On 2/24/23 5:59 AM, Guillaume Lederrey wrote:

On Thu, 23 Feb 2023 at 22:56, Kingsley Idehen <kidehen@openlinksw.com> wrote:

On 2/23/23 3:09 PM, Guillaume Lederrey wrote:

On Thu, 23 Feb 2023 at 16:39, Kingsley Idehen <kidehen@openlinksw.com> wrote:

On 2/22/23 3:28 AM, Guillaume Lederrey wrote:

On Wed, 22 Feb 2023 at 00:03, Kingsley Idehen via Wikidata <wikidata@lists.wikimedia.org> wrote:

On 2/21/23 4:05 PM, Guillaume Lederrey wrote:
> 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
>
Hi Guillaume,

Are there plans to decouple WDQS from the back-end database? Doing that
provides more resilient architecture for Wikidata as a whole since you
will be able to swap and interchange SPARQL-compliant backends.

It depends what you mean by decoupling. The coupling points as I see them are:

* update process

* UI

* exposed SPARQL endpoint

The update process is mostly decoupled from the backend. It is producing a stream of RDF updates that is backend independent, with a very thin Blazegraph specific adapted to load the data into Blazegraph.

Does that mean that we could integrate the RDF stream into our setup re keeping our Wikidata instance up to date, for instance?

That data stream isn't exposed publicly. There are a few tricky part about the stream needing to be synchronized with a specific Wikidata dump that makes it not entirely trivial to reuse outside of our internal use case. But if there is enough interest, we could potentially work on making that stream public.

I suspect there's broad interest in this matter since it contributes to the overarching issue of loose-coupling re Wikidata's underlying infrastructure.

For starters, offering a public stream would be very useful to 3rd party Wikidata hosts.

The UI is mostly backend independant. It relies on Search for some features. And of course, the queries themselves might depend on Blazegraph specific features.

Can WDQS, based on what's stated above, work with a generic SPARQL back-end like Virtuoso, for instance? By that I mean dispatch SPARQL queries input by a user (without alteration) en route to server processing?

The WDQS UI is managed by WMDE, my knowledge is limited. Maybe someone from WMDE could jump in and add more context. That being said, as far as I know, pointing it to a different backend is just a configuration option. Feel free to have a look at the code (https://gerrit.wikimedia.org/g/wikidata/query/gui).

I'll take a look.

It should be reasonably easy to deploy another WDQS UI instance somewhere else, which points to whatever backend you'd like.

Okay, I assume that in the current state it would be sending Blazegraph-specific SPARQL?

Again, not my area of expertise, but I assume that the UI itself is issuing fairly standard SPARQL. Of course, user queries will use whatever they want. It does have dependencies on our Search interface as well, so that would have to be replicated.

You mean WDQS has a Text Search interface component that's intertwined with the Query Service provided by the Wikidata SPARQL Endpoint?

As a policy, we don't send traffic to any third party, so we will not setup such an instance.

The exposed SPARQL endpoint is at the moment a direct exposition of the Blazegraph endpoint, so it does expose all the Blazegraph specific features and quirks.

Is there a Query Service that's separated from the Blazegraph endpoint? The crux of the matter here is that WDQS benefits more by being loosely- bound to endpoints rather than tightly-bound to the Blazegraph endpoint.

It depends what you mean by Query Service. My definition of a Query Service in this context is a SPARQL endpoint with a specific data set.

Yes, but in the case of Wikidata that's a combination of both a SPARQL Query Service (query processor and endpoint) and WDQS query solution rendering services.

That SPARQL endpoint at the moment is Blazegraph. I'm not entirely clear what kind loose bound you'd like to see in this context. We might have different definitions of the same words here.

Loose-coupling, in the context I am describing, would comprise the following:

1. WDQS that can be bolted on to any SPARQL endpoint, just like YASGUI

In this context, I would say "WDQS UI can be bolted to any SPARQL endpoint". In term of SPARQL itself, that should already be mostly the case. I think there is a dependency on Search as well.

As per my earlier comment, I don't quite understand what you are referring to regarding the Search (Free Text Querying) intermingling. Does this relate to SPARQL Query Patterns comprising literal objects? If so, WDQS should be able to constrain such behavior to Blazegraph instances -- by way of configuration that informs introspection.

WDQS UI relies on a Search endpoint (backed by Elasticsearch) for auto completion. The requirements of low latency and reasonable ranking are something that Elasticsearch (or another Search oriented backend) does really well. But I would not expect an RDF backend to offer good ranking heuristics.