This weekend we have been trying SPARQL federation queries. If we do https://data.pdok.nl directly ( http://tinyurl.com/y9hl3vd5 ) it fails, if we federate it through another SPARQL server it works ( http://tinyurl.com/y9bc3clv ). It also works from the SPARQL test instance on wmflabs and a curl from the production Blazegraph server.
|Open||None||T197530 [tracking] federation query issues on Wikidata Query Server|
|Resolved||Smalyshev||T195203 Federated SPARQL queries https://data.pdok.nl/sparql failing with error 500|
The end of the stack trace, for future reference:
Caused by: com.bigdata.rdf.sail.webapp.client.HttpException: Status Code=500, Status Line=Internal Server Error, Response=<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <html><head> <title>500 Internal Server Error</title> </head><body> <h1>Internal Server Error</h1> <p>The server encountered an internal error or misconfiguration and was unable to complete your request.</p> <p>Please contact the server administrator, root@localhost and inform them of the time the error occurred, and anything you might have done that may have caused the error.</p> <p>More information about this error may be available in the server error log.</p> </body></html> at com.bigdata.rdf.sail.webapp.client.RemoteRepositoryBase.checkResponseCode(RemoteRepositoryBase.java:102) at com.bigdata.rdf.sail.webapp.client.RemoteRepositoryManager.tupleResults(RemoteRepositoryManager.java:2178) at com.bigdata.rdf.sparql.ast.service.RemoteServiceCallImpl.call(RemoteServiceCallImpl.java:153) at com.bigdata.rdf.sparql.ast.service.RemoteServiceCallImpl.call(RemoteServiceCallImpl.java:51) at com.bigdata.bop.controller.ServiceCallJoin$ChunkTask$ServiceCallTask.doNonBigdataSesameServiceCall(ServiceCallJoin.java:839) at com.bigdata.bop.controller.ServiceCallJoin$ChunkTask$ServiceCallTask.doRemoteServiceCall(ServiceCallJoin.java:803) at com.bigdata.bop.controller.ServiceCallJoin$ChunkTask$ServiceCallTask.doServiceCall(ServiceCallJoin.java:717) ... 6 more
If this gets stuck on debugging on our side, I could probably ask around for contact details for the people running https://data.pdok.nl/sparql . On https://www.wikidata.org/wiki/Wikidata:SPARQL_query_service/Federation_report several other endpoints are also failing. Solving the pdok.nl problem might fix some of the others too.
We probably can't fix it until we migrate to Jetty that supports ALPN. On the other side, it may be possible to fix it if the server would handle non-APLN clients properly but I am not sure how to configure and test it, unfortunately... I don't have too much knowledge into inner workings of ALPN and I have no idea which software data.pdok.nl runs.