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  • 1.
    Cheng, Sijin
    et al.
    Linköpings universitet, Institutionen för datavetenskap, Databas och informationsteknik. Linköpings universitet, Tekniska fakulteten.
    Hartig, Olaf
    Linköpings universitet, Institutionen för datavetenskap, Databas och informationsteknik. Linköpings universitet, Tekniska fakulteten.
    OPT plus : A Monotonic Alternative to OPTIONAL in SPARQL2019Inngår i: Journal of Web Engineering, ISSN 1540-9589, E-ISSN 1544-5976, Vol. 18, nr 1-3, s. 169-206Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Due to the OPTIONAL operator, the core fragment of the SPARQL query language is non-monotonic. That is, some solutions of a query result can be returned to the user only after having consulted all relevant parts of the queried dataset(s). This property presents an obstacle when developing query execution approaches that aim to reduce responses times rather than the overall query execution times. Reducing the response times?i.e., returning as many solutions as early as possible? is important in particular in Web-based client-server query processing scenarios in which network latencies dominate query execution times. Such scenarios are typical in the context of integration of Web data sources where a data integration component executes queries over a decentralized federation of such data sources. In this paper we introduce an alternative operator that is similar in spirit to OPTIONAL but without causing non-monotonicity. We show fundamental properties of this operator and observe that the downside of achieving the desired monotonicity property is a potentially significant increase in query result sizes. We study the extend of this trade-off in practice.

  • 2.
    Hartig, Olaf
    Linköpings universitet, Institutionen för datavetenskap, Databas och informationsteknik. Linköpings universitet, Tekniska fakulteten.
    Foundations of RDF* and SPARQL*: (An Alternative Approach to Statement-Level Metadata in RDF)2017Inngår i: Proceedings of the 11th Alberto Mendelzon International Workshop on Foundations of Data Management and the Web 2017 / [ed] Juan Reutter, Divesh Srivastava, Juan Reutter, Divesh Srivastava , 2017, Vol. 1912, artikkel-id 12Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The standard approach to annotate statements in RDF with metadatahas a number of shortcomings including data size blow-up and unnecessarilycomplicated queries. We propose an alternative approach that is based on nestingof RDF triples and of query patterns. The approach allows for a more compactrepresentation of data and queries, and it is backwards compatible with the standard.In this paper we present the formal foundations of our proposal and ofdifferent approaches to implement it. More specifically, we formally capture thenecessary extensions of the RDF data model and its query language SPARQL,and we define mappings based on which our extended notions can be convertedback to ordinary RDF and SPARQL. Additionally, for such type of mappings wedefine two desirable properties, information preservation and query result equivalence,and we show that the introduced mappings possess these properties.

  • 3.
    Hartig, Olaf
    et al.
    Linköpings universitet, Institutionen för datavetenskap, Databas och informationsteknik. Linköpings universitet, Tekniska fakulteten. Hasso Plattner Institute, University of Potsdam, Potsdam, Germany.
    Bull-Aranda, Carlos
    Informatics Department, Universidad Técnica Federico Santa María, Valparaíso, Chile.
    Bindings-restricted triple pattern fragments2016Inngår i: On the Move to Meaningful Internet Systems: OTM 2016 Conferences, Springer Berlin/Heidelberg, 2016, Vol. 10033, s. 762-769Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The Triple Pattern Fragment (TPF) interface is a recent proposal for reducing server load in Web-based approaches to execute SPARQL queries over public RDF datasets. The price for less overloaded servers is a higher client-side load and a substantial increase in network load (in terms of both the number of HTTP requests and data transfer). In this paper, we propose a slightly extended interface that allows clients to attach intermediate results to triple pattern requests. The response to such a request is expected to contain triples from the underlying dataset that do not only match the given triple pattern (as in the case of TPF), but that are guaranteed to contribute in a join with the given intermediate result. Our hypothesis is that a distributed query execution using this extended interface can reduce the network load (in comparison to a pure TPF-based query execution) without reducing the overall throughput of the client-server system significantly. Our main contribution in this paper is twofold: we empirically verify the hypothesis and provide an extensive experimental comparison of our proposal and TPF.

  • 4.
    Hartig, Olaf
    et al.
    Linköpings universitet, Institutionen för datavetenskap, Databas och informationsteknik. Linköpings universitet, Tekniska fakulteten.
    Curé, Olivier
    Université Paris-Est Marne la Vallée Paris, France.
    Semantic Data Management in Practice2017Inngår i: WWW '17 Companion: Proceedings of the 26th International Conference on World Wide Web Companion, 2017, International World Wide Web Conferences Steering Committee , 2017, s. 901-904Konferansepaper (Fagfellevurdert)
    Abstract [en]

    After years of research and development, standards and technologiesfor semantic data are suciently mature to be usedas the foundation of novel data science projects that employsemantic technologies in various application domains such asbio-informatics, materials science, criminal intelligence, andsocial science. Typically, such projects are carried out bydomain experts who have a conceptual understanding of semantictechnologies but lack the expertise to choose and toemploy existing data management solutions for the semanticdata in their project. For such experts, including domainfocuseddata scientists, project coordinators, and projectengineers, our tutorial delivers a practitioner's guide to se-mantic data management. We discuss the following importantaspects of semantic data management and demonstratehow to address these aspects in practice by using mature,production-ready tools: i) storing and querying semanticdata; ii) understanding, iii) searching, and iv) visualizingthe data; v) automated reasoning; vi) integrating externaldata and knowledge; and vii) cleaning the data.

  • 5.
    Hartig, Olaf
    et al.
    Linköpings universitet, Institutionen för datavetenskap, Databas och informationsteknik. Linköpings universitet, Tekniska fakulteten.
    Perez, Jorge
    University of Chile, Chile; Chilean Centre Semant Web Research, Chile.
    LDQL: A query language for the Web of Linked Data2016Inngår i: Journal of Web Semantics, ISSN 1570-8268, E-ISSN 1873-7749, Vol. 41Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Web of Linked Data is composed of tons of RDF documents interlinked to each other forming a huge repository of distributed semantic data. Effectively querying this distributed data source is an important open problem in the Semantic Web area. In this paper, we propose LDQL, a declarative language to query Linked Data on the Web. One of the novelties of LDQL is that it expresses separately (i) patterns that describe the expected query result, and (ii) Web navigation paths that select the data sources to be used for computing the result. We present a formal syntax and semantics, prove equivalence rules, and study the expressiveness of the language. In particular, we show that LDQL is strictly more expressive than all the query formalisms that have been proposed previously for Linked Data on the Web. We also study some computability issues regarding LDQL. We first prove that when considering the Web of Linked Data as a fully accessible graph, the evaluation problem for LDQL can be solved in polynomial time. Nevertheless, when the limited data access capabilities of Web clients are considered, the scenario changes drastically; there are LDQL queries for which a complete execution is not possible in practice. We formally study this issue and provide a sufficient syntactic condition to avoid this problem; queries satisfying this condition are ensured to have a procedure to be effectively evaluated over the Web of Linked Data. (C) 2016 Elsevier B.V. All rights reserved.

  • 6.
    Hartig, Olaf
    et al.
    Linköpings universitet, Institutionen för datavetenskap, Databas och informationsteknik. Linköpings universitet, Tekniska fakulteten.
    Perez, Jorge
    Univ Chile, Chile.
    Semantics and Complexity of GraphQL2018Inngår i: WEB CONFERENCE 2018: PROCEEDINGS OF THE WORLD WIDE WEB CONFERENCE (WWW2018), ASSOC COMPUTING MACHINERY , 2018, s. 1155-1164Konferansepaper (Fagfellevurdert)
    Abstract [en]

    GraphQL is a recently proposed, and increasingly adopted, conceptual framework for providing a new type of data access interface on the Web. The framework includes a new graph query language whose semantics has been specified informally only. This has prevented the formal study of the main properties of the language. We embark on the formalization and study of GraphQL. To this end, we first formalize the semantics of GraphQL queries based on a labeled-graph data model. Thereafter, we analyze the language and show that it admits really efficient evaluation methods. In particular, we prove that the complexity of the GraphQL evaluation problem is NL-complete. Moreover, we show that the enumeration problem can be solved with constant delay. This implies that a server can answer a GraphQL query and send the response byte-by-byte while spending just a constant amount of time between every byte sent. Despite these positive results, we prove that the size of a GraphQL response might be prohibitively large for an internet scenario. We present experiments showing that current practical implementations suffer from this issue. We provide a solution to cope with this problem by showing that the total size of a GraphQL response can be computed in polynomial time. Our results on polynomial-time size computation plus the constant-delay enumeration can help developers to provide more robust GraphQL interfaces on the Web.

  • 7.
    Hartig, Olaf
    et al.
    Linköpings universitet, Institutionen för datavetenskap, Databas och informationsteknik. Linköpings universitet, Tekniska fakulteten. University of Potsdam, Germany.
    Pirro, Giuseppe
    Italian National Research Council ICAR CNR, Italy.
    SPARQL with property paths on the Web2017Inngår i: Semantic Web, ISSN 1570-0844, E-ISSN 2210-4968, Vol. 8, nr 6, s. 773-795Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Linked Data on the Web represents an immense source of knowledge suitable to be automatically processed and queried. In this respect, there are different approaches for Linked Data querying that differ on the degree of centralization adopted. On one hand, the SPARQL query language, originally defined for querying single datasets, has been enhanced with features to query federations of datasets; however, this attempt is not sufficient to cope with the distributed nature of data sources available as Linked Data. On the other hand, extensions or variations of SPARQL aim to find trade-offs between centralized and fully distributed querying. The idea is to partially move the computational load from the servers to the clients. Despite the variety and the relative merits of these approaches, as of today, there is no standard language for querying Linked Data on theWeb. A specific requirement for such a language to capture the distributed, graph-like nature of Linked Data sources on the Web is a support of graph navigation. Recently, SPARQL has been extended with a navigational feature called property paths (PPs). However, the semantics of SPARQL restricts the scope of navigation via PPs to single RDF graphs. This restriction limits the applicability of PPs for querying distributed Linked Data sources on the Web. To fill this gap, in this paper we provide formal foundations for evaluating PPs on the Web, thus contributing to the definition of a query language for Linked Data. We first introduce a family of reachability-based query semantics for PPs that distinguish between navigation on the Web and navigation at the data level. Thereafter, we consider another, alternative query semantics that couples Web graph navigation and data level navigation; we call it context-based semantics. Given these semantics, we find that for some PP-based SPARQL queries a complete evaluation on the Web is not possible. To study this phenomenon we introduce a notion of Web-safeness of queries, and prove a decidable syntactic property that enables systems to identify queries that areWeb-safe. In addition to establishing these formal foundations, we conducted an experimental comparison of the context-based semantics and a reachability- based semantics. Our experiments show that when evaluating a PP-based query under the context-based semantics one experiences a significantly smaller number of dereferencing operations, but the computed query result may contain less solutions.

  • 8.
    Hartig, Olaf
    et al.
    Linköpings universitet, Institutionen för datavetenskap, Databas och informationsteknik. Linköpings universitet, Tekniska fakulteten.
    Pérez, Jorge
    Department of Computer Science, Universidad de Chile, Chile.
    An Initial Analysis of Facebook’s GraphQL Language2017Inngår i: Proceedings of the 11th Alberto Mendelzon International Workshop on Foundations of Data Management and the Web. / [ed] Juan Reutter, Divesh Srivastava, Juan Reutter, Divesh Srivastava , 2017, Vol. 1912, artikkel-id 11Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Facebook’s GraphQL is a recently proposed, and increasingly adopted,conceptual framework for providing a new type of data access interface on theWeb. The framework includes a new graph query language whose semantics hasbeen specified informally only. The goal of this paper is to understand the propertiesof this language. To this end, we first provide a formal query semantics.Thereafter, we analyze the language and show that it has a very low complexityfor evaluation. More specifically, we show that the combined complexity ofthe main decision problems is in NL (Nondeterministic Logarithmic Space) and,thus, they can be solved in polynomial time and are highly parallelizable.

  • 9.
    Hartig, Olaf
    et al.
    Linköpings universitet, Institutionen för datavetenskap, Databas och informationsteknik. Linköpings universitet, Tekniska fakulteten.
    Tamer Ozsu, M.
    University of Waterloo, Canada.
    Walking Without a Map: Ranking-Based Traversal for Querying Linked Data2016Inngår i: SEMANTIC WEB - ISWC 2016, PT I, Springer-Verlag New York, 2016, Vol. 9981, s. 305-324Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The traversal-based approach to execute queries over Linked Data on the WWW fetches data by traversing data links and, thus, is able to make use of up-to-date data from initially unknown data sources. While the downside of this approach is the delay before the query engine completes a query execution, user perceived response time may be improved significantly by returning as many elements of the result set as soon as possible. To this end, the query engine requires a traversal strategy that enables the engine to fetch result-relevant data as early as possible. The challenge for such a strategy is that the query engine does not know a priori which of the data sources discovered during the query execution will contain result-relevant data. In this paper, we investigate 14 different approaches to rank traversal steps and achieve a variety of traversal strategies. We experimentally study their impact on response times and compare them to a baseline that resembles a breadth-first traversal. While our experiments show that some of the approaches can achieve noteworthy improvements over the baseline in a significant number of cases, we also observe that for every approach, there is a non-negligible chance to achieve response times that are worse than the baseline.

  • 10.
    Knuth, Magnus
    et al.
    Hasso Plattner Institute, University of PotsdamPotsdamGermany.
    Hartig, Olaf
    Linköpings universitet, Institutionen för datavetenskap, Databas och informationsteknik. Linköpings universitet, Tekniska fakulteten.
    Scheduling Refresh Queries for Keeping Results from a SPARQL Endpoint Up-to-Date2016Inngår i: On the Move to Meaningful Internet Systems: OTM 2016 Conferences, 2016, Vol. 10033, s. 780-791Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Many datasets change over time. As a consequence, long-running applications that cache and repeatedly use query results obtained from a SPARQL endpoint may resubmit the queries regularly to ensure up-to-dateness of the results. While this approach may be feasible if the number of such regular refresh queries is manageable, with an increasing number of applications adopting this approach, the SPARQL endpoint may become overloaded with such refresh queries. A more scalable approach would be to use a middle-ware component at which the applications register their queries and get notified with updated query results once the results have changed. Then, this middle-ware can schedule the repeated execution of the refresh queries without overloading the endpoint. In this paper, we study the problem of scheduling refresh queries for a large number of registered queries by assuming an overload-avoiding upper bound on the length of a regular time slot available for testing refresh queries. We investigate a variety of scheduling strategies and compare them experimentally in terms of time slots needed before they recognize changes and number of changes that they miss.

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