Wikidata community members,
Thank you for all of your work helping Wikidata grow and improve over the
years. In the spirit of better communication, we would like to take this
opportunity to share some of the current challenges Wikidata Query Service
(WDQS) is facing, and some strategies we have for dealing with them.
WDQS currently risks failing to provide acceptable service quality due to
the following reasons:
1.
Blazegraph scaling
1.
Graph size. WDQS uses Blazegraph as our graph backend. While
Blazegraph can theoretically support 50 billion edges
<https://blazegraph.com/>, in reality Wikidata is the largest graph
we know of running on Blazegraph (~13 billion triples
<https://grafana.wikimedia.org/d/000000489/wikidata-query-service?viewPanel=7&orgId=1&refresh=1m>),
and there is a risk that we will reach a size
<https://www.w3.org/wiki/LargeTripleStores#Bigdata.28R.29_.2812.7B.29>limit
of what it can realistically support
<https://phabricator.wikimedia.org/T213210>. Once Blazegraph is maxed
out, WDQS can no longer be updated. This will also break Wikidata tools
that rely on WDQS.
2.
Software support. Blazegraph is end of life software, which is no
longer actively maintained, making it an unsustainable backend
to continue
moving forward with long term.
Blazegraph maxing out in size poses the greatest risk for catastrophic
failure, as it would effectively prevent WDQS from being updated further,
and inevitably fall out of date. Our long term strategy to address this is
to move to a new graph backend that best meets our WDQS needs and is
actively maintained, and begin the migration off of Blazegraph as soon as a
viable alternative is identified <https://phabricator.wikimedia.org/T206560>
.
In the interim period, we are exploring disaster mitigation options for
reducing Wikidata’s graph size in the case that we hit this upper graph
size limit: (i) identify and delete lower priority data (e.g. labels,
descriptions, aliases, non-normalized values, etc); (ii) separate out
certain subgraphs (such as Lexemes and/or scholarly articles). This would
be a last resort scenario to keep Wikidata and WDQS running with reduced
functionality while we are able to deploy a more long-term solution.
1.
Update and access scaling
1.
Throughput. WDQS is currently trying to provide fast updates, and
fast unlimited queries for all users. As the number of SPARQL queries
grows over time
<https://www.mediawiki.org/wiki/User:MPopov_(WMF)/Wikimania_2021_Hackathon>alongside
graph updates, WDQS is struggling to sufficiently keep up
<https://grafana.wikimedia.org/d/000000489/wikidata-query-service?viewPanel=8&from=now-6M&to=now&refresh=1d>
in each dimension of service quality without compromising anywhere. For
users, this often leads to timed out queries.
2.
Equitable service. We are currently unable to adjust system behavior
per user/agent. As such, it is not possible to provide equitable
service to
users: for example, a heavy user could swamp WDQS enough to hinder
usability by community users.
In addition to being a querying service for Wikidata, WDQS is also part of
the edit pipeline of Wikidata (every edit on Wikidata is pushed to WDQS to
update the data there). While deploying the new Flink-based Streaming
Updater <https://phabricator.wikimedia.org/T244590> will help with
increasing throughput of Wikidata updates, there is a substantial risk that
WDQS will be unable to keep up with the combination of increased querying
and updating, resulting in more tradeoffs between update lag and querying
latency/timeouts.
In the near term, we would like to work more closely with you to determine
what acceptable trade-offs would be for preserving WDQS functionality while
we scale up Wikidata querying. In the long term, we will be conducting more
user research to better understand your needs so we can (i) optimize
querying via SPARQL and/or other methods, (ii) explore better user
management that will allow us to prevent heavy use of WDQS that does not
align with the goals of our movement and projects, and (iii) make it easier
for users to set up and run their own query services.
Though this information about the current state of WDQS may not be a total
surprise to many of you, we want to be as transparent with you as possible
to ensure that there are as few surprises as possible in the case of any
potential service disruptions/catastrophic failures, and that we can
accommodate your work as best as we can in the future evolution of WDQS. We
plan on doing a session on WDQS scaling challenges during WikidataCon this
year at the end of October.
Thanks for your understanding with these scaling challenges, and for any
feedback you have already been providing. If you have new concerns,
comments and questions, you can best reach us at this talk page
<https://www.wikidata.org/wiki/Wikidata_talk:Query_Service_scaling_update_Aug_2021>.
Additionally, if you have not had a chance to fill out our survey
<https://docs.google.com/forms/d/e/1FAIpQLSe1H_OXQFDCiGlp0QRwP6-Z2CGCgm96MWBBmiqsMLu0a6bhLg/viewform?usp=sf_link>
yet, please tell us how you use the Wikidata Query Service (see privacy
statement
<https://foundation.wikimedia.org/wiki/WDQS_User_Survey_2021_Privacy_Statement>)!
Whether you are an occasional user or create tools, your feedback is needed
to decide our future development.
Best,
WMF Search + WMDE
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