A hypermedia application offers its users much freedom to navigate through a large hyperspace. For authors finding a good compromise between offering navigational freedom and offering guidance is difficult, especially in applications that target a broad audience. Adaptive hypermedia (AH) offers (automatically generated) personalized content and navigation support, so the choice between freedom and guidance can be made on an individual basis. Many adaptive hypermedia systems (AHS) are tightly integrated with one specific application. In this paper we study design issues for general-purpose adaptive hypermedia systems, built according to an application-independent architecture. We use the Dexter-based AHAM reference model for adaptive hypermedia [7] to describe the functionality of such systems at the conceptual level. We concentrate on the architecture and behavior of a general-purpose adaptive engine. Such an engine performs adaptation and updates the user model according to a set of adaptation rules specified in an adaptation model. In our study of the behavior of such a system we concentrate on the issues of termination and confluence, which are important to detect potential problems in an adaptive hypermedia application. We draw parallels with static rule analysis in active database systems [1,2]. By using common properties of AHS we are able to obtain more precise (less conservative) results for AHS than for active databases in general, especially for the problem of termination.

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We present a hierarchically structured transaction-oriented concept for a rule-based active database system. In [LL94, LHL95], we have proposedStatelog as a uni ed framework for active and deductive rules. Following the need for better structuring capabilities, we introduce procedures as a means to group semantically related rules and to encapsulate their behavior. In addition to executing elementary updates, procedures can be called, thereby de ning (sub)transactions which may perform complex computations. A Statelog procedure is a set of ECAstyle Datalog rules together with an import/export interface. Systemimmanent frame and procedure rules ensure both propagation of facts and processing of results of committed subtransactions. Thus, Statelog programs specify a nested transaction model which allows a much more structured and natural modeling of complex transactions than previous approaches. Two equivalent semantics for a Statelog program P are given: (i) a logic programming style semantics by a compilation into a logic program, and (ii) a model-theoretic Kripke-style semantics. While (ii) serves as a conceptual model of active rule behavior and allows to reason about properties of the speci ed transactions, (i) -- together with the appropriate execution model -- yields an operational semantics and can be used as an implementation of P.

ing with credit is permitted. To copy otherwise, to republish, to post on servers, to redistribute to lists, or to use any component of this work in other works, requires prior specific permission and/or a fee. Permissions may be requested from Publications Dept, ACM Inc., 1515 Broadway, New York, NY 10036 USA, fax +1 (212) 869-0481, or permissions@acm.org. This is a preliminary release of an article accepted by ACM Transactions on Database Systems. The definitive version is currently in production at ACM and, when released, will supersede this version. 2 \Delta E. Baralis and J. Widom 1. INTRODUCTION An active database system is a conventional database system extended with a facility for managing active rules (or triggers). Incorporating active rules into a conventional database system has raised considerable interest both in the scientific community and in the commercial world: A number of prototypes that incorporate active rules into relational and object-oriented database system...

After brie y reviewing the basic notions and terminology of active rules and relating them to production rules and deductive rules, respectively, we survey a number of formal approaches to active rules. Subsequently, we present our own state-oriented logical approach to active rules which combines the declarative semantics of deductive rules with the possibility to de ne updates in the style of production rules and active rules. The resulting language Statelog is surprisingly simple, yet captures many features of active rules including composite event detection and di erent coupling modes. Thus, it can be used for the formal analysis of rule properties like termination and expressive power. Finally, we showhow nested transactions can be modeled in Statelog, both from the operational and the model-theoretic perspective.

Rules in active database systems can be very difficult to program, due to the unstructured and unpredictable nature of rule processing. We provide static analysis techniques for predicting whether a given rule set is guaranteed to terminate, and whether rule execution is confluent (guaranteed to have a unique final state). Our methods are based on previous techniques for analyzing rules in active database systems. We improve considerably on the previous techniques by providing analysis criteria that are much less conservative: our methods often determine that a rule set will terminate or is confluent when previous methods could not make this determination. Our improved analysis is based on a "propagation" algorithm, which uses an extended relational algebra to accurately determine when the action of one rule can affect the condition of another, and to determine when rule actions commute. We consider both Condition-Action rules and Event-Condition-Action rules, making our approach widel...

XML is a now a dominant standard for storing and exchanging information. With its increasing use in areas such asdatawarehousing and e-commerce, there is a rapidly growing need for rule-based technology to support reactive functionality on XML repositories. Eventcondition-action (ECA) rules automatically perform actions in response to events and are a natural facility to support such functionality. In this paper, we study ECA rules in the context of XML data. We de ne a simple language for specifying ECA rules on XML repositories. The language is illustrated by means of some examples, and its syntax and semantics are then speci ed more formally. We theninvestigate methods for analysing and optimising these ECA rules, a task which has added complexity in this XML setting compared with conventional active databases.

We study action rule based systems with two forms of negation, namely classical negation and "negation as failure to find a course of actions". We show by several examples that adding negation as failure to such systems increase their expressiveness, in the sense that real life problems can be represented in a natural and simple way. Then, we address the problem of providing a formal declarative semantics to these extended systems, by adopting an argumentation based approach, which has been shown to be a simple unifying framework for understanding the declarative semantics of various non-monotonic formalisms. In this way, we naturally define the grounded (well-founded), stable and preferred semantics for production systems with negation as failure. Next, we characterize the class of stratified production systems, which enjoy the properties that the above mentioned semantics coincide and that negation as failure to find a course of actions can be computed by a simple bottom-up ...

Two different approaches have been traditionally considered for dealing with the process of integrity constraints enforcement: integrity checking and integrity maintenance. However, while previous research in the first approach has mainly addressed efficiency issues, research in the second approach has been mainly concentrated in being able to generate all possible repairs that falsify an integrity constraint violation. In this paper we address efficiency issues during the process of integrity maintenance. In this sense, we propose a technique which improves efficiency of existing methods by defining the order in which maintenance of integrity constraints should be performed. Moreover, we use also this technique for being able to handle in an integrated way the integrity constraints enforcement approaches mentioned above. 1. Introduction Database updating has attracted a lot of research during last years [Abi88]. Several problems may arise when updating a deductive database [TU95]. O...

We focus on well-behaved Adaptive Hypermedia Systems, which means the adaptation engine that executes adaptation rules always terminates and produces predictable (confluent) adaptation results. Unfortunately termination and confluence are undecidable in general. In this paper we discuss sufficient conditions to help authors to write adaptation rules that satisfy termination and confluence.