the limits of hardware solutions. In order for the Internet's growth to continue, we must efficiently distribute server load and reduce the network traffic generated by its various services. Traditional wide-area caching schemes are client initiated. Decisions on where and when to cache information are made without the benefit of the server's global knowledge of the situation. We introduce a technique---push- caching---that is server initiated; it leaves caching decisions to the server. The server uses its knowledge of network topology, geography, and access patterns to minimize network traffic and server load. The World Wide Web is an example of a large-scale distributed information system that will benefit from this ge...

Epidemic protocols have been heralded as appropriate for wireless sensor networks. The nodes in such networks have limited battery resources. In this paper, we investigate the use of power in three styles of epidemic protocols: basic epidemics, neighborhood flooding epidemics, and hierarchical epidemics. Basic epidemics turn out to be highly power hungry, and are not appropriate for power-aware applications. Both neighborhood and hierarchical epidemics can be made to use power judiciously, but a trade-off exist between scalability and latency.

s Service Paul Taylor October 1995 Distributed Systems Group Department of Computer Science University of Dublin Trinity College, Dublin 2, Ireland. Fax: +353-1-6772204 Tel: +353-1-6081531 Email: pftaylor@dsg.cs.tcd.ie Abstract CaberNet is the ESPRIT network of excellence in distributed systems consisting of several European research groups. CaberNet has industrial affiliates who receive regular information about the research activities of CaberNet members. Most CaberNet members produce technical reports which are of interest to the industrial affiliates and other researchers worldwide. This document describes the design and implementation of a unified technical report service. A contributing CaberNet site just has to make a bibliography available on a local machine. This bibliography is retrieved by a central site and any new or revised records are placed into a database. These records are processed by a professional library cataloguer who ensures that the information is relevant, ...

Optimistic replication allows mobile workspaces to remain accessible during interrupted network access, but require consecutive handling of any conflicts. When conflict handling requires manual intervention by the user, the system must not require that conflicts are resolved immediatley after their detection. We present a log-based algorithm for reconciling changes to replicated data that supports deferred manual conflict resolution, while sharing the favorable scalability properties of log-based approaches of small storage and communication overhead. Simulation results validate our design and show that it compares favorably in the relevant metrics with version vector-based designs.

Wireless access is increasingly ubiquitous while mobile devicesthatusethemareresourcerich.Thesetrends allow wireless users to collaborate with each other. We investigate various group communication paradigms that underly collaboration applications. We synthesize durations when members collaborate using wireless device availability traces. Wireless users operate from a variety of locations. Hence, we analyzed the behavior of wireless users in universities, corporations, conference venues, and city-wide hotspots. We show that the availability durations are longer in corporations followed by university and then in hotspots. The number of simultaneously available wireless users is small in all the scenarios. The session lengths are becoming smaller while the durations between sessions are becoming larger. We observed user churn in all the scenarios. We show that synchronous mechanisms require less effort to maintain update synchronicity among the group members. However, distributed mechanisms require a large number of replicas in order to propagate updates among the users. For asynchronous mechanisms, we show that pull-based mechanisms naturally randomize the times when updates are propagated and thus achieve better performance than push based mechanisms. We develop an adaptive approach that customizes the update frequencyusingthelastsessiondurationandshow that this mechanism exhibits goodperformancewhentherequiredupdatefrequencyintervalsarelarge.We also show that for a given number of gossips, it is preferable to propagate updates to all available nodes rather than increasing the frequency while correspondingly reducing the number of nodes to propagate updates. We develop a middleware to illustrate the practicality of our approach. Keywords: Wireless LAN, group communication mechanisms