RDF FOR LIBRARIANS
JENN RILEY METADATA LIBRARIAN DIGITAL LIBRARY PROGRAM
DLP Brown Bag Series, September 22, 2010
From Wikipedia
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A collection of RDF statements intrinsically represents a labeled, directed multi-graph.
<http://en.wikipedia.org/wiki/Resource_Description_Framework>
Learning the lingo is important; however for us its probably better to start with understanding how some of the basic concepts are different than what were used to. Thats what were going to do today.
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Structural differences
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Libraries have records
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RDF has statements and graphs
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This is a statement, aka a triple. A statement is made in a particular direction. predicate subject
object
Statements combine to form graphs. A graph is of no fixed size and contains no predetermined types of statements. (The graph is the real RDF model; Figures from RDF Primer a triple is a secondary representation.) <http://www.w3.org/TR/rdf-primer/>
More about statements
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subject: must be a URI
predicate: must be a URI object: can be a URI, or a literal
Figure from RDF Primer <http://www.w3.org/TR/rdf-primer/>
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The centrality of URIs in RDF
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RDF uses URIs to identify: individuals, e.g., Eric Miller, identified by http:// www.w3.org/People/EM/contact#me kinds of things, e.g., Person, identified by http:// www.w3.org/2000/10/swap/pim/contact#Person properties of those things, e.g., mailbox, identified by http://www.w3.org/2000/10/swap/pim/ contact#mailbox values of those properties, e.g. mailto:em@w3.org as the value of the mailbox property (RDF also uses character strings such as "Eric Miller", and values from other datatypes such as integers and dates, as the values of properties)
Figure and text from RDF Primer <http://www.w3.org/TR/rdf-primer/>
Implications of statement structure (1)
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"!
Subjects are always formalized; know exactly what is being talked about
!! Theyre
only implicit in library metadata !! Makes moot the 1:1 problem !! Might still have content vs. carrier problem
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Predicates are always formalized
!! Maybe
not all that different than library/archive/ museum metadata models !! More obviously reusable
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Implications of statement structure (2)
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"!
Representation of objects is flexible
!! Using
a URI facilitates connecting this value into a larger graph !! Literals allow more limited functionality; used when the value isnt likely to be useful as part of a vocabulary !! URI vs. literal a design choice; note the previous example used a literal where a URI might be better!
Making a statement that two different URIs refer to the same thing can join two graphs "! Note, URIs arent necessarily dereferencable
"!
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Blank nodes
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This blank node stands in for John Smiths address
Figure from RDF Primer <http://www.w3.org/TR/rdf-primer/>
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Properties vs. elements
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Remember, a statements predicate represents a property of the resource (subject) being described
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Resource=subject; Property=predicate. I dont know why there are different terms for the model than for individual statements. Lets move on.
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On the surface, an RDF property looks like the same thing as an XML element or a database field But the underlying formal model is different
Explicit subjects and directionality of statements !! Formality of RDF model places additional restrictions but allows more explicit meaning
!!
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Therefore inferences you can draw for elements and fields are not as strong as you can for properties
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Graph vs. tree
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XML documents are trees This is as good as you can do inferring a graph from a tree:
Tree vs. graph figure from XML2RDF documentation DLP Brown Bag Series <http://rhizomik.net/html/redefer/xml2rdf/>
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Model vs. its syntax(es)
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"!
Theres a difference between:
!! Model
of information representation !! Property or element definitions !! Binding of the information into a specific syntax
MARC is both the record structure (syntax) and content designations (element definitions) "! RDF model has many encoding syntaxes "! RDF and MARC operate at different levels in this landscape; but that doesnt mean MARCs structure is capable of representing the RDF model
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Some realities of RDF that scare us
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"! "!
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No predetermined set of properties to care about No guarantee that the same person/item/place/whatever are always referred to with the same URI No inherent mechanism/requirement for vetting properties, URIs, etc But lets be frank here. Are our library/archive/museum records really:
Complete? !! Authoritative? !! Consistent? !! Accurate? !! All that functional for what we want to do?
!!
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More RDF concepts
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"!
Class
!! A
statement can say a subject is of a certain type or category known as a class. !! Classes can be formally defined in RDF Schema documents
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Domain and Range
!! Specify
what classes subjects and objects of statements (respectively) using a given property can be !! More than one domain or range statement can be made for any given property
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These features allow inferencing to deduce statements that arent actually made
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But both have
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Raging debate over how precise you have to be in indicating what it is youre describing An interest in mechanisms to allow consumers of data to evaluate the authority of statements A tendency to over-model (see The Modeller blog post on handout) Other similarities
!! Application
profile notion applicable to both XML and RDF models, though would be implemented differently !! XML elements repeatable; RDF makes no restriction on how many statements about the same resource use the same predicate.
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Terminology differences
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Subject
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In libraries, what an information resource is about "! In RDF, what a statement is about
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Vocabulary
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In libraries, implies a controlled vocabulary of a certain sort
!! Authorized
terms !! Lead-in terms (see references, etc.) !! Sometimes, hierarchical structure !! Sometimes, related terms
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In RDF, much looser definition
!! Includes
formal definitions of classes and properties
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Class
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In libraries, a classification scheme indicating the general topic or area of knowledge covered by an information resource "! In RDF, a type or category that any type of object or resource belongs to
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Schema
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XML Schema defines a set of elements intended to be used together "! RDF Schema defines classes and properties intended to be used anywhere, alone or in combination
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So, wait, why should I learn this?
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Libraries are moving in this direction
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At least a little bit "! Increasing viability and acceptance of interoperating with data from outside of libraries "! RDA gives us an opportunity to fundamentally rethink some features of our data "! Semantic Web activities have been given new life with the grassroots Linked Data movement
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Linked Data design issues
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Use URIs as names for things "! Use HTTP URIs so that people can look up those names. "! When someone looks up a URI, provide useful information, using the standards (RDF, SPARQL) "! Include links to other URIs. so that they can discover more things.
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Tim Berners-Lee <http://www.w3.org/DesignIssues/LinkedData.html>
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How much planning do we need to do?
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There are other cases where the easiest thing for somebody to do is to just put data up in whatever form it's available.
Tim Berners-Lee <http://www.readwriteweb.com/archives/ interview_with_tim_berners-lee_part_1.php>
Lets review some of what libraries have done so far.
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id.loc.gov
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German National Library
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Virtual International Authority File
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LIBRIS Swedish National Library
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Open Library
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Registering FRBR and RDA properties
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What about incorporating data from other sources?
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BBC Wildlife Finder <http://www.bbc.co.uk/wildlifefinder/> Slide by Thomas Baker, What Makes the Linked Data Approach Different, NISO DCMI Webinar 2010. <http://dublincore.org/resources/training/ NISO_Webinar_20100825/dcmi-webinar-02.pdf>
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Challenges to implementing RDF in practice
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How do you
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find all the triples you need? "! know what predicates and URIs to use when creating new triples? "! know what predicates and URIs to use when processing data?
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What infrastructure needs to be built in order to
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negate an erroneous statement? "! say that a statement is time-dependent? "! judge the likely validity of a statement made by someone else?
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actually end up with machine-understandable data in the end???
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Dont run away!
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These uncertainties do not give librarians permission to write off the entire model. Incorporating new ideas doesnt mean we have to give up our core principles. We have an opportunity here:
!! to
create classes and properties that represent data as we think it should be. !! to bring new features to the model and to its implementations. !! to provide some of the infrastructure thats missing. !! to participate in the discussion.
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Lets go!
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Photo by Gregory Moine <http://www.flickr.com/photos/gregory-moine/4713276677/> Creative Commons Attribution 2.0 Generic <http://creativecommons.org/licenses/by/2.0/deed.en>
For more information
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"! "!
<http://www.dlib.indiana.edu/~jenlrile/presentations/bbfall10/ rdf/rdfForLibrarians.pptx> <http://www.dlib.indiana.edu/education/brownbags/> <http://www.dlib.indiana.edu/~jenlrile/presentations/bbfall10/ rdf/handout.pdf>
jenlrile@indiana.edu These presentation slides:
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Recording of talk on DLP Brown Bag page: More resources on handout:
Thank you!
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