rdf Stream Processing (rsp) is a rapidly evolving area of research that focuses on extensions of the Semantic Web in order to model and process Web data streams. While state-of-the-art approaches concentrate on server-side processing of rdf streams, we investigate the Triple Pattern Fragments Query Streamer (tpf-qs) method for server-side publishing of rdf streams, which moves the workload of continuous querying to clients. We formalize tpf-qs in terms of the rsp-ql reference model in order to formally compare it with existing rsp query languages.
The runtime optimization of federated SPARQL query engines is of central importance to ensure the usability of the Web of Data in real-world applications. The efficient selection of sources (SPARQL endpoints in our case) as well as the generation of optimized query plans belong to the most important optimization steps in this respect. This paper presents CostFed, an index-assisted federation engine for federated SPARQL query processing. CostFed makes use of statistical information collected from endpoints to perform efficient source selection and cost-based query planning.