TL/DR: We're selected
BlazeGraph to back the next Wikidata Query Service.
After Titan evaporated about a month ago we went back to the
drawing board on back ends for a new Wikidata Query Service. We took four weeks (including a planed
trip to Berlin) to settle on a backend. As you can see from the
spreadsheet
we've really blown out the number of options. As you can also see we
didn't finish filling them all out. But we've still pretty much settled
on
BlazeGraph anyway. Let me first explain what
BlazeGraph is and then defend our decision to stop spreadsheet work.
BlazeGraph
is a GPLed RDF triple store that natively supports SPARQL 1.1, RDFS,
some OWL, and some extensions. Those are all semantic web terms and
they translate into a "its a graph database with an expressive, mostly
standardized query language and support for inferring stuff as data is
added and removed to the graph". It also has some features that you'd
recognize from nice relational databases: join order rewriting, smart query planner,
hash and nested loop joins, query rewrite rules, group by, order by, and aggregate functions.
These
are all cool features - really the kind of things that we thought we
need but they come with an "interesting" price. Semantic Web is a very
old thing that's had a really odd degree of success. If you have an
hour and half Jim Hendler can
explain
it to you. The upshot is that _tons_ of people have _tons_ of
opinions. The W3C standardizes RDF, SPARQL, RDFS, OWL, and about a
billion other things. There are (mostly non-W3C) standards for talking about
people,
social connections, and
music.
And they all have rules. And Wikidata doesn't. Not like these rules. One thing I've learned from this project is that this lack of prescribed rules is one of Wikidata's founding principles. Its worth it to allow openness. So
you _can_ set gender to "Bacon" or put GeoCoordinants on
Amber. Anyway! I argue that, at least for now, we should ignore many of these standards. We need to think of Wikidata Query Service as a
tool to answer questions instead of as a some grand statement about the
semantic web. Mapping existing ontologies onto Wikidata is a task for another day.
I feel like these semantic web
technologies and BlazeGraph in particular are good fits for this project
mostly because the quality of our "but what about X?" questions is very
very high. "How much inference should we do instead of query
rewriting?" instead of "Can we do inference? Can we do query
rewriting?" And "Which standard vocabularies should think about mapping
to Wikidata?" Holy cow! In any other system there aren't "standard
vocabularies" to even talk about mapping, much less a mechanism for
mapping them. Much less two! Its almost an overwhelming wealth and as I
elude to above it can be easy to bikeshed.
We've been reasonably careful to reach out people we know are familiar with this space. We're well aware of projects like the Wikidata Toolkit and its RDF exports. We've been using those for testing. We've talked to so many people about so many things. Its really consumed a lot more time then I'd expected and made the search for the next backend very long. But I feel comfortable that we're in a good place. We don't know all the answers but we're sure there _are_ answers.
The BlazeGraph upstream has been super active with us. They've spent
hours with us over hangouts, had me out to their office (a house an hour
and half from mine) to talk about data modeling, and spent a ton of time
commenting on Phabricator tickets. They've offered to donate a
formal support agreement as well. And to get together with us about
writing any features we might need to add to BlazeGraph. And they've
added me as a committer (I told them I had some typos to fix but I have
yet to actually commit them). And their code is well documented.
So
by now you've realized I'm a fan. I believe that we should stop on the
spreadsheet and just start work against
BlazeGraph
because I think we
have phenomenal momentum with upstream. And its a pretty clear winner
on the spreadsheet at this point. But there are two other triple
stores which we haven't fully filled out that might be viable: OpenLink
Virtuoso Open Source and
Apache Jena. Virtuoso is open core so I'm really loath to go too deep
into it at his point. Their HA features are not open source which
implies that we'd have trouble with them as an upstream. Apache Jena
just isn't
known to scale to data as large as
BlazeGraph and Virtuoso. So I argue
that these are systems that, in the unlikely event that
BlazeGraph goes
the way of Titan, we should start our third round of investigation
against. As it stands now I think we have a winner.
3. Some folks have identified update rate as a risk. Not upstream, but others familiar with triple stores in general.
Our
plans is to work on #2 over the next weeks because it really informs #1
because there are lots of working set size vs cpu time tradeoffs to
investigate. We'll start on #1 shortly as well. #3 is a
potential risk area so we'll be sure to investigate it soon.
I admit I'm not super happy to leave the
spreadsheet in the format its current unfilled-out state but I'm
excited to have something to work with and think its the right thing to
do right now.
So thanks for reading all of this. Please reply with comments.