Bernhard Schandl

Replication and versioning of partial RDF graphs

The sizes of datasets available as RDF (e.g., as part of the Linked Data cloud) are increasing continuously. For instance, the recent DBpedia version consists of nearly 500 millions triples. A common strategy to avoid problems that arise e.g., from limited network connectivity or lack of bandwidth is to replicate data locally, therefore making them accessible for applications without depending on a network connection. For mobile devices with limited capabilities, however, the replication and synchronization of billions of triples is not feasible. To overcome this problem, we propose an approach to replicate parts of an RDF graph to a client. Applications may then apply changes to this partial replica while being offline; these changes are written back to the original data source upon reconnection. Our approach does not require any kind of additional logic (e.g., change logging) or data structures on the client side, and hence is suitable to be applied on devices with limited computing power and storage capacity.

Interweaving OAI-PMH data sources with the linked data cloud

The Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH) has found widespread adoption for exchanging bibliographic metadata. In parallel, the W3Cs Linking Open Data Initiative exposes and interlinks structured data from a variety of data sources on the web. Since many of these data sources contain valuable information for institutional repositories (e.g., shared concept definitions, thesauri, etc.), we believe that institutions that currently expose their data via OAI-PMH can benefit if they integrate their metadata with the data available in the Linked Data cloud. To achieve such an integration, we must bridge the OAI-PMH-specific protocol characteristics that currently prevent OAI-PMH metadata from being interoperable with the Linked Data approach of exposing data. As first contribution of this paper, we describe a possible solution for exposing OAI-PMH metadata on the web as part of the Linked Data cloud. As a second contribution, we present a rule-based mechanism for linking these metadata with other relevant data sources together with a case study that describes possible linking scenarios for three representative OAI-PMH data providers. Finally, we discuss certain quality criteria that OAI-PMH metadata must meet to benefit from data exposed by other Linked Data sources.

Linked Data and multimedia: the state of affairs

Linked Data is a way of exposing and sharing data as resources on the Web and interlinking them with semantically related resources. In the last three years significant amounts of data have been generated, increasingly forming a globally connected, distributed data space. For multimedia content, metadata are a key factor for efficient management, organization, and retrieval. However, the relationship between multimedia and Linked Data has been rarely studied, leading to a lack of mutual awareness and, as a consequence thereof, technological deficiencies. This article introduces the basic concepts of Linked Data in the context of multimedia metadata, and discusses techniques to generate, expose, discover, and consume Linked Data. It shows that a large amount of data sources exist, which are ready to be exploited by multimedia applications. The benefit of Linked Data in two multimedia-related applications is discussed and open research issues are outlined with the goal of bringing the research fields of multimedia and Linked Data closer together in order to facilitate mutual benefit.

A framework for context-driven RDF data replication on mobile devices

With the continuously growing amount of structured data available on the Semantic Web there is an increasing desire to replicate such data to mobile devices. This enables services and applications to operate independently of the network connection quality. Traditional replication strategies cannot be properly applied to mobile systems because they do not adopt to changing user information needs, and do not consider the technical, environmental, and infrastructural restrictions of mobile devices. Therefore, it is reasonable to consider contextual information, gathered from physical and logical sensors, in the replication process, and replicate only data that are actually needed by the user. In this paper we present a framework that uses Semantic Web technologies to build comprehensive descriptions of the users information needs based on contextual information, and employs these descriptions to selectively replicate data from external sources. In consequence, the amounts of replicated data are reduced, while a maximum share of relevant data are continuously available to be used by applications, even in situations with limited or no network connectivity.

Context-driven RDF data replication on mobile devices

With the continuously growing amount of structured data available on the Semantic Web there is an increasing desire to replicate such data to mobile devices. This enables services and applications to operate independently of the network connection quality. Traditional replication strategies cannot be properly applied to mobile systems because they do not adopt to changing user information needs, and they do not consider the technical, environmental, and infrastructural restrictions of mobile devices. Therefore, it is reasonable to consider contextual information, gathered from physical and logical sensors, in the replication process, and replicate only data that are actually needed by the user. In this paper we present a framework that uses Semantic Web technologies to build comprehensive descriptions of the users information needs based on contextual information, and employs these descriptions to selectively replicate data from external sources. In consequence, the amount of replicated data is reduced, while a maximum share of relevant data are continuously available to be used by applications, even in situations with limited or no network connectivity.

The SemDAV project: metadata management for unstructured content

Despite the evolution of complex storage facilities for different application domains (e.g. multimedia-enabled databases, or RDF triple stores), the file system is still the major repository paradigm for unstructured content that is created, stored and managed by end-users. Many desktop file systems support the addition of metadata to files, but there exists no platform-independent agreement on how to retrieve and manage metadata, be it in a users private storage or in shared data repositories.In this paper, we point out why the file system has significant drawbacks regarding efficient management of metadata, and how extensions to the well-known WebDAV protocol can be designed so that metadata can be retrieved, stored, managed and utilized for increased user benefit in searching and browsing situations. We describe the drawbacks of traditional file systems and strategies how to overcome their limitations. Finally, we express ideas regarding how the file system API for end user applications can be extended so that applications are enabled to annotate content with attention metadata, and describe how we plan to realize these ideas in the course of the SemDAV project.

iweightings: Enhancing Structure-based Ontology Alignment by Enriching Models with Importance Weighting

Structural ontology matching methods analyze mainly two factors: entity labels and relationships among entities. We propose to additionally consider an importance and relevance factor, which is determined by two indicators automatically calculated by a (simple) weighting method. This weighting factor represents the importance of a concept based on its information significance in the modeling context and, additionally, its relevance for structure-based alignment depending on the number of relationships this concept participates in quantified by the rweighting indicator. The method starts via a manually weighting annotation of relationships among concepts conducted by ontology engineers during the ontology development process. Our approach is an assistance mechanism to improve the ontology alignment process and to enhance the cognitive support for users. Thus, ontology alignment becomes already important ex ante when the ontology development process starts, unlike other alignment techniques, which consider only ex post knowledge.

Functions over RDF Language Elements

RDF data are usually accessed using one of two methods: either, graphs are rendered in forms perceivable by human users (e.g., in tabular or in graphical form), which are difficult to handle for large data sets. Alternatively, query languages like SPARQL provide means to express information needs in structured form; hence they are targeted towards developers and experts. Inspired by the concept of spreadsheet tools, where users can perform relatively complex calculations by splitting formulas and values across multiple cells, we have investigated mechanisms that allow us to access RDF graphs in a more intuitive and manageable, yet formally grounded manner. In this paper, we make three contributions towards this direction. First, we present RDFunctions, an algebra that consists of mappings between sets of RDF language elements (URIs, blank nodes, and literals) under consideration of the triples contained in a background graph. Second, we define a syntax for expressing RDFunctions, which can be edited, parsed and evaluated. Third, we discuss Tripcel, an implementation of RDFunctions using a spreadsheet metaphor. Using this tool, users can easily edit and execute function expressions and perform analysis tasks on the data stored in an RDF graph.