[Wikidata] Scaling Wikidata Query Service

Hello all! There has been a number of concerns raised about the performance and scaling of Wikdata Query Service. We share those concerns and we are doing our best to address them. Here is some info about what is going on: In an ideal world, WDQS should: * scale in terms of data size * scale in terms of number of edits * have low update latency * expose a SPARQL endpoint for queries * allow anyone to run any queries on the public WDQS endpoint * provide great query performance * provide a high level of availability Scaling graph databases is a "known hard problem", and we are reaching a scale where there are no obvious easy solutions to address all the above constraints. At this point, just "throwing hardware at the problem" is not an option anymore. We need to go deeper into the details and potentially make major changes to the current architecture. Some scaling considerations are discussed in [1]. This is going to take time. Reasonably, addressing all of the above constraints is unlikely to ever happen. Some of the constraints are non negotiable: if we can't keep up with Wikidata in term of data size or number of edits, it does not make sense to address query performance. On some constraints, we will probably need to compromise. For example, the update process is asynchronous. It is by nature expected to lag. In the best case, this lag is measured in minutes, but can climb to hours occasionally. This is a case of prioritizing stability and correctness (ingesting all edits) over update latency. And while we can work to reduce the maximum latency, this will still be an asynchronous process and needs to be considered as such. We currently have one Blazegraph expert working with us to address a number of performance and stability issues. We are planning to hire an additional engineer to help us support the service in the long term. You can follow our current work in phabricator [2]. If anyone has experience with scaling large graph databases, please reach out to us, we're always happy to share ideas! Thanks all for your patience! Guillaume [1] https://wikitech.wikimedia.org/wiki/Wikidata_query_service/ScalingStrategy [2] https://phabricator.wikimedia.org/project/view/1239/ -- Guillaume Lederrey Engineering Manager, Search Platform Wikimedia Foundation UTC+2 / CEST