Driven by initiatives like Schema.org, the amount of semantically annotated data is expected to grow steadily towards massive scale, requiring cluster-based solutions to query it. At the same time, Hadoop has become dominant in the area of Big Data processing with large infrastructures being already deployed and used in manifold application fields. For Hadoop-based applications, a common data pool (HDFS) provides many synergy benefits, making it very attractive to use these infrastructures for semantic data processing as well. Indeed, existing SPARQL-on-Hadoop (MapReduce) approaches have already demonstrated very good scalability, however, query runtimes are rather slow due to the underlying batch processing framework. While this is acceptable for data-intensive queries, it is not satisfactory for the majority of SPARQL queries that are typically much more selective requiring only small subsets of the data. In this paper, we present Sempala, a SPARQL-over-SQL-on-Hadoop approach designed with selective queries in mind. Our evaluation shows performance improvements by an order of magnitude compared to existing approaches, paving the way for interactive-time SPARQL query processing on Hadoop.

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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.

RDF datasets with billions of triples are no longer unusual and continue to grow constantly (e.g. LOD cloud) driven by the inherent flexibility of RDF that allows to represent very diverse datasets, ranging from highly structured to unstructured data. Because of their size, understanding and processing RDF graphs is often a difficult task and methods to reduce the size while keeping as much of its structural information become attractive. In this paper we study bisimulation as a means to reduce the size of RDF graphs according to structural equivalence. We study two bisimulation algorithms, one for sequential execution using SQL and one for distributed execution using MapReduce. We demonstrate that the MapReduce-based implementation scales linearly with the number of the RDF triples, allowing to compute the bisimulation of very large RDF graphs within a time which is by far not possible for the sequential version. Experiments based on synthetic benchmark data and real data (DBPedia) exhibit a reduction of more than 90% of the size of the RDF graph in terms of the number of nodes to the number of blocks in the resulting bisimulation partition.

Abstract. The MapReduce programming model has gained traction in different application areas in recent years, ranging from the analysis of log files to the computation of the RDFS closure. Yet, for most users the MapReduce abstraction is too low-level since even simple computations have to be expressed as Map and Reduce phases. In this paper we propose RDFPath, an expressive RDF path query language geared towards casual users that benefits from the scaling properties of the MapReduce framework by automatically transforming declarative path queries into MapReduce jobs. Our evaluation on a real world data set shows the applicability of RDFPath for investigating typical graph properties like shortest paths.