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 https://docs.google.com/a/wikimedia.org/spreadsheets/d/1MXikljoSUVP77w7JKf9EXN40OB-ZkMqT8Y5b2NYVKbU/edit#gid=0 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 http://www.blazegraph.com/ 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 https://www.youtube.com/watch?v=oKiXpO2rbJM 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 http://xmlns.com/foaf/spec/, social connections http://rdfs.org/sioc/spec/, and music http://musicontology.com/specification/. 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 https://www.wikidata.org/wiki/Q1053330. 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 http://www.w3.org/wiki/LargeTripleStores#Jena_TDB_.281.7B.29 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.
We created a phabricator task https://phabricator.wikimedia.org/T90101 with lots of children to run down our remaining questions. The biggest remaining questions revolve around three areas: 1. Operational issues like "how should the cluster be deployed?" "do we use HA at all?" "how are rolling restarts done in HA?" 2. How should we represent the data in the database? BlazeGraph (and only BlazeGraph) has an extension that *could* us called RDR. Should we use it? 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.
Thanks again,
Nik