Mission Possible: Deploying Government Linked Data (Pt2)

Sandro Hawke, (sandro@w3.org), W3C/MIT, @sandhawke
John L. Sheridan, @johnlsheridan
gov 2.0 expo, May 25-26, 2010, Washington DC
http://www.w3.org/2010/Talks/0525-rdf-vocabularies (wiki)

Part 2

Viewing Your Data as Triples

Good URIs
Properties (relationships and attributes)
Overlap and Competition
Classes (Description Logic)
Finding and/or Creating Vocabularies

Vocabulary as Interface

How do programs communicate via triples?

Alice and Bob publish triples, Charlie's software tries to use their data.

It's all about the specific URIs, the vocabulary.

Essentially, when using RDF, the vocabulary is the syntax, the API.

Use Case: Crime Reports

Data Providers:

Springfield Police Department "Police Blotter"
Springfield FBI Field Office
Springfield Citizen's Watch

Data Consumers:

Journalists
Mobile Apps (AreYouSafe)
Real Estate Listings
Driving Directions

Identify Your Subjects

What are the things your data is about?

(items, entities, objects, individuals, resources)

in scenario: incident, location, suspect/convict, trial, stolen/damaged property, victim

See UML, Database Records, your web site

Assign Good URIs

give them good, long-term URI names

Pick URIs that:

no one will want for something else
that may become unfashionable, but wont become wrong
that someone will web-serve forever

Such as:

See Designing URI Sets for the UK Public Sector

Some Bad (Linked Data) URIs

Misleading Names

http://example.gov/recent_crime_downtown
- fine if it's a list of recent crimes downtown
- lousy identifier for a particular crime

Can't Derefernce

tag:hawke.org,2001:Sandro_Hawke
urn:isbn:0451450523
urn:uuid:6e8bc430-9c3a-11d9-9669-0800200c9a66

Dereference Doesn't Work

http://hawke.org/Sandro_Hawke

Conflating Item and Page About Item

http://en.wikipedia.org/wiki/Tim_Berners-Lee identifies a web page, not a person
- (It can be used indirectly, but that's different)

Unreadable names (usually):

http://hawke.org/6e8bc430-9c3a-11d9-9669-0800200c9a66

Namespaces

Shared Leading Prefix:

Namespace name: "http://dbpedia.org/resource/"

Confusingly similar to XML namespaces; sometimes the same, sometimes different.

Namespace document:

dereference http://dbpedia.org/resource/
dereference http://xmlns.com/foaf/0.1/
dereference http://vocab.deri.ie/dcat#

Often the same as dereferencing things in namespace.

URI Lifecycle

Stage 1: Unstable

Coordinate with all users about every change in meaning
- Easy when you're the only user
- Gets harder, slower as user community grows
- Some users will avoid unstable terms

Stage 2: Stable

No incompatible changes in meaning.
- okay to improve/rewrite documentation
- don't break people's running code
- maybe change meaning to resolve harmful ambiguity
  - (break minority code)

URI Lifecycle (cont)

Stage 3: Deprecated

Still stable, still served, but not recommended for use
Should no longer be produced; still consumed for a while.

Stage 4: Dead

URI can no longer be de-referenced. Best to avoid this.

URIs in a namespace can be at different stages.

Don't version namespaces. Don't be fooled by:

(They were chosen before we knew this, and now they're stuck.)

Properties

Essential to understand a triple, the middle part

Also known as:

relation, relationship,
predicate
column, column-name
attribute
field, member, slot (sort of)

Property As Question

Each triple states the answer to a question.

The subject: the item the question is about
- you, me, Massachusetts, the moon, crude oil, ... whatever
The property: the question
- who created it? when was it created? where is it located?
The value: the answer to the question

Object and Data Properties

A Data Property:

value is a literal (string, number, date, etc)
- data_created, height, weight, name, name_of_owner

An Object Property:

value is another first-class entity (not just a literal)
- owner, near, friend, hometown, capital

Data Types

RDF uses some XML Schema Part 2: Datatypes, usually these ones.

Each datatype is a mapping:

from character strings, eg "3" or "3.0" or "0003"
to their values, eg the number three

xs:decimal
- xs:integer
  - xs:int
    - xs:byte
xs:time
xs:dayTimeDuration

... etc

Some Well-Known Properties

@@@ make these be links to real documentation

rdfs:label

rdfs:comment

owl:sameAs

foaf:name

dc:creator

Overlapping, Competing Vocabularies

two terms for the same thing

owl:sameAs

owl:equivalentProperty

dc:creator owl:equivalentProperty dcterms:creator
foaf:name owl:equivalentProperty vcard:FN

two terms for possibly-identical things. splitting hairs:

eg: Peter Pan (various fictional works, productions, editions, copies, characters)

Compare: City of Boston (politcal entity) City of Boston (geographic region)

The Pedantic Web

Subproperty

(dc refines)

rdfs:subPropertyOf

sandro contact john
- sandro friend john
  - sandro recentFriend john
- sandro presentedWith john

friend rdfs:subPropertyOf contact
recentFriend rdfs:subPropertyOf friend
presentedWith rdfs:subPropertyOf contact

geographically near, overlapping, contained-within

Messy Overlap

Not all related properties are equiv or sub

foaf:firstName, foaf:givenName, foaf:lastName, foaf:familyName, foaf:name

Conversion Rules

        if { ?x foaf:firstName ?first;
                foaf:lastName ?last }
        then
           { ?x foaf:familyName ?last;
                foaf:givenName ?first;
                foaf:name func:string-join(?first " " ?last)
           }

        if { ?x foaf:name ?name } and
           pred:contains(?name, " ")
        then
           # incorrect if lastname has space, like Hillary Rodham Clinton
           { ?x foaf:firstName func:string-before(?name, " ");
                foaf:lastName func:string-after(?name, " ")
           }

See Rule Interchange Format

Applying Rules

In custom code, or using rules engine
In producer (publish using many vocabs)
In consumer (accept many vocabs)

Advice?

The world is full of competing standards. That's good, but painful.

Probably best to follow Postel's Law:

 Be conservative in what you do; be liberal in what you accept from others.

Research topic: automatic downloading of conversion rules

Classes and Subclasses

Sets of objects with something in common.

Instances / rdf:type

Subclass hierarchy

plant / large_plant / tree / mature_horse_chestnut, the one in my back yard

Domain and Range

domain: the class of things which might have this property.

range: the class of possible values for this property

Domain	Property	Range
foaf:Person	foaf:firstName	(string)
dcat:Catalog	dcat:record	dcat:CatalogRecord
eg:Parent	eg:daughter	eg:Female
rdfs:Property	rdfs:domain	rdfs:Class

OWL

Powerful way of declaring how properties, classes, and individuals relate to each other.

"Ontologies"

http://www.w3.org/TR/owl2-primer/

class expressions ( US_Citizen and Irish_Citizen and not Minor )
ChildofSandro (anythin with 'parent' being Sandro)
BigTree (trees with height over 30 feet)
inverse properties
- Sandro child Gregorian
- Gregorian parent Sandro
- => parent owl:inverseOf child
negative assertions (some triple is false)

OWL can be conveyed in triples, but also has some easier-to-read syntaxes. I suggest Manchester, when you don't need triples.