Ensuring License Compliance in Linked Data with Query Relaxation

When two or more licensed datasets participate in evaluating a federated query, to be reusable, the query result must be protected by a license compliant with each license of the involved datasets. Due to incompatibilities or contradictions among licenses, such a license does not always exist, leading to a query result that cannot be licensed nor reused on a legal basis. We propose to deal with this issue during the federated query processing by dynamically discarding datasets of conflicting licenses. However, this solution may generate an empty query result. To face this problem, we use query relaxation techniques. Our problem statement is, given a SPARQL query and a federation of licensed datasets, how to guarantee a relevant and non-empty query result whose license is compliant with each license of involved datasets? To detect and prevent license conflicts, we propose FLiQue, a license-aware query processing strategy for federated query engines. Our challenge is to limit communication costs when the query relaxation process is necessary. Experiments show that FLiQue guarantees license compliance, and if necessary, can find relevant relaxed federated queries with a limited overhead in terms of execution time.