2.1. Introduction

The Semantic Web is rapidly growing, generating large volumes of Resource Description Framework (RDF) data [W3C 14] stored in the Linked Open Data (LOD) cloud. With data sets ranging from hundreds of millions to billions of triples, RDF triple stores are expected to meet properties such as scalability, high availability, automatic work distribution and fault tolerance. This chapter is dedicated to the problem of evaluating SPARQL queries over large RDF datasets. Section 2.2 introduces the RDF data model and the SPARQL query language. The challenges and solutions for efficiently processing SPARQL queries and in particular basic graph pattern (BGP) expressions are presented in section 2.3. The specific solution using the MapReduce framework for processing SPARQL graph patterns [DEA 04] is introduced in section 2.4. The chapter concludes with section 2.5, describing the use of Apache Spark and explaining the importance of the physical data layers for the query performance...