Marvin M. Theimer

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.

Providing Location Information in a Ubiquitous Computing Environment

To take full advantage of the promise of ubiquitous computing requires the use of location information, yet people should have control over who may know their whereabouts. We present an architecture that achieves these goals for an interesting set of applications. Personal information is managed by User Agents, and a partially decentralized Location Query Service is used to facilitate location-based operations. This architecture gives users primary control over their location information, at the cost of making more expensive certain queries, such as those wherein location and identity closely interact. We also discuss various extensions to our architecture that offer users additional trade-offs between privacy and efficiency. Finally, we report some measurements of the unextended system in operation, focusing on how well the system is actually able to track people. Our system uses two kinds of location information, which turn out to provide partial and complementary coverage.

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.

Architectural considerations for scalable, secure, mobile computing with location information

To take full advantage of the promise of mobile/ubiquitous computing requires the availability and use of location information about the various physical objects and persons in an environment. At the same time, indiscriminant use of location information for people can result in an invasion of privacy and provides the potential for abuse by third parties. In this article we describe a suite of useful location-based applications and discuss architectural considerations for supporting them in a scalable, secure fashion.<<ETX>>

Operating system issues for PDAs

Personal digit assistants (PDAs) are small, mobile computing devices whose functionality ranges somewhere between that of an electronic organizer and that of a full-function portable computer. Since PDAs are too small in size to be used for extended work sessions they must justify their existence mostly by providing services that are tailored to people on the go. However, we observe that PDAs are also starting to acquire the ability to communicate and believe that in the long run much of their value will come from being able to interact with external services and devices. The goal of the paper is to relate a view of the ways in which operating systems for this new breed of small ubiquitous computing device will be different from the operating systems of today. The viewpoint is based partly on thoughts about what a ubiquitous computing world will look like and partly on experience with building and using various parts of a ubiquitous computing testbed. In particular, ParcTab computers and ParcPad notebook computers. Both are stylus-based and have onboard communications facilities.<<ETX>>

Finding idle machines in a workstation-based distributed system

The authors describe the design and performance of scheduling facilities for finding idle hosts in a workstation-based distributed system. They focus on the tradeoffs between centralized and decentralized architectures with respect to scalability, fault tolerance, and simplicity of design, as well as several implementation issues of interest when multicast communication is used. They conclude that the principal tradeoff between the two approaches is that a centralized architecture can be scaled to a significantly greater degree and can more easily monitor global system statistics whereas a decentralized architecture is simpler to implement. >

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.

Providing location information in a ubiquitous computing environment (panel session)

To take full advantage of the promise of ubiquitous computing requires the use of location information, yet people should have control over who may know their whereabouts. We present an architecture that achieves these goals for an interesting set of applications. Personal information is managed by User Agents, and a partially decentralized Location Query Service is used to facilitate location-based operations. This architecture gives users primary control over their location information, at the cost of making more expensive certain queries, such as those wherein location and identity closely interact. We also discuss various extensions to our architecture that offer users additional trade-offs between privacy and efficiency. Finally, we report some measurements of the unextended system in operation, focusing on how well the system is actually able to track people. Our system uses two kinds of location information, which turn out to provide partial and complementary coverage.

Heterogeneous process migration by recompilation

An approach to heterogeneous process migration that involves building a machine-independent migration program that specifies the current code and data state of the process to be migrated is described. When this program is compiled and executed on the target machine, it will first reconstruct the processs state and then continue the normal execution of the now-migrated process. The principal advantage of this approach is that it hides the details of code and data translation in the compilers for each machine.<<ETX>>