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.

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.