A program denotes computations in some universe of objects. Abstract interpretation of programs consists in using that denotation to describe computations in another universe of abstract objects, so that the results of abstract execution give some information on the actual computations. An intuitive example (which we borrow from Sintzoff [72]) is the rule of signs. The text -1515 * 17 may be understood to denote computations on the abstract universe {(+), (-), (±)} where the semantics of arithmetic operators is defined by the rule of signs. The abstract execution -1515 * 17 → -(+) * (+) → (-) * (+) → (-), proves that -1515 * 17 is a negative number. Abstract interpretation is concerned by a particular underlying structure of the usual universe of computations (the sign, in our example). It gives a summary of some facets of the actual executions of a program. In general this summary is simple to obtain but inaccurate (e.g. -1515 + 17 → -(+) + (+) → (-) + (+) → (±)). Despite its fundamentally incomplete results abstract interpretation allows the programmer or the compiler to answer questions which do not need full knowledge of program executions or which tolerate an imprecise answer, (e.g. partial correctness proofs of programs ignoring the termination problems, type checking, program optimizations which are not carried in the absence of certainty about their feasibility, …).

/pdf/abstract-interpretation-a-unified-lattice-model-for-static-2jfaix0dre.pdf

Abstract interpretation: a unified lattice model for static analysis of programs by construction or approximation of fixpoints

The concept of an agent has become important in both Artificial Intelligence (AI) and mainstream computer science. Our aim in this paper is to point the reader at what we perceive to be the most important theoretical and practical issues associated with the design and construction of intelligent agents. For convenience, we divide these issues into three areas (though as the reader will see, the divisions are at times somewhat arbitrary). Agent theory is concerned with the question of what an agent is, and the use of mathematical formalisms for representing and reasoning about the properties of agents. Agent architectures can be thought of as software engineering models of agents;researchers in this area are primarily concerned with the problem of designing software or hardware systems that will satisfy the properties specified by agent theorists. Finally, agent languages are software systems for programming and experimenting with agents; these languages may embody principles proposed by theorists. The paper is not intended to serve as a tutorial introduction to all the issues mentioned; we hope instead simply to identify the most important issues, and point to work that elaborates on them. The article includes a short review of current and potential applications of agent technology.

/pdf/intelligent-agents-theory-and-practice-5dchj7p8pc.pdf

Intelligent Agents: Theory and Practice

Collaborative filters help people make choices based on the opinions of other people. GroupLens is a system for collaborative filtering of netnews, to help people find articles they will like in the huge stream of available articles. News reader clients display predicted scores and make it easy for users to rate articles after they read them. Rating servers, called Better Bit Bureaus, gather and disseminate the ratings. The rating servers predict scores based on the heuristic that people who agreed in the past will probably agree again. Users can protect their privacy by entering ratings under pseudonyms, without reducing the effectiveness of the score prediction. The entire architecture is open: alternative software for news clients and Better Bit Bureaus can be developed independently and can interoperate with the components we have developed.

http://pages.cpsc.ucalgary.ca/~saul/601.13/readings/p175-resnick.pdf

GroupLens: an open architecture for collaborative filtering of netnews

OceanStore is a utility infrastructure designed to span the globe and provide continuous access to persistent information. Since this infrastructure is comprised of untrusted servers, data is protected through redundancy and cryptographic techniques. To improve performance, data is allowed to be cached anywhere, anytime. Additionally, monitoring of usage patterns allows adaptation to regional outages and denial of service attacks; monitoring also enhances performance through pro-active movement of data. A prototype implementation is currently under development.

/pdf/oceanstore-an-architecture-for-global-scale-persistent-4qe9kbb44y.pdf

OceanStore: an architecture for global-scale persistent storage

Groupware reflects a change in emphasis from using the computer to solve problems to using the computer to facilitate human interaction. This article describes categories and examples of groupware and discusses some underlying research and development issues. GROVE, a novel group editor, is explained in some detail as a salient groupware example

/pdf/groupware-some-issues-and-experiences-3o63d5bi23.pdf

Groupware: some issues and experiences

Data sharing is fundamental to computer-supported cooperative work: People share information through explicit communication channels and through their coordinated use of shared databases. This paper examines the data management requirements of group work applications on the basis of experience with three prototype systems and on observations from the literature. Database and object management technologies that support these requirements are briefly surveyed, and unresolved issues in the particular areas of access control and concurrency control are identified for future research.

Data sharing in group work

This paper is about the design and implementatbn of a repllwted database that forms the basis for the N&s*gray, communication system The system supports groups of people working on shared sets of donunents and is intended jor use in a p e r s o ~ l computer network enulronment in which the database servers are 'tarely connected". Most algorithms for guamnteetng amsistency across repllcas require more reliable network amnectbns between serwrs for adequate performance. Analysis of many group oommunkatfon applicatbns, however, revealed relatlwly weak consistency requirements across copies of the database. These requirements can be met b y a simple replication algorlthrn that works well in rarely connected environments. TRls kind of replication has been used prevfously for a limited set of applications such as name directory replicatfon: we haw applied this technique to a much larger class of applfcatlons. Our characterfzatbn ojthfs class of applications suggests that this technique generalizes to support dfstrlbuted database Lmplementatlons of other gray, Luork systems, tncluding computer oonfeenclng and bulletin board systems. Presented at the Second Conference on ComputerSupporId Coopontive Work. Portland, Oregon, September 26-28, 1988 Notes is a group communication system that is used by people to share textual. numerlc, and graphical information. The system operates on personal computers in local-area and wide-area networks, and provides end-users the ability to design and create document databases for specfflc applications. This paper focuses on the Notes document manager. which supports replicated databases with a number of interesting characteristics, particularly when compared with replication technology typical in transaction-oriented databases. In tmnsactlon-processing applications for record-oriented database management systems. replication algorithms must meet strict consistency criteria. usually defined in terms of serializabflity of transactions [Gray]. Most implementations of replication algorithms that provide strict consistency depend on high likelihood of continuous connection between database server machines. Notes has a strong requirement to support workgroups that cannot afford continuously available inter-network connections. We refer to such networks as rarelyamteckd networks. This level of connectMty is typical for PC users. Local area networks that connect small workgroups who share printers often arc not intemomected to support cross-group collaboration. Dial-up line connections that cross organizational boundaries an expensive and have low bandwidth. In mly connected environments, replication that guarantees eertallzabillty would at best be possible only at enormous cost in performance. However, Notes applications do not requirt this level of consistency across replicas. As a result we arc able to rely on a simple replication algorithm. It provides the replication that is crucial to the viability of the p d u c t in the PC environment. at a cost that is acceptable in that environment. The replication capability has had several additional benefits for the product, since it also prwldes a means of doing static load balancing on a single network, automatic backup of databases, and support of home or portable computers that operate in a standalone mode. Similar approaches to replicated databases have been previously used for distributed directory services [Oppen] [Smith]. Our work extends this approach to a broader class of applications, including document sharing, electronic mail, and conferencing. The implementation is new as well, because it Is optimized to work well wer low bandwidth, dial up h e s . Section 2 briefly outllnes the characteristics of typical applications. In section 3 we review the design goals for the document manager and replication pwess. Section 4 presents the algorlthms and implementation. Section 5 provides some data on current usage of replication. The concluding section reviews our approach to bringing shared document databases to the PC user. Thts approach meets a wide-range of user needs, not feast of which is the need to collaborate despite the reality of poor network connectMty for PC users in ad hoc workgroups. 2. Application Characteristics Notes is based on a shared document database system that can be tailored to the needs of specific workgroups. A user might participate in a number of workgroup actMties each supported by a different shared database. A database is a collection of related forms or semi-structured [Malonel documents, organized through views that sort or categorize information. Users build specialized applications by tailoring the database to store. organhe and present speclllc kinds of information. For example. a group managing a software development project would want a variety of different documents in the database: bug reports. bug fix notices. comments and suggestions, progress reports, etc. The documents would be organized to make it easy for a reader to h d new items. and so that comments related to particular topics were grouped together. The format of documents and appearance of information can vary from application to application: graphs, images, pictures and numerical information can be intennfxcd with textual information: layout and use of color can give individual applications very distinctive looks. Databases have been designed to support group applications such as:

Replicated document management in a group communication system

Data sharing is fundamental to computer-supported cooperative work: 'people share information through explicit communication channels and through their coordinated use of shared databases. Database support tools are therefore critical to the effective implementation of software for group work. This paper surveys data sharing requirements for group work, highlighting new database technologies that are especially likely to affect our ability to build computer systems supporting group work.

This paper describes our experience implementing CES, a distributed Collaborative Editing System written in Argus, a language that includes facilities for managing long-lived distributed data. Argus provides atomic actions, which simplify the handling of concurrency and failures, and mechanisms for implementing atomic data types, which ensure serializability and recoverability of actions that use them. This paper focuses on the support for atomicity in Argus, especially the support for building new atomic types. Overall the mechanisms in Argus made it relatively easy to build CES; however, we encountered interesting problems in several areas. For example, much of the processing of an atomic action in Argus is handled automatically by the run-time system; several examples are presented that illustrate areas where more explicit control in the implementations of atomic types would be useful.

Atomic data abstractions in a distributed collaborative editing system

Work on PLANNER-73 and actors has led to the development of a basis for semantics of programming languages. Its value in describing programs with side-effects, parallelism, and synchronization is discussed. Formal definitions are written and explained for sequences, cells, and a simple synchronization primitive. In addition there is discussion of the implications of actor semantics for the controversy over elimination of side-effects.

Irene Greif

Papers

Data sharing in group work

Replicated document management in a group communication system

Data sharing in group work

Atomic data abstractions in a distributed collaborative editing system

Actor semantics of PLANNER-73