Hello all!

TL;DR: We expect to successfully complete the recent data reload on Wikidata Query Service soon, but we've encountered multiple failures related to the size of the graph, and anticipate that this issue may worsen in the future. Although we succeeded this time, we cannot guarantee that future reload attempts will be successful given the current trend of the data reload process. Thank you for your understanding and patience..

Longer version:

WDQS is updated from a stream of recent changes on Wikidata, with a maximum delay of ~2 minutes. This process was improved as part of the WDQS Streaming Updater project to ensure data coherence[1] . However, the update process is still imperfect and can lead to data inconsistencies in some cases[2][3]. To address this, we reload the data from dumps a few times per year to reinitialize the system from a known good state.

The recent reload of data from dumps started in mid-December and was initially met with some issues related to download and instabilities in Blazegraph, the database used by WDQS[4]. Loading the data into Blazegraph takes a couple of weeks due to the size of the graph, and we had multiple attempts where the reload failed after >90% of the data had been loaded. Our understanding of the issue is that a "race condition" in Blazegraph[5], where subtle timing changes lead to corruption of the journal in some rare cases, is to blame.[6]

We want to reassure you that the last reload job was successful on one of our servers. The data still needs to be copied over to all of the WDQS servers, which will take a couple of weeks, but should not bring any additional issues. However, reloading the full data from dumps is becoming more complex as the data size grows, and we wanted to let you know why the process took longer than expected. We understand that data inconsistencies can be problematic, and we appreciate your patience and understanding while we work to ensure the quality and consistency of the data on WDQS.

Thank you for your continued support and understanding!


    Guillaume


[1] https://phabricator.wikimedia.org/T244590
[2] https://phabricator.wikimedia.org/T323239
[3] https://phabricator.wikimedia.org/T322869
[4] https://phabricator.wikimedia.org/T323096
[5] https://en.wikipedia.org/wiki/Race_condition#In_software
[6] https://phabricator.wikimedia.org/T263110

--

Guillaume Lederrey (he/him) Engineering Manager Wikimedia Foundation

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