SPARQL federated query debugging tool

Gaining insight into a complex problem often requires combining data from multiple datasets. For this reason, feder- ated SPARQL query support is an important technology. However, several pitfalls have been encountered in practice, significantly complicating the use of federated SPARQL queries. These challenges include uninformative error responses, performance bot- tlenecks and unintended semantic changes introduced by SPARQL endpoints. To address these pitfalls, this paper introduces a newly implemented federated SPARQL query debugger, which is available as a web application at https://sparql-debugger.elixir- czech.cz. It has been developed for the purpose of monitoring, in real time, the execution of federated SPARQL queries. This monitoring is crucial for error detection and performance optimization. Detailed service execution data (such as SPARQL re- quests and responses, durations, etc.) can help identify the specific instance of a service responsible for a problem, even if it is deeply nested within the service execution tree. The tool is based on the principle of redirecting all requests to a debugging proxy server, so it can be used with all SPARQL-compliant endpoints without the need for their modification. The debugging tool presented in the paper enables the identification and resolution of issues that are otherwise difficult to address and has proven its effectiveness in practice.