Marie-Christine Rousset

Combining Horn rules and description logics in CARIN

Abstract We describe carin , a novel family of representation languages, that combine the expressive power of Horn rules and of description logics. We address the issue of providing sound and complete inference procedures for such languages. We identify existential entailment as a core problem in reasoning in carin , and describe an existential entailment algorithm for the ALCNR description logic. As a result, we obtain a sound and complete algorithm for reasoning in non-recursive carin ALCNR knowledge bases, and an algorithm for rule subsumption over ALCNR . We show that in general, the reasoning problem for recursive carin - ALCNR knowledge bases is undecidable, and identify the constructors of ALCNR causing the undecidability. We show two ways in which carin - ALCNR knowledge bases can be restricted while obtaining sound and complete reasoning.

TreeFinder: a first step towards XML data mining

In this paper we consider the problem of searching frequent trees from a collection of tree-structured data modeling XML data. The TreeFinder algorithm aims at finding trees, such that their exact or perturbed copies are frequent in a collection of labelled trees. To cope with complexity issues, TreeFinder is correct but not complete: it finds a subset of actually frequent trees. The default of completeness is experimentally investigated on artificial medium size datasets; it is shown that TreeFinder reaches completeness or falls short for a range of experimental settings.

THE USE OF CARIN LANGUAGE AND ALGORITHMS FOR INFORMATION INTEGRATION: THE PICSEL SYSTEM

PICSEL is an information integration system over sources that are distributed and possibly heterogeneous. The approach which has been chosen in PICSEL is to define an information server as a knowledge-based mediator in which CARIN is used as the core logical formalism to represent both the domain of application and the contents of information sources relevant to that domain. In this paper, we describe the way the expressive power of the CARIN language is exploited in the PICSEL information integration system, while maintaining the decidability of query answering. We illustrate it on examples coming from the tourism domain, which is the first real case that we have to consider in PICSEL, in collaboration with the travel agency Degriftour. see

Rewriting queries using views in description logics

The problem of rewriting queries using views is to iind a query expression that uses only a set of views V and is equivalent to (or maximally contained in) a given query Q. Rewriting queries using views is important for query optimization and for applications such as information integration and data warehousing. Description logics are a family of logics that were developed for modeling complex hierarchical structures, and can also be viewed as a query language with an interesting tradeoff between complexity and expressive power. We consider the problem of rewriting queries using views expressed in description logics and conjunctive queries over description logics. We show that if the view definitions do not contain existential variables, then it is always possible to find a rewriting that is a union of conjunctive queries, and furthermore, this rewriting produces the maximal set of answers possible from the views. If the views have existential variables, the rewriting may be recursive. We present an algorithm for producing a recursive rewriting, that is guaranteed to be a maximal one when the underlying database forms a tree of constants. We show that in general, it is not always be possible to find a maximal rewriting.

Distributed reasoning in a peer-to-peer setting: application to the semantic web

In a peer-to-peer system, there is no centralized control or hierarchical organization: each peer is equivalent in functionality and cooperates with other peers in order to solve a collective task. Such systems have evolved from simple keyword-based peer-to-peer file sharing systems like Napster and Gnutella to schema-based peer data management systems like Edutella [3] or Piazza [2], which handle semantic data description and support complex queries for data retrieval. In this paper, we are interested in peer-to-peer inference systems in which each peer can answer queries by reasoning from its local (propositional) theory but can also ask queries to some other peers with which it is semantically related by sharing part of its vocabulary. This framework encompasses several applications like peer-to-peer information integration systems or intelligent agents, in which each peer has its own knowledge (about its data or its expertise domain) and some partial knowledge about some other peers. In this setting, when it is solicited to perform a reasoning task and if it cannot solve completely that task locally, a peer must be able to distribute appropriate reasoning subtasks among its acquainted peers. The contribution of this paper is the first consequence finding algorithm in a peer-to-peer setting: it is anytime and computes consequences gradually from the solicited peer to peers that are more and more distant. We have exhibited a sufficient condition on the acquaintance graph of the peer-to-peer inference system for guaranteeing the completeness of this algorithm. Our algorithm splits clauses if they involve vocabularies of several peers. Each piece of a splitted clause is transmitted to the corresponding theory to find its consequences. The consequences that are found for each piece of splitted clause must be recomposed to get the consequences of that clause.

Combining a Logical and a Numerical Method for Data Reconciliation

The reference reconciliation problem consists in deciding whether different identifiers refer to the same data, i.e. correspond to the same real world entity. In this article we present a reference reconciliation approach which combines a logical method for reference reconciliation called L2R and a numerical one called N2R. This approach exploits the schema and data semantics, which is translated into a set of Horn FOL rules of reconciliation. These rules are used in L2R to infer exact decisions both of reconciliation and non-reconciliation. In the second method N2R, the semantics of the schema is translated in an informed similarity measure which is used by a numerical computation of the similarity of reference pairs. This similarity measure is expressed in a non linear equation system, which is solved by using an iterative method. The experiments of the methods made on two different domains, show good results for both recall and precision. They can be used separately or in combination. We have shown that their combination allows to improve runtime performance.

Answering queries using views: A KRDB perspective for the semantic Web

In this article, we investigate a first step towards the long-term vision of the Semantic Web by studying the problem of answering queries posed through a mediated ontology to multiple information sources whose content is described as views over the ontology relations. The contributions of this paper are twofold. We first offer a uniform logical setting which allows us to encompass and to relate the existing work on answering and rewriting queries using views. In particular, we make clearer the connection between the problem of rewriting queries using views and the problem of answering queries using extensions of views. Then we focus on an instance of the problem of rewriting conjunctive queries using views through an ontology expressed in a description logic, for which we exhibit a complete algorithm.

Semantic integration in Xyleme: a uniform tree-based approach

Xyleme is a huge warehouse integrating XML data of the Web. Xyleme considers a simple data model with data trees and tree types for describing the data sources, and a simple query language based on tree queries with boolean conditions. The main components of the data model are a mediated schema modeled by an abstract tree type, as a view of a set of tree types associated with actual data trees, called concrete tree types, and a mapping expressing the connection between the mediated schema and the concrete tree types. The first contribution of this paper is formal: we provide a declarative model-theoretic semantics for Xyleme tree queries, a way of checking tree query containment, and a characterization of tree queries as a composition of branch queries. The other contributions are algorithmic and handle the potentially huge size of the mapping relation which is a crucial issue for semantic integration and query evaluation in Xyleme. First, we propose a method for pre-evaluating queries at compile time by storing some specific meta-information about the mapping into map translation tables. These map translation tables summarize the set of all the branch queries that can be generated from the mediated schema and the set of all the mappings. Then, we propose different operators and strategies for relaxing queries which, having an empty map translation table, will have no answer if they are evaluated against the data. Finally, we present a method for semi-automatically generating the mapping relation.

SomeWhere in the semantic web

In this paper, we describe the SomeWhere semantic peer-to-peer data management system that promotes a “small is beautiful” vision of the Semantic Web based on simple personalized ontologies (e.g., taxonomies of classes) but which are distributed at a large scale. In this vision of the Semantic Web, no user imposes to others his own ontology. Logical mappings between ontologies make possible the creation of a web of people in which personalized semantic marking up of data cohabits nicely with a collaborative exchange of data. In this view, the Web is a huge peer-to-peer data management system based on simple distributed ontologies and mappings.

The Xyleme project

Abstract The current development of the Web and the generalization of XML technology [ http://www.w3.org/ ] provides a major opportunity to radically change the management of distributed data. We have developed a prototype of a dynamic warehouse for XML data , namely Xyleme. In the present paper, we briefly present some motivation and important aspects of the work performed in the framework of the Xyleme project. (A short preliminary version of this paper appeared in [IEEE Data Engng. Bull. 24 (2) (2001) 40].) The project was completed at the end of 2000 with the implementation of a prototype. The prototype was then turned into a product by a start-up company also called Xyleme [ http://www.xyleme.com/ ].