Federating queries in SPARQL 1.1: Syntax, semantics and evaluation.

Abstract

Given the sustained growth that we are experiencing in the number of SPARQL endpoints available,the need to be able to send federated SPARQL queries across these has also grown. To address this usecase, the W3C SPARQL working group is defining a federation extension for SPARQL 1.1 which allowsfor combining graph patterns that can be evaluated over several endpoints within a single query. In thispaper, we describe the syntax of that extension and formalize its semantics. Additionally, we describehow a query evaluation system can be implemented for that federation extension, describing some staticoptimization techniques and reusing a query engine used for data-intensive science, so as to deal withlarge amounts of intermediate and final results. Finally we carry out a series of experiments that showthat our optimizations speed up the federated query evaluation process.