Introduction to linked data

Gordon DunsirePresented at the Cataloguing and Indexing Group

Scotland seminar “Linked data and the Semantic Web: what have libraries got to do with it?”, Edinburgh,

National Library of Scotland, 17 June 2011

Overview

Relational recordsInfluence of RDA vocabulariesDisaggregated, distributed “records”Logical conclusion: simple metadata statement

RDFTriples, etc.

Linked dataChains, clusters

Title: Cataloguing is fun!

Author: Mary MacDonald

Content type:

Carrier type:

LCSH:

microfiche

text

Cataloging

Bibliographic record: 12345 Name authority record: 8765

Heading: MacDonald, Mary

Place of birth: Edinburgh

LCSH authority record: 5432

Heading: Cataloging

See also: Books

RDA content type record: 1234

Term: text

Definition: Content expressed through a form of notation for language intended to be perceived visually.

RDA carrier type record: 5432

Term: microfiche

Definition:A sheet of film bearing a number of microimages in a two-dimensional array.

8765

5432

1234

5432

9876

65443

Title: Cataloguing is fun!

Author:

Content type:

Carrier type:

LCSH:

Bibliographic record: 12345

8765

5432

1234

5432

Name authority record: 8765

Heading: MacDonald, Mary

Place of birth: 9876

12345 8765Author

8765 Place of birth 9876

8765 Heading “MacDonald, Mary”

9876 Name “Edinburgh”

9876 Country 4567

Stop! Ambiguous: link not safe.

Identifier: ok to link.

Linked data is not a new idea!

It extends concepts of authority control“Preferred” labelsChange once; link many times

Re-use of metadataMore than one “attribute” associated with a “heading”

E.g. Place of birth of person with name heading

Concepts can be applied to authority recordsAs well as bibliographic description records

Full extension leads to “record” dis-aggregationAll “records” in bibliographic control systems

Linked data and RDF

Resource Description Framework (RDF)Designed for machine-processing of metadata

at global scale24/7/365Trillions of operations per second

Everything must be dis-ambiguatedMachines are dumb

Simplicity helps!Machine-readable identifiers

RDF triple

Metadata expressed as “atomic” statementsA simple, single, irreducible statement

The title of this book is “Cataloguing is fun!”

Constructed in 3 parts“Triple”

The title of this book is “Cataloguing is fun!”Subject of the statement = Subject: This bookNature of the statement = Predicate: has titleValue of the statement = Object: “Cataloguing is fun!”

This book – has title – “Cataloguing is fun!”subject – predicate - object

Identifiers

Need unambiguous way of identifying each part of the triple for efficient machine-processingHuman labels (“This book”, “has title”) no good

Same thing, different labels; different things, same label

Exploit the utility of the URLMachine-readable, regular syntax, unambiguous

Uniform Resource Identifier (URI)

Uniform Resource Identifier

Can be any unique combination of numbers and lettersNo intrinsic meaning; it’s just an identifying label

Can look like a URLhttp://iflastandards.info/ns/isbd/elements/P1001But does not lead to a Web page (in principle ...)

RDF requires the subject and predicate of triple to be URIsObject can be a URI, or a literal string (“Cataloguing is

fun!”)

Namespaces

URI can be constructed from a base plus a unique, identifying suffixhttp://iflastandards.info/ns/isbd/elements/+ P1001

Base is known as a namespaceCan be abbreviated by human programmer

“isbd” = http://iflastandards.info/ns/isbd/elements/isbd:P1001

Machine expands abbreviation for processing

Everything as triples in RDF

Every aspect of the metadata must be expressed in RDF to be machine-processableMetadata about real-world objects (books,

people, etc.)Metadata about the predicates (definition, label,

scope, etc.)Common predicates apply to many types of thing

(human-readable label, etc.)High-level RDF namespaces (rdfs, owl, skos)

RDF is expressed in RDF (“bootstrap”)

RDF properties

Predicates are called properties in RDF“Verbal” part of the metadata statement

E.g. “A has title ...”, “B is author of C”, “D is embodiment of E”

Properties link specific instances of two thingsA = a specific book, B = a specific person, etc.... = a specific label, character string, annotation

=> a “literal”

Properties are the links in linked data, the pathways through the Semantic Web

Domains and ranges

A property can specify the types of thing it linksE.g. Bibliographic resources, Persons, Places, etc.Types of thing are RDF classes

A domain is the class of the subject of the propertyE.g. The domain of “is embodiment of” is Expression

(FRBR)A range is the class of the object of the property

E.g. The range of “is embodiment of” is Manifestation (FRBR)

Inferencing

RDF enables semantic inferencingDeducing additional, unstated triples from an

existing statement or set of statementsE.g. “D is embodiment of E” + “(is

embodiment of) has domain Expression” => “D is a Expression”

And “D is embodiment of E” + “(is embodiment of) has range Manifestation” => “E is a Manifestation”

The truth

There is no test of veracity for a single triple in RDFAnybody can say Anything about Anything (AAA)

Inferencing only tests for logical inconsistencyE.g. If it results in “E is a Manifestation” + “E is not a

Manifestation”Library linked data must choose and apply its

properties/links with careTo maintain our reputation for reliability, quality, etc.In a web of user-, machine-, and politically-generated

metadata

Thank you

To be continued ...