Paul Wilton

For those of you that have decoded the title, then the Q1415055
[https://www.wikidata.org/entity/Q1415055] is probably self evident. Having
spent several weeks building a practical application upon Wikidata
[http://wikidata.org], I thought it was time for an honest assessment. There are
indeed, good, bad and ugly facets to Wikidata. I have my tin hat on.

The good
Always best to start with the positives. Wikidata does was it says on the tin.
It does work. It is a broad, extensive knowledge base, based on the principles
of linked data, a superset of the data, machine-extracted from wikipedia. It is
crowd and bot curated. Anybody can contribute via its RDF enhanced Mediawiki UI.
In the true spirit of linked data it has outbound links to other open knowledge
bases, Google knowledge graph, BBC Things, Geonames, and MusicBrainz to name a
few, and also links to the information and images served up by its older
sisters, Wikipedia and Wikimedia.

Provenance claims can be asserted on any wikipedia statement/fact - in the form
of references and sources.

It gets better - Wikidata has a public SPARQL 1.1 endpoint, and a rather nice UI
[https://query.wikidata.org] for querying it. Again this works, and seems to
deal with contention well. Over the course of several weeks and a lot of
querying on this endpoint, I never experienced any issues.

However...

The bad
It’s the model. I am struggling to work out what was the thought process that
went into this. For those of you familiar with RDF
[https://en.wikipedia.org/wiki/Resource_Description_Framework], you will
understand RDF is a W3C framework and standard for creating models that describe
some domain (ontologies), and describing things in that domain using statements
that conform to those models. The RDF schema [https://www.w3.org/TR/rdf-schema/] 
has patterns and rules for defining classes of Things, and the properties that
describe them. Statements such as
<A> rdf:type <C> declares that resource <A> is an instance of some class <C>.
<D> rdfs:subClassOf <C> declares <D> to a be a subclass of <C> .
Object Oriented software developers have a natural affinity to this form of
modelling

Wikidata is built upon the RDF framework. But instead of describing wikidata
resources (things) using the RDF schema, wikidata has built its own resource
description framework out of RDF (the resource description framework), thus
creating an entire abstraction layer between its own model of the world and RDF.

Wikidata resources are divided essentially into two distinct sets. A set of
property resources and a set of instance resources, known as items. The items
set is the set of all the things in the realm of human knowledge. The set of
properties is the collection of semantic predicates that items can be described
by. The property resources all have unique IDs prefixed with P and the item IDs
are prefixed with Q.

Now here is the kicker. Instead of using the RDF schema to define the properties
and classes, wikidata have defined their own schema that sort of mirrors the RDF
schema. There is a wikidata property P31
[https://www.wikidata.org/wiki/Property:P31] “instance of” that is semantically
equivalent to rdf:type. Property P279
[https://www.wikidata.org/wiki/Property:P279] “subclass of” is semantically
equivalent to rdfs:subClassOf. Classes themselves are declared as items, and
items are then described using these properties (not using the RDF schema).

If you are now confused, you probably should be, as anyone with any experience
of RDF being introduced to wikidata will be. Why does wikidata just not use RDF
schema you will ask yourself. I did and have no good answer (yet). This
abstraction layer, means wikidata loses all the benefits of RDF schema, and
gains a frankly painful amount of confusion. Any consumer of wikidata first
needs to understand the wikidata schema. Semantics in wikidata have been
redefined - badly in my opinion.

and the ugly
It’s the obfuscation. So what’s wrong with opaque identifiers I hear you say.
Indeed nothing, it makes perfect sense that all the Items (the things), should
be identified by globally unique opaque IDs - the dataset is large and
crowd-curated, conciseness and consistency works, names of things may change, 
hackability
[http://www.jenitennison.com/2009/07/25/opaque-uris-unreadable-uris.html] is not
a concern. In fact the one in the title of this article I have no problem with.
But I do have a problem with the wikidata properties being obfuscated too. The
entire set of wikidata properties including those that are the schema itself
(the one that is not the RDF schema) are opaque and not human understandable.
This makes building queries and applications on wikidata really painful and time
consuming.
Ontology models are like a programming language, informing the user how to build
things using its model. Obfuscating the model is akin to obfuscating a
programming language. Imagine if all the lemmas, grammars, and reserved words in
Java or Python were not human readable, but encoded as a set of opaque IDs. Yes,
it is the same with ontologies, they need to be human understandable too (at
least until human developers are entirely replaced by AIs). Wikidata is
essentially a giant obfuscated graph of nodes and edges. You have no idea of the
semantic meaning of an edge without dereferencing the edge via a query or the
wiki.
The reason we write code that is self-documenting is well understood. Thoughtful
naming of variables and functions is critical in delivering code that is deemed
to be self-documenting. The same principle should apply to ontologies.

For a few dollars more..
More bad - There is some fairly unusual ontology model when you dig below the
surface. For example Clint Eastwood (Q43203
[https://www.wikidata.org/entity/Q43203]), an instance of human (Q5
[https://www.wikidata.org/entity/Q5]) has occupation (P106
[https://www.wikidata.org/wiki/Property:P106]) actor (Q33999
[https://www.wikidata.org/entity/Q33999]). Actor is an instance of profession -
all good so far. But actor (Q33999) is also a subclass of artist, creator and
person. Thus semantically the range of occupation includes the class “person”.
Clint Eastwood is an instance of human, and has the job of being a person.

More good - back to the rather good SPARQL query UI. The UI provides very nice
hover and autocomplete features on wikidata properties and resources that
de-obfuscates the opaque IDs. This provides some mitigation. If it wasn’t for
this building queries would be difficult.

The SPARQL service (provided by the graph database, Blazegraph
[https://www.blazegraph.com/] that wiki data is served from) has some very handy
plugin services enabled, not least the label & description binding service. This
allows you to write a SPARQL query and bind instance labels very easily, in any
available language, without having to write all the joins and language filters
in your query. For example :


# get actors in the film "The Good, The Bad, and the Ugly"
SELECT ?actor ?actorLabel ?actorDescription WHERE {
  ?actor wdt:P106 wd:Q33999 .
  wd:Q41483 wdt:P161 ?actor .
  SERVICE wikibase:label { bd:serviceParam wikibase:language [AUTO_LANGUAGE],en,es,it" }
}



In the query above, (try it
[https://query.wikidata.org/#select%20%3Factor%20%3FactorLabel%20%3FactorDescription%20where%20%7B%0A%20%20%3Factor%20wdt%3AP106%20wd%3AQ33999%20.%0A%20%20wd%3AQ41483%20wdt%3AP161%20%3Factor%20.%0A%20%20SERVICE%20wikibase%3Alabel%20%7B%20bd%3AserviceParam%20wikibase%3Alanguage%20%22%5BAUTO_LANGUAGE%5D%2Cen%2Ces%2Cit%22%20%7D%0A%7D]
) the label service automatically binds label and description variables to any
named variable in the query. If you a querying for many variables, this is a big
time saver.

Roll credits..
I do have reservations about allowing the crowd to curate the underlying model
as well as the knowledge itself. The model is something that needs strong
governance, and solid grasp of information architecture to ensure quality and
semantic correctness.

All up, I have enjoyed working with wikidata, if you can invest the time in
getting to grips with the schema and model, it is a superb, extensive source of
machine readable information, the best of the current breed, with comprehensive
links to open linked data sets (the primary reason we used it as knowledge
source). If only the model was self-documenting, and it used RDF for the purpose
it was designed.

We have been working with linked data since our inception and have helped global brands with linked data strategy and implementation. It is a core discipline at Data Language. Linked data is a W3C backed pattern for publishing structured data using vocabularies like schema.org that can be connected together and interpreted by machines.

We have been working with linked data since our inception and have helped global brands with linked data strategy and implementation.

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