RDF has become very popular for semantic data publishing due to its flexible and universal graph-like data model. Thus, the ever-increasing size of RDF data collections raises the need for scalable distributed approaches. We endorse the usage of existing infrastructures for Big Data processing like Hadoop for this purpose. Yet, SPARQL query performance is a major challenge as Hadoop is not intentionally designed for RDF processing. Existing approaches often favor certain query pattern shapes while performance drops significantly for other shapes. In this paper, we introduce a novel relational partitioning schema for RDF data called ExtVP that uses a semi-join based preprocessing, akin to the concept of Join Indices in relational databases, to efficiently minimize query input size regardless of its pattern shape and diameter. Our prototype system S2RDF is built on top of Spark and uses SQL to execute SPARQL queries over ExtVP. We demonstrate its superior performance in comparison to state of the art SPARQL-on-Hadoop approaches.

Navigational queries are among the most natural query patterns for RDF data, but yet most existing RDF query languages fail to cover all the varieties inherent to its triple-based model, including SPARQL 1.1 and its derivatives. As a consequence, the development of more expressive RDF languages is of general interest. With TriAL* [14], there exists an expressive algebra which subsumes many previous approaches, while adding novel features that are not expressible in most other RDF query languages based on the standard graph model. However, its algebraic notation is inappropriate for practical usage and it is not supported by any existing RDF triple store. In this paper, we propose TriAL-QL, an easy to write and grasp language for TriAL*, preserving its compositional algebraic structure. We present an implementation based on Impala, a massive parallel SQL query engine on Hadoop, using an optimized semi-naive evaluation for the recursive fragments of TriAL*. This way, we support both data-intensive ETL-like workloads and explorative ad-hoc style queries. To demonstrate the scalability and expressiveness of our approach, we conducted experiments on generated social networks with up to 1.8 billion triples and compared different execution strategies to a Hive-based solution.

Taste in music is of highly subjective nature, making the recommending of music tracks a challenging research task. With TRECS, our live prototype system, we present a weighted hybrid recommender approach that amalgamates three diverse recommender te

In recent times, it has been widely recognized that, due to their inherent scalability, frameworks based on MapReduce are indispensable for so-called "Big Data" applications. However, for Semantic Web applications using SPARQL, there is still a demand for sophisticated MapReduce join techniques for processing basic graph patterns, which are at the core of SPARQL. Renowned for their stable and efficient performance, sort-merge joins have become widely used in DBMSs. In this paper, we demonstrate the adaptation of merge joins for SPARQL BGP processing with MapReduce. Our technique supports both n-way joins and sequences of join operations by applying merge joins within the map phase of MapReduce while the reduce phase is only used to fulfill the preconditions of a subsequent join iteration. Our experiments with the LUBM benchmark show an average performance benefit between 15% and 48% compared to other MapReduce based approaches while at the same time scaling linearly with the RDF dataset size.

Taste in music is of highly subjective nature, making the recommending of music tracks a challenging research task. With TRecS, our live prototype system, we present a weighted hybrid recommender approach that amalgamates three diverse recommender techniques into one comprehensive score. Moreover, our system peppers the generated result list with recommendations based on a simple serendipity heuristic. This way, users can benefit from recommendations aligned with their current taste in music while gaining some exploratory diversification. An explanation feature helps the user understand the rationale behind each of the tracks being recommended to him. Empirical evaluations of the live system, based on an online evaluation, assess the overall recommendation quality as well as the impact of each of the three sub-recommenders.