Karin Petersen

Flexible update propagation for weakly consistent replication

Bayous anti-entropy protocol for update propagation between weakly consistent storage replicas is based on pair-wise communication, the propagation of write operations, and a set of ordering and closure constraints on the propagation of the writes. The simplicity of the design makes the protocol very flexible, thereby providing support for diverse networking environments and usage scenarios. It accommodates a variety of policies for when and where to propagate updates. It operates over diverse network topologies, including low-bandwidth links. It is incremental. It enables replica convergence, and updates can be propagated using floppy disks and similar transportable media. Moreover, the protocol handles replica creation and retirement in a light-weight manner. Each of these features is enabled by only one or two of the protocols design choices, and can be independently incorporated in other systems. This paper presents the anti-entropy protocol in detail, describing the design decisions and resulting features.

An overview of the PARCTAB ubiquitous computing experiment

The PARCTAB system integrates a palm-sized mobile computer into an office network. The PARCTAB project serves as a preliminary testbed for ubiquitous computing, a philosophy originating at Xerox PARC that aims to enrich our computing environment by emphasizing context sensitivity, casual interaction and the spatial arrangement of computers. This article describes the ubiquitous computing philosophy, the PARCTAB system, user interface issues for small devices, and our experience in developing and testing a variety of mobile applications.

Session guarantees for weakly consistent replicated data

Four per-session guarantees are proposed to aid users and applications of weakly consistent replicated data: read your writes, monotonic reads, writes follow reads, and monotonic writes. The intent is to present individual applications with a view of the database that is consistent with their own actions, even if they read and write from various, potentially inconsistent servers. The guarantees can be layered on existing systems that employ a read-any/write-any replication scheme while retaining the principal benefits of such a scheme, namely high availability, simplicity, scalability, and support for disconnected operation. These session guarantees were developed in the context of the Bayou project at Xerox PARC in which we are designing and building a replicated storage system to support the needs of mobile computing users who may be only intermittently connected.<<ETX>>

The Bayou Architecture: Support for Data Sharing Among Mobile Users

The Bayou System is a platform of replicated, highly-available, variable-consistency, mobile databases on which to build collaborative applications. This paper presents the preliminary system architecture along with the design goals that influenced it. We take a fresh, bottom-up and critical look at the requirements of mobile computing applications and carefully pull together both new and existing techniques into an overall architecture that meets these requirements. Our emphasis is on supporting application-specific conflict detection and resolution and on providing application controlled inconsistency.

The Parctab Ubiquitous Computing Experiment

The ParcTab system integrates a palm-sized mobile computer into an office network. This project serves as a preliminary testbed for Ubiquitous Computing, a philosophy originating at Xerox PARC that aims to enrich our computing environment by emphasizing context sensitivity, casual interaction and the spatial arrangement of computers. This paper describes the Ubiquitous Computing philosophy, the ParcTab system, user-interface issues for small devices, and our experience developing and testing a variety of mobile applications.

Designing and implementing asynchronous collaborative applications with Bayou

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 Bayou architecture.

Bayou: replicated database services for world-wide applications

The Bayou architecture provides scalability, availability, extensibility, and adaptability features that address database storage needs of world-wide applications. In addition to discussing these features, this paper presents Bayous mechanisms for permitting the replicas of a database to vary dynamically without global coordination. Key is the use of weak consistency replication among autonomous machines and strict adherence to the tenet that no operation should involve more than two machines.

Dealing with server corruption in weakly consistent, replicated data systems

Providing high availability and the ability to share data despite the weak connectivity of mobile computing raises the problem of trusting replicated data servers that may be corrupt. This is because servers must be run on portable computers, and these machines are less secure and thus less trustworthy than those traditionally used to run servers. We describe the kinds of problems one must be prepared to deal with, noting that even users of secured, non‐portable computers are at risk if servers trust all authorized peers. We show that high availability through data replication on portable computers need not be mutually exclusive with various levels of data security one might want. We give three solutions to this trust problem for a simple example architecture, achieving progressively higher levels of security with progressively higher costs. We then show how to solve this trust problem for the more complex architecture of Bayou, a weakly consistent replicated data system we built at Xerox PARC.

A programming model for active documents

Traditionally, designers organize software system as active end-points (e.g. applications) linked by passive infrastructures (e.g. networks). Increasingly, however, networks and infrastructures are becoming active components that contribute directly to application behavior. Amongst the various problems that this presents is the question of how such active infrastructures should be programmed. We have been developing an active document management system called Placeless Documents. Its programming model is organized in terms of properties that actively contribute to the functionality and behavior of the documents to which they are attached. This paper discusses active properties and their use as a programming model for active infrastructures. We have found that active properties enable the creation of persistent, autonomous active entities in document systems, independent of specific repositories and applications, but present challenges for managing problems of composition.

Dealing with Tentative Data Values in Disconnected Work Groups

This paper describes a problem of weakly-consistent replicated data systems used in support of disconnected groups of people. The problem concerns actions and updates derived from tentative data updates that are ultimately determined to be in conflict. While some such actions and updates can be automatically resolved, many require human intervention. FurthemOre, although some file and database systems support internal conflict detection and resolution. derived actions may be external to those systems, implying that human users must ensure that proper consistency is maintained between independent components of the system. The entire problem becomes exascerbated when disconnected work groups are taken into account, where tentative data values may be seen and acted upon by multiple people.