A Framework for Federated SPARQL Query Processing over Heterogeneous Linked Data Fragments

In recent years, the number of Linked Data Fragment (LDF) interfaces to query RDF data on the Web has increased. These interfaces differ in the SPARQL expressions they can evaluate and metadata they provide. Client-side query processing approaches have been proposed and are optimized to evaluate queries over individual interfaces. Moreover, federated query processing has focused on federations with a single type of LDF interface only, typically SPARQL endpoints. In this work, we address the challenges of SPARQL query processing over federations with heterogeneous LDF interfaces. To this end, we formalize the concept of federations of Linked Data Fragment services and propose a framework for federated querying over heterogeneous federations with different LDF interfaces. The framework comprises query decomposition, query planning, and physical operators adapted to the particularities of different LDF interfaces. Further, we propose an approach for each component of our framework and evaluate these approaches in an experimental study on the well-known FedBench benchmark. The results show a substantial improvement in performance that can be achieved by devising these interface-aware approaches to exploit the capabilities of heterogeneous interfaces in the federation.