@dcausse @Gehel @WolfgangFahl QLever can now also produce application/sparql-results+xml. Here is an example:

@dcausse I am confused, where does https://data.nlg.gr/sparql come from? I thought the endpoint in question were https://qlever.cs.uni-freiburg.de/api/dblp and https://qlever.cs.uni-freiburg.de/api/wikidata, where the following command lines work just fine:

@Gehel Thanks for the reply! But to clarify, what I am asking is not to do something different for different deferation endpoints. It's the same for every federation endpoint, namely sending the header

Is it possible to configure Blazegraph to send the following Accept header:

To add to this, the two-index approach has another rather beautiful property:

@Pulquero AFAIK the two main problems with Blazegraph are:

@Hannah_Bast maybe this interests you? Do you think this system would perform well considering the load on WDQS and the type of queries we have?

@Pulquero Thank you for this interesting piece of information. I have a few questions:

In T206560#7562538, @Fnielsen wrote:

I am taking the liberty to polute the thread with a reference to "MillenniumDB: A Persistent, Open-Source, Graph Database" https://arxiv.org/pdf/2111.01540.pdf from November 2021. Millennium may have some serious limitations in terms of requirements that can be setup, but interestingly they write "However, MillenniumDB was designed with the complete version of Wikidata – including qualifiers, references, etc. – in mind." and their benchmarks seems strong. They compare against Blazegraph, Jena, Virtuoso and Neo4J.

In T206560#7430950, @AndreasKuczera wrote:

I imported the wikidata-DB into neo4j and it works quite well.

@DD063520: You find some details at https://github.com/ad-freiburg/qlever/blob/master/docs/quickstart.md .

Oh, ok. Could you give an example of a query that has no "highly selective triples" so I can test it on QLever vs. BG?

@Justin0x2004 Thanks, Justin. QLever already supports something like named subqueries. You can simply have the same subquery in multiple places and it will be evaluated only once and for the other occurrences, the result will be reused.

@So9q I have commented on your comments concerning Rya in the "Evaluate Apache Rya as alternative to Blazegraph": https://phabricator.wikimedia.org/T289561#7393732

I have now revised QLever's Quickstart page: https://github.com/ad-freiburg/qlever

We looked a bit into Apache Rya. A couple of observations:

I will provide a detailed reply later today, also to the other thread. Four things for now:

It's of course up to you (the Wikidata team) to decide this. But I wouldn't dismiss this idea so easily.

Wikibase doesn’t store data in RDF, so dumping the data set means parsing the native representation (JSON) and writing it out again as RDF, including some metadata for each page.

Can you or anyone else explain why the data dump takes so long, Lukas? One would expect that it is much easier to dump a (snapshot of a) dataset than to build a complex data structure from it. Also, dumping and compression are easily parallelized. And the pure volume isn't that large (< 100 GB compressed).

I agree with Kingsley that you don't need a distributed SPARQL engine when the knowledge graph fits on a single machine and will do so also in the future. Which is clearly the case for Wikidata, since it's even the case for the ten times larger UniProt (which at the time of this writing already contains over 90 billion triples).

For whoever is interested, I wrote more about the QLever SPARQL engine on this thread: https://phabricator.wikimedia.org/T290839 .

PS: Note that large query throughputs are not a problem for a SPARQL engine that runs on a single standard PC or server. Depending on the overall demand, you can just run multiple instances on separate machines and trivially distribute the queries. What's more important, I think, is the processing time for individual queries because you cannot easily distribute the processing of an individual query. And it does make quite a difference for the user experience whether a query takes seconds, minutes, or hours. The current SPARQL endpoint for Wikidata (realized using Blazegraph) times out a lot when the queries are a bit harder.

Yes, QLever is developed in our group at the University of Freiburg. I presented it to the Wikidata team in March. You can try out a demo on the complete Wikidata on https://qlever.cs.uni-freiburg.de/wikidata . You can also select other interesting large knowledge graphs there, for example, the complete OpenStreetMap data.

I have already talked about Sage with Lukas last November. I don't think that Sage is an option for Wikidata. The focus of Sage is on the ability to pause and resume SPARQL queries (which is a very useful feature), not on efficiency. For example, if you run the people-professions query from https://phabricator.wikimedia.org/T206560 on their demo instance of Wikidata http://sage.univ-nantes.fr/#query (which has only 2.3B triples), it takes forever. Also simple queries are quite slow. For example, the following query (all humans) produces results at a rate of around a thousand rows per second:

Would it be an option that one of these two backends uses a SPARQL engine that does not support the SPARQL Update operation, but instead rebuilds its index periodically, for example, every 24 hours?

Thanks, Kingsley, that explains it!

[1] Our Live Wikidata SPARQL Query Endpoint