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. Furthermore, 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

Providing Location Abstract Information in a Ubiquitous Computing Environment

To take full advantage of the promise of ubiquitous computing requires the use of location information, yet peo-ple should have control over who may know their where-abouts. We present an architecture that achieves these goals for an interesting set of applications. Personal in-formation is managed by User Agents, and a partially decentralized Location Query Service is used to facili-tate 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 sys-tem uses two kinds of location information, which turn out to provide partial and complementary coverage.

On modeling and tolerating incorrect software

Distributed systems have to deal with the following scenarios in practice: bugs in components; incorrect specifications of components and, therefore, incorrect use of components; unanticipated faults due to complex interactions or to not containing the effects of faults in lower-level components; and evolution of components. Extant fault tolerance models deal with such scenarios in only a limited manner. In particular, we point out that state corruption is inevitable in practice and that therefore one must accept it and seek to correct it. The well-known concepts of detectors and correctors can be used to find and repair state corruption. However, these concepts have traditionally been employed to immediately detect and correct errors caused by misbehaving system components. Immediate detection and correction is often too expensive to perform and hence we consider the implications of running detectors and correctors only intermittently. More specifically, we address issues that must be dealt with when state corruption may persist within a system for a period of time. We show how to both detect and correct state corruption caused by infrequently occurring “transient ” errors despite the ability for it to actively spread to other parts of the system. We also show how to eventually detect all state corruption, even in cases where continually recurring errors are constantly introducing new state corruption. Finally, we discuss the minimum set of capabilities needed from a trusted base of software in order to guarantee the correctness of our algorithms. This work was partially sponsored by Microsof

Disseminating Active Map Information to Mobile Hosts

Mobile computing differs from desk-top computing because of the dynamic nature of system state: as users move, the sets of stationary and mobile objects they control and the types of information they wish to access change. Navigating a mobile environment can be aided by active maps that describe the location and characteristics of objects within some region as they change over time. We describe an active map service that keeps clients informed of changes in their environment. The primary issue driving our design is the question of scale: an active map service must be able to handle updates and queries over sufficiently large regions of space to satisfy clients' interests and must be able to handle peak loads that can occur when everyone in a region is moving around, for example, to attend a meeting. Our solution detects sets of clients that wish to receive the same active map information and then dynamically assigns multicast groups to them. To guarantee clients that they and the..

health10.fm

ABSTRACT Making effective use of a computer system requires that user needs are matched to system behavior; this in turn requires an understanding of that behavior. However, in distributed computing environments, it is increasingly hard for end users to gain such understandings, both in general and in particular. We have been developing tools to help end users understand the state of their own distributed applications. We present an architecture for managing and sharing information about system state, and show how we exploit this architecture to provide reliable tools that run alongside (or even within) user applications and present users with an view of the behavior and performance of the systems on which they depend

A Reverse Address Resolution Protocol

Status of this Memo This RFC suggests a method for workstations to dynamically find their protocol address (e.g., their Internet Address), when they know only their hardware address (e.g., their attached physical network address). This RFC specifies a proposed protocol for the ARPA Internet community, and requests discussion and suggestions for improvements. I