Hello Guillaume,

Le ven. 7 févr. 2020 à 14:33, Guillaume Lederrey
<glederrey@wikimedia.org> a écrit :
>
> Hello all!
>
> First of all, my apologies for the long silence. We need to do better in terms of communication. I'll try my best to send a monthly update from now on. Keep me honest, remind me if I fail.
>

It will be nice to have some feedback on my grant request at:

  https://meta.wikimedia.org/wiki/Grants:Project/Future-proof_WDQS

Or one of the other threads on the very same mailing list.

> Another attempt to get update lag under control is to apply back pressure on edits, by adding the WDQS update lag to the Wikdiata maxlag [6]. This is obviously less than ideal (at least as long as WDQS updates are lagging as often as they are), but does allow the service to recover from time to time. We probably need to iterate on this, provide better granularity, differentiate better between operations that have an impact on update lag and those which don't.
>
> On the slightly better news side, we now have a much better understanding of the update process and of its shortcomings. The current process does a full diff between each updated entity and what we have in blazegraph. Even if a single triple needs to change, we still read tons of data from Blazegraph. While this approach is simple and robust, it is obviously not efficient. We need to rewrite the updater to take a more event streaming / reactive approach, and only work on the actual changes.

When it will be done, it will be still a short term solution

> This is a big chunk of work, almost a complete rewrite of the updater,

> and we need a new solution to stream changes with guaranteed ordering (something that our kafka queues don't offer). This is where we are focusing our energy at the moment, this looks like the best option to improve the situation in the medium term. This change will probably have some functional impacts [3].

Guaranteed ordering in a multi-party distributed setting has no easy
solution, and apparently it is not provided by Kafka.  For a
non-technical person, you can read
https://en.wikipedia.org/wiki/Two_Generals%27_Problem

> Some longer term thoughts:
>
> Keeping all of Wikidata in a single graph is most probably not going to work long term.

:(

> We have not found examples of public SPARQL endpoints with > 10 B triples and there is probably a good reason for that.

Because Wikimedia is the only non-profit in the field?

> We will probably need to split the graphs at some point.

:(

> We don't know how yet

:(

> (that's why we loaded the dumps into Hadoop, that might give us some more insight).

:(

> We might expose a subgraph with only truthy statements. Or have language-specific graphs, with only language-specific labels.

:(

> Or something completely different.

:)

> Keeping WDQS / Wikidata as open as they are at the moment might not be possible in the long term. We need to think if / how we want to implement some form of authentication and quotas.

With blacklists and whitelists, but this is huge anyway.

> Potentially increasing quotas for some use cases, but keeping them strict for others. Again, we don't know how this will look like, but we're thinking about it.

> What you can do to help:
>
> Again, we're not sure. Of course, reducing the load (both in terms of edits on Wikidata and of reads on WDQS) will help. But not using those services makes them useless.

What about making the lag part of the service.  I mean, you could
reload WDQS periodically, for instance daily, and drop the updater
altogether. Who needs to see the updates live in WDQS as soon as edits
are done in wikidata?

> We suspect that some use cases are more expensive than others (a single property change to a large entity will require a comparatively insane amount of work to update it on the WDQS side). We'd like to have real data on the cost of various operations, but we only have guesses at this point.
>
> If you've read this far, thanks a lot for your engagement!
>
>   Have fun!
>

Will do.

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