Olaf Hartig

Olaf Hartig
Linköping University | LiU · Department of Computer and Information Science (IDA)

PhD

About

102
Publications
14,875
Reads
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2,752
Citations
Citations since 2017
29 Research Items
1504 Citations
2017201820192020202120222023050100150200250
2017201820192020202120222023050100150200250
2017201820192020202120222023050100150200250
2017201820192020202120222023050100150200250
Additional affiliations
October 2018 - present
Linköping University
Position
  • Professor (Associate)
August 2016 - October 2018
Linköping University
Position
  • Professor (Assistant)
August 2016 - present
Linköping University
Position
  • Professor (Assistant)
Description
  • Database Technologies courses for undergraduates and graduate students (TDDD12, TDDD37, TDDD46, TDDD74, TDDD81), Advanced Data Models course for graduate students (TDDD43)
Education
October 2007 - June 2014
Humboldt-Universität zu Berlin
Field of study
  • Computer Science
August 2000 - June 2006
Humboldt-Universität zu Berlin
Field of study
  • Computer Science

Publications

Publications (102)
Article
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 nove...
Conference Paper
Full-text available
The World Wide Web currently evolves into a Web of Linked Data where content providers publish and link data as they have done with hypertext for the last 20 years. While the declarative query language SPARQL is the de facto for querying a-priory defined sets of data from the Web, no language exists for querying the Web of Linked Data itself. Howev...
Conference Paper
Full-text available
The Web of Linked Data forms a single, globally distributed dataspace. Due to the openness of this dataspace, it is not possible to know in advance all data sources that might be relevant for query answering. This openness poses a new challenge that is not addressed by traditional research on federated query processing. In this paper we present an...
Article
Full-text available
The publication of Linked Open Data on the Web has gained tremendous momentum over the last five years. This development makes possible (and interesting) the execution of queries using up-to-date data from multiple, automatically discovered data sources. As a result, we currently witness the emergence of a new research area that focuses on an onlin...
Conference Paper
Full-text available
Today a large amount of RDF data is published on the Web. However, the openness of the Web and the ease to combine RDF data from different sources creates new challenges. The Web of data is missing a uniform way to assess and to query the trustworthiness of information. In this paper we present tSPARQL, a trust-aware extension to SPARQL. Two additi...
Article
Full-text available
Amazon Neptune is a graph database service that supports two graph models: W3C’s Resource Description Framework (RDF) and Labeled Property Graphs (LPG). Customers choose one or the other model. This choice determines which data modeling features can be used and – perhaps more importantly – which query languages are available. The choice between the...
Chapter
GraphQL is a popular new approach to build Web APIs that enable clients to retrieve exactly the data they need. Given the growing number of tools and techniques for building GraphQL servers, there is an increasing need for comparing how particular approaches or techniques affect the performance of a GraphQL server. To this end, we present LinGBM, a...
Preprint
Full-text available
GraphQL is a popular new approach to build Web APIs that enable clients to retrieve exactly the data they need. Given the growing number of tools and techniques for building GraphQL servers, there is an increasing need for comparing how particular approaches or techniques affect the performance of a GraphQL server. To this end, we present LinGBM, a...
Chapter
A federation of RDF data sources offers enormous potential when answers or insights of queries are unavailable via a single data source. As various interfaces for accessing RDF data are proposed, one challenge for querying such a federation is that the federation members are heterogeneous in terms of the type of data access interfaces. There does n...
Preprint
Full-text available
Today's space of graph database solutions is characterized by two main technology stacks that have evolved separate from one another: on one hand, there are systems that focus on supporting the RDF family of standards; on the other hand, there is the Property Graph category of systems. As a basis for bringing these stacks together and, in particula...
Article
Full-text available
Ensuring the success of big graph processing for the next decade and beyond.
Chapter
Full-text available
RDF Stream Processing (RSP) has been proposed as a way of bridging the gap between the Complex Event Processing (CEP) paradigm and the Semantic Web standards. Uncertainty has been recognized as a critical aspect in CEP, but it has received little attention within the context of RSP. In this paper, we investigate the impact of different RSP optimiza...
Preprint
Full-text available
Graphs are by nature unifying abstractions that can leverage interconnectedness to represent, explore, predict, and explain real- and digital-world phenomena. Although real users and consumers of graph instances and graph workloads understand these abstractions, future problems will require new abstractions and systems. What needs to happen in the...
Chapter
Full-text available
RDF Stream Processing (RSP) has been proposed as a candidate for bringing together the Complex Event Processing (CEP) paradigm and the Semantic Web standards. In this paper, we investigate the impact of explicitly representing and processing uncertainty in RSP for the use in CEP. Additionally, we provide a representation for capturing the relevant...
Preprint
Full-text available
Federations of RDF data sources provide great potential when queried for answers and insights that cannot be obtained from one data source alone. A challenge for planning the execution of queries over such a federation is that the federation may be heterogeneous in terms of the types of data access interfaces provided by the federation members. Thi...
Book
This book constitutes the proceedings of the satellite events held at the 17th Extended Semantic Web Conference, ESWC 2020, in May/June 2020. The conference was planned to take place in Heraklion, Crete, Greece, but changed to an online format due to the COVID-19 pandemic. ESWC is a major venue for presenting and discussing the latest scientific re...
Chapter
Full-text available
RSP-QL was developed by the W3C RDF Stream Processing (RSP) community group as a common way to express and query RDF streams. However, RSP-QL does not provide any way of annotating data on the statement level, for example, to express the uncertainty that is often associated with streaming information. Instead, the only way to provide such informati...
Conference Paper
Full-text available
GraphQL is a highly popular new approach to build Web APIs. An important component of this approach is the GraphQL schema definition language (SDL). The original purpose of this language is to define a so-called GraphQL schema that specifies the types of objects that can be queried when accessing a specific GraphQL Web API. This paper focuses on th...
Book
This book constitutes the thoroughly refereed post-conference proceedings of the Satellite Events of the 16th Extended Semantic Web Conference, ESWC 2019, held in Portorož, Slovenia, in June 2019. The volume contains 38 poster and demonstration papers, 2 workshop papers,5 PhD symposium papers, and 3 industry track papers, selected out of a total of...
Conference Paper
Full-text available
This paper provides an overview of a model for capturing properties of client-server-based query computation setups. This model can be used to formally analyze different combinations of client and server capabilities, and compare them in terms of various fine-grain complexity measures. While the motivations and the focus of the presented work are r...
Conference Paper
Full-text available
The runtime optimization of federated SPARQL query engines is of central importance to ensure the usability of the Web of Data in real-world applications. The efficient selection of sources (SPARQL endpoints in our case) as well as the generation of optimized query plans belong to the most important optimization steps in this respect. This paper pr...
Conference Paper
Full-text available
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...
Article
Full-text available
The runtime optimization of federated SPARQL query engines is of central importance to ensure the usability of the Web of Data in real-world applications. The efficient selection of sources (SPARQL endpoints in our case) as well as the generation of optimized query plans belong to the most important optimization steps in this respect. This paper pr...
Conference Paper
Full-text available
The Linked Data Fragment (LDF) framework has been proposed as a uniform view to explore the trade-offs of consuming Linked Data when servers provide (possibly many) different interfaces to access their data. Every such interface has its own particular properties regarding performance, bandwidth needs, caching, etc. Several practical challenges aris...
Conference Paper
Full-text available
The standard approach to annotate statements in RDF with metadata has a number of shortcomings including data size blow-up and unnecessarily complicated queries. We propose an alternative approach that is based on nesting of RDF triples and of query patterns. The approach allows for a more compact representation of data and queries, and it is backw...
Conference Paper
Full-text available
Facebook's 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. The goal of this paper is to understand the properties of this language. To this end, we first p...
Conference Paper
After years of research and development, standards and technologies for semantic data are sufficiently mature to be used as the foundation of novel data science projects that employ semantic technologies in various application domains such as bio-informatics, materials science, criminal intelligence, and social science. Typically, such projects are...
Book
The chapter 'Scholia, Scientometrics and Wikidata' is available open access under a CC BY 4.0 license via link.springer.com.
Book
The two volumes LNCS 10249 and 10250 constitute the refereed proceedings of the 14th International Semantic Web Conference, ESWC 2017, held in Portorož, Slovenia. The 51 revised full papers presented were carefully reviewed and selected from 183 submissions. In addition, 10 PhD papers are included, selected out of 14 submissions. The papers are or...
Book
The two volumes LNCS 10249 and 10250 constitute the refereed proceedings of the 14th International Semantic Web Conference, ESWC 2017, held in Portorož, Slovenia. The 51 revised full papers presented were carefully reviewed and selected from 183 submissions. In addition, 10 PhD papers are included, selected out of 14 submissions. The papers are or...
Conference Paper
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...
Conference Paper
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...
Conference Paper
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 si...
Technical Report
Full-text available
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...
Technical Report
Full-text available
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...
Article
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...
Technical Report
Full-text available
The emergence of Linked Data on the WWW has spawned research interest in an online execution of declarative queries over this data. A particularly interesting approach is traversal-based query execution which fetches data by traversing data links and, thus, is able to make use of up-to-date data from initially unknown data sources. The downside of...
Book
In recent years, an increasing number of organizations and individuals have contributed to the Semantic Web by publishing data according to the Linked Data principles. In addition, a significant body of Semantic Web research exists that studies various aspects of knowledge representation and automated reasoning over collections of such data. Howeve...
Article
Billions of Linked Data triples exist in thousands of RDF knowledge graphs on the Web, but few of those graphs can be queried live from Web applications. Only a limited number of knowledge graphs are available in a queryable interface, and existing interfaces can be expensive to host at high availability. To mitigate this shortage of live queryable...
Article
Full-text available
Billions of Linked Data triples exist in thousands of RDF knowledge graphs on the Web, but few of those graphs can be queried live from Web applications. Only a limited number of knowledge graphs are available in a queryable interface, and existing interfaces can be expensive to host at high availability. To mitigate this shortage of live queryable...
Conference Paper
Full-text available
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 nove...
Technical Report
Full-text available
In this paper, we propose LDQL, that is, a language to query Linked Data on the World Wide Web. The novelty of LDQL is that it enables a user to express 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 show that LDQL is strictly more e...
Conference Paper
Full-text available
As of today, there exists no standard language for querying Linked Data on the Web, where navigation across distributed data sources is a key feature. A natural candidate seems to be SPARQL, which recently has been enhanced with navigational capabilities thanks to the introduction of property paths (PPs). However, the semantics of SPARQL restricts...
Conference Paper
Full-text available
Recent advances in Linked Data Management and the Semantic Web have led to a rapid increase in both the quantity as well as the variety of Web applications that rely on the SPARQL interface to query RDF data. Thus, RDF data management systems are increasingly exposed to workloads that are far more diverse and dynamic than what these systems were de...
Conference Paper
Full-text available
In this paper, we propose LDQL, that is, a language to query Linked Data on the Web. The novelty of LDQL is that it enables a user to express 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. As a downside of this expressiveness, we find t...
Conference Paper
Full-text available
As of today, there exists no standard language for querying Linked Data on the Web, where navigation across distributed data sources is a key feature. A natural candidate seems to be SPARQL, which recently has been enhanced with navigational capabilities thanks to the introduction of property paths (PPs). However, the semantics of SPARQL restricts...
Article
During recent years a set of best practices for publishing and connecting structured data on the World Wide Web (WWW) has emerged. These best practices are referred to as the Linked Data principles and the resulting form of Web data is called Linked Data. The increasing adoption of these principles has lead to the creation of a globally distributed...
Technical Report
Full-text available
Both the notion of Property Graphs (PG) and the Resource Description Framework (RDF) are commonly used models for representing graph-shaped data. While there exist some system-specific solutions to convert data from one model to the other, these solutions are not entirely compatible with one another and none of them appears to be based on a formal...
Conference Paper
Full-text available
The Resource Description Framework (RDF) is a standard for conceptually describing data on the Web, and SPARQL is the query language for RDF. As RDF data continue to be published across heterogeneous domains and integrated at Web-scale such as in the Linked Open Data (LOD) cloud, RDF data management systems are being exposed to queries that are far...
Conference Paper
For publishers of Linked Open Data, providing queryable access to their dataset is costly. Those that offer a public sparql endpoint often have to sacrifice high availability; others merely provide non-queryable means of access such as data dumps. We have developed a client-side query execution approach for which servers only need to provide a ligh...
Conference Paper
As the Web of Data is growing at an ever increasing speed, the lack of reliable query solutions for live public data becomes apparent. SPARQL implementations have matured and deliver impressive performance for public SPARQL endpoints, but poor availability—especially under high loads—prevents their use in real-world applications. We propose to tack...
Thesis
Full-text available
In recent years, an increasing number of organizations and individuals have contributed to the Semantic Web by publishing data according to the Linked Data principles. In addition, a significant body of Semantic Web research exists that studies various aspects of knowledge representation and automated reasoning over collections of such data. Howeve...
Thesis
In den letzten Jahren haben sich spezielle Prinzipien zur Veröffentlichung strukturierter Daten im World Wide Web (WWW) etabliert. Diese Prinzipien erlauben es, von den jeweils angebotenen Daten auf weitere, nach den selben Prinzipien veröffentlichten Daten zu verweisen. Die daraus resultierende Form von Web-Daten wird entsprechend als Linked Data...
Technical Report
Full-text available
This document defines extensions of the RDF data model and of the SPARQL query language that capture an alternative approach to represent statement-level metadata. While this alternative approach is backwards compatible with RDF reification as defined by the RDF standard, the approach aims to address usability and data management shortcomings of RD...
Conference Paper
Full-text available
Traversal-based approaches to execute queries over data on the Web have recently been studied. These approaches make use of up-to-date data from initially unknown data sources and, thus, enable applications to tap the full potential of the Web. While existing work focuses primarily on implementation techniques, a principled analysis of subwebs that...
Conference Paper
The publication of Linked Open Data on the Web has gained tremendous momentum over the last six years. As a consequence, we currently witness the emergence of a new research area that focuses on an online execution of Linked Data queries; i.e., declarative queries that range over Web data that is made available using the Linked Data publishing prin...
Conference Paper
Full-text available
The World Wide Web (WWW) currently evolves into a Web of Linked Data where content providers publish and link their data as they have done with hypertext for the last 20 years. We understand this emerging dataspace as a huge, distributed database which is -at best- partially known to query execution systems. To tap the full potential of the Web, su...
Book
This document specifies how to use standard Web protocols, including HTTP, to obtain information about the provenance of resources on the Web. We describe both simple access mechanisms for locating provenance records associated with web pages or resources, and provenance query services for more complex deployments. This is part of the larger W3C PR...
Conference Paper
Full-text available
Nowadays, more and more datasets are published on the Web adhering to the Linked Data principles. Our tutorial provides a beginners' introduction on how to query this data using the query language SPARQL.
Conference Paper
Full-text available
The emergence of a Web of Linked Data [2] enables new forms of application that require expressive query access, for which mature, Web-scale information retrieval techniques may not be suited. Rather than attempting to deliver expressive query capabilities at Web-scale, we propose the use of smaller, pre-populated data caches whose contents are per...
Technical Report
Full-text available
Query execution over the Web of Linked Data has attracted much attention recently. A particularly interesting approach is link traversal based query execution which proposes to integrate the traversal of data links into the construction of query results. Hence -in contrast to traditional query execution paradigms- this approach does not assume a fi...
Conference Paper
Full-text available
Query execution over the Web of Linked Data has attracted much attention recently. A particularly interesting approach is link traversal based query execution which proposes to integrate the traversal of data links into the creation of query results. Hence -in contrast to traditional query execution paradigms- this does not assume a fixed set of re...
Conference Paper
Full-text available
The emergence of a Web of Data enables new forms of application that require expressive query access, for which mature, Web-scale information retrieval techniques may not be suited. Rather than attempting to deliver expressive query capabilities at Web-scale, this paper proposes the use of smaller, pre-populated data caches whose contents are perso...
Conference Paper
Full-text available
The World Wide Web currently evolves into a Web of Linked Data where content providers publish and link data as they have done with hypertext for the last 20 years. While the declarative query language SPARQL is the de facto for querying a-priory defined sets of data from the Web, no language exists for querying the Web of Linked Data itself. Howev...
Technical Report
Full-text available
This document specifies how to use standard Web protocols, including HTTP, to obtain information about the provenance of Web resources. We describe both simple access mechanisms for locating provenance information associated with web pages or resources, and provenance query services for more complex deployments. This is part of the larger W3C Prov...
Conference Paper
Full-text available
Provenance provides vital information for evaluating quality and trustworthiness of information on the Web. To achieve this we must have access to semantically interchangeable provenance information and an agreement on where and how this information is to be located. The ongoing W3C Prove-nance Working Group provides a promise towards leverag-ing t...
Technical Report
Full-text available
Research on querying the Web of Data is still in its infancy. In this paper, we provide an initial set of general features that we envision should be considered in order to define a query language for the Web of Data. Furthermore, for each of these features, we pose questions that have not been addressed before in the context of querying the Web of...
Conference Paper
Full-text available
Link traversal based query execution is a new query execution paradigm for the Web of Data. This approach allows the execution engine to discover potentially relevant data during the query execution and, thus, enables users to tap the full potential of the Web. In earlier work we propose to implement the idea of link traversal based query execution...
Conference Paper
Full-text available
A possible approach to query Linked Data combines the actual evaluation of a query with the traversal of data links in order to discover and retrieve potentially relevant data. An implementation of this idea requires approaches that support an efficient and flexi-ble management of temporary, ad hoc data collections that emerge during query executio...
Conference Paper
Full-text available
Link traversal based query execution is a novel query approach which enables applications that exploit the Web of Data to its full potential. This approach makes use of the characteristics of Linked Data: During query execution it traverses data links to discover data that may contribute to query results. Once retrieved from the Web, the data can b...
Conference Paper
Full-text available
The World Wide Web evolves into a Web of Data, a huge, globally distributed dataspace that contains a rich body of machine- processable information from a virtually unbound set of providers cov- ering a wide range of topics. However, due to the openness of the Web little is known about who created the data and how. The fact that a large amount of t...
Conference Paper
Full-text available
In order to enable a reliable and approved consumption and processing of Linked Data in applications it requires various information about the consumed data (e.g. licensing and provenance). Since a large amount of this information is available to the publishers of the data it should become a practice to provide this information as metadata. We demo...
Article
Full-text available
During recent years an increasing number of data providers adopted the Linked Data principles for publishing and connecting structured data on the Web, thus creating a globally distributed dataspace—the Web of Data. While the execution of structured, SQL-like queries over this dataspace opens possibilities not conceivable before, query execution on...
Conference Paper
Full-text available
Existing research on trust in the Semantic Web extensively studies trustworthiness and trust in the context of active entities such as persons and agents. However, few work exist that focus on the content in the Semantic Web and that study trustworthiness as an information quality criterion. Hence, computer systems that use the trustworthiness of S...
Conference Paper
Full-text available
In the past two years, the amount of data published in RDF and following the Linked Data principles has increased dramatically. Everyday people are publishing datasets as Linked Data. However, applications that consume Linked Data are not mainstream yet. To overcome this issue, we present a beginners tutorial on consuming Linked Data. We will discu...
Conference Paper
Full-text availa