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Scalable Distributed Indexing and Query Processing over Linked Data
"... Linked Data is becoming the core part of modern Web applications and thus efficient access to structured information expressed in RDF gains paramount importance. A number of efficient local RDF stores exist already, while distributed indexing and distributed query processing over Linked Data with si ..."
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Linked Data is becoming the core part of modern Web applications and thus efficient access to structured information expressed in RDF gains paramount importance. A number of efficient local RDF stores exist already, while distributed indexing and distributed query processing over Linked Data with similar efficiency and data management features as known from traditional database and data integration systems are only starting to develop. Distributed approaches will necessarily co-exist with centralized schemes, as data will be owned by different stakeholders who may not want to provide their complete data sets to a central place. Additionally, central / integrated storage may be prohibited for organizational or legal reasons in certain areas. To support decentralized schemes, only a few attempts in this direction exist so far, but they are limited in terms of capabilities and the degree of distribution vs. efficiency, query expressivity, and scalability. To remedy this situation, the approach and proof-of-concept prototype presented in this paper provides a solution for these open challenges. As we argue for widely distributed systems as a possible answer to scalability issues, we first identify and discuss the main challenges and based on this analysis, we propose an approach for efficient and scalable query processing over distributed Linked Data sources, taking into account the latest advances in database technology. Our system is based on a layered architecture that makes use of the advantages of decentralized indexing and query processing approaches, which have been researched and matured over the last decade. Our approach is based on a logical algebra for queries over RDF data and a related physical query algebra to enable optimization, both on the logical and physical layers in
IOS Press A Systemic Approach for Effective Semantic Access to Cultural Content
"... Abstract. A large on-going activity for digitization, dissemination and preservation of cultural heritage is taking place in Europe and the United States, which involves all types of cultural institutions, i.e., galleries, libraries, museums, archives and all types of cultural content. The developme ..."
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Abstract. A large on-going activity for digitization, dissemination and preservation of cultural heritage is taking place in Europe and the United States, which involves all types of cultural institutions, i.e., galleries, libraries, museums, archives and all types of cultural content. The development of Europeana, as a single point of access to European Cultural Heritage, has probably been the most important result of the activities in the field till now. Semantic interoperability is a key issue in these developments. This paper presents a system that provides content providers and users with the ability to map, in an effective way, their own meta-data schemas to common domain standards and the Europeana (ESE, EDM) data models. Based on these mappings, semantic enrichment and query answering techniques are proposed as a means for providing effective access of users to digital cultural heritage. An experimental study is presented involving content from national and thematic content aggregators in Europeana, which illustrates the proposed system capabilities.
Constrained regular expressions for answering RDF-path queries modulo RDFS
"... Abstract. The standard SPARQL query language is currently defined for querying RDF graphs without RDFS semantics. Several extensions of SPARQL to RDFS semantics have been proposed. In this paper, we discuss extensions of SPARQL that use regular expressions to navigate RDF graphs and may be used to a ..."
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Abstract. The standard SPARQL query language is currently defined for querying RDF graphs without RDFS semantics. Several extensions of SPARQL to RDFS semantics have been proposed. In this paper, we discuss extensions of SPARQL that use regular expressions to navigate RDF graphs and may be used to answer queries considering RDFS semantics. In particular, we present and compare nSPARQL and our proposal CPSPARQL. We show that CPSPARQL is expressive enough to answer full SPARQL queries modulo RDFS. Finally, we compare the expressiveness and complexity of both nSPARQL and the corresponding frag-ment of CPSPARQL, that we call cpSPARQL. We show that both languages have the same complexity through cpSPARQL, being a proper extension of SPARQL graph patterns, is more expressive than nSPARQL.
Everything YouAlwaysWanted to KnowAbout BlankNodes ∗
"... In this paper we thoroughly cover the issue of blank nodes, which have been defined in RDF as ‘existential variables’. We first introduce the theoretical precedent for existential blank nodes from first order logic and incomplete information in database theory. We then cover the different (and somet ..."
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In this paper we thoroughly cover the issue of blank nodes, which have been defined in RDF as ‘existential variables’. We first introduce the theoretical precedent for existential blank nodes from first order logic and incomplete information in database theory. We then cover the different (and sometimes incompatible) treatment of blank nodes across the W3C stack of RDF-related standards. We present an empirical survey of the blank nodes present in a large sample of RDF data published on the Web (the BTC–2012 dataset), where we find that 25.7 % of unique RDF terms are blank nodes, that 44.9 % of documents and 66.2 % of domains featured use of at least one blank node, and that aside from one Linked Data domain whose RDF data contains many “blank node cycles”, the vast majority of blank nodes form tree structures that are efficient to compute simple entailment over. With respect to the RDF-merge of the full data, we show that 6.1 % of blank-nodes are redundant under simple entailment. The vast majority of non-lean cases are isomorphisms resulting from multiple blank nodes with no discriminating information being given within an RDF document or documents being duplicated in multiple Web locations. Although simple entailment is NP-complete and leanness-checking is coNP-complete, in computing this latter result, we demonstrate that in practice, real-world RDF graphs are sufficiently “rich ” in ground information for problematic cases to be avoided by non-naive algorithms.
