Federated SPARQL Queries Processing with Replicated Fragments

Federated query engines provide a unified query interface to federations of SPARQL endpoints. Replicating data fragments from different Linked Data sources facilitates data re-organization to better fit federated query processing needs of data consumers. However, existing federated query engines are not designed to support replication and replicated data can negatively impact their performance. In this paper, we formulate the source selection problem with fragment replication (SSP-FR). For a given set of endpoints with replicated fragments and a SPARQL query, the problem is to select the endpoints that minimize the number of tuples to be transferred. We devise the FEDRA source selection algorithm that approximates SSP-FR. We implement FEDRA in the state-of-the-art federated query engines FedX and ANAPSID, and empirically evaluate their performance. Experimental results suggest that FEDRA efficiently solves SSP-FR, reducing the number of selected SPARQL endpoints as well as the size of query intermediate results.