Federation and Navigation in SPARQL 1.1

Abstract

SPARQL is now widely used as the standard query language for RDF. Since the release of its first version in 2008, the W3C group in charge of the standard has been working on extensions of the language to be included in the new version, SPARQL 1.1. These extensions include several interesting and very useful features for querying RDF. In this paper, we survey two key features of SPARQL 1.1: Federation and navigation capabilities. We first introduce the SPARQL standard presenting its syntax and formal semantics. We then focus on the formalization of federation and navigation in SPARQL 1.1. We analyze some classical theoretical problems such as expressiveness and complexity, and discuss algorithmic properties. Moreover, we present some important recently discovered issues regarding the normative semantics of federation and navigation in SPARQL 1.1, specifically, on the impossibility of answering some unbounded federated queries and the high computational complexity of the evaluation problem for queries including navigation functionalities. Finally, we discuss on possible alternatives to overcome these issues and their implications on the adoption of the standard.