Opportunistic Linked Data Querying through Approximate Membership Metadata

Between uri dereferencing and the sparql protocol lies a largely unexplored axis of possible interfaces to Linked Data, each with its own combination of trade-offs. One of these interfaces is Triple Pattern Fragments, which allows clients to execute sparql queries against low-cost servers, at the cost of higher bandwidth. Increasing a client’s efficiency means lowering the number of requests, which can among others be achieved through additional metadata in responses. We noted that typical sparql query evaluations against Triple Pattern Fragments require a significant portion of membership subqueries, which check the presence of a specific triple, rather than a variable pattern. This paper studies the impact of providing approximate membership functions, i.e., Bloom filters and Golombcoded sets, as extra metadata. In addition to reducing http requests, such functions allow to achieve full result recall earlier when temporarily allowing lower precision. Half of the tested queries from a WatDiv benchmark test set could be executed with up to a third fewer http requests with only marginally higher server cost. Query times, however, did not improve, likely due to slower metadata generation and transfer. This indicates that approximate membership functions can partly improve the client-side query process with minimal impact on the server and its interface.