Answers to continuous queries over data streams are often delayed until some relevant input arrives through the data stream. These delays may turn answers when they arrive, obsolete to users who sometimes have to make decisions with no help whatsoever. Therefore, it can be useful to provide hypothetical answers—“given the current information, it is possible that \(X\) will become true at time \(t\) ”—instead of no information at all. In this work, we present a semantics for queries and corresponding answers that cover such hypothetical answers, together with an incremental online algorithm for updating the set of facts that are consistent with the currently available information. Our framework also works in a language supporting negation.

Continuous queries over data streams often delay answers until some relevant input arrives through the data stream. These delays may turn answers, when they arrive, obsolete to users who sometimes have to make decisions with no help whatsoever. Therefore, it can be useful to provide hypothetical answers – “given the current information, it is possible that X will become true at time t” – instead of no information at all. In this paper we present a semantics for queries and corresponding answers that covers such hypothetical answers, together with an online algorithm for updating the set of facts that are consistent with the currently available information.

Originally proposed in the mid-90s, design patterns for software development played a key role in objectoriented programming not only in increasing software quality, but also by giving a better understanding of the power and limitations of this paradigm. Since then, several authors have endorsed a similar task for other programming paradigms, in the hope of achieving similar benefits. In this paper we discuss design patterns for hybrid semantic web systems combining several description logic knowledge bases via a logic program. We introduce eight design patterns, grouped in three categories: three elementary patterns, which are the basic building blocks; four derived patterns, built from these; and a more complex pattern, the study of which can shed some insight in future syntactic developments of the underlying framework. These patterns are extensively applied in a natural way in a large-scale example that illustrates how their usage greatly simplifies some programming tasks, at the level of both development and extension. We work in a generalization of dl-programs that supports several (possibly different) description logics, but the results presented are easily adaptable to other existing frameworks such as multi-context systems.

Repairing an inconsistent knowledge base is a well known problem for which several solutions have been proposed and implemented in the past. In this paper, we start by looking at databases with active integrity constraints - consistency requirements that also indicate how the database should be updated when they are not met - as introduced by Caroprese et al.We show that the different kinds of repairs considered by those authors can be effectively computed by searching for leaves of specific kinds of trees. Although these computations are in general not very efficient (deciding the existence of a repair for a given database with active integrity constraints is NP-complete), on average the algorithms we present make significant reductions on the number of nodes in the search tree. Finally, these algorithms also give an operational characterization of different kinds of repairs that can be used when we extend the concept of active integrity constraints to the more general setting of knowledge bases.