Map-Side Merge Joins for Scalable SPARQL BGP Processing

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.