RDF, RSS and all that

Silterra, April 2004

RDF, RSS and all that

THREADING THE

RDF MAZE

http://www.georgetown.edu/labyrinth/search.html


Outline of Discussion

• What is RDF?

• How is RDF different from XML?

• Some simple RDF Descriptions.

• Some simple RDF Schemas.

• Some simple RDF tools to combine data.


What is RDF

• Resource Description Format

• Tool of the Semantic Web

• History: First Recommended Model and Syntax – 1999.

• RDF Schema Specification 2000


Recent Specifications

• Feb 10, 2004

• RDF/XML Syntax Specification

• RDF Vocabulary Description Language 1.0: RDF Schema

• RDF Primer

• Resource Description Framework (RDF): Concepts and Abstract Syntax


Goals of RDF

• Web metadata– Content Rating, like PICS

• PICS, the Platform for Internet Content Selection [PICS96], [PICSSYS96] is a system for associating metadata (PICS "labels") with Internet content

• Categorize and classify – step up from simple term searching to concept searching.

• Descriptions of capabilities, like CC/PP


RDF? XML?

• Goals of RDF seem like those of XML

• Share data between across the Web.

• Reuse data between applications.

• XML is a data format standard.

• The structure of an xml document can get in the way of analyzing the information.

• An xml document is a tree.


RDF model

• RDF consists of assertions about Web resources.

• RDF models can be represented, and built on top of XML.

• RDF and XML are complementary.


Key Concepts of RDF

• Expression of simple facts

• Graph data model

• URI-based vocabulary

• Datatypes

• Literals

• Different syntaxes for representing the assertions, though RDF/XML is one.


RDF Assertions, Examples• The Author of ISBN 0152038655 is Carl Sandburg.• The Title of ISBN 0152038655 is Arithmetic.• The Author of the web page is Ora• The Subject of www.library.cornell.edu/mayantislavery

is “Slavery”.

• <rdf:Description rdf:about="uri:isbn:0152038655"

• mods:title="Arithmetic"• mods:creator="Sandburg,Carl" />


RDF Assertions, Cont'd

• <rdf:Description rdf:about="http://example.com">

• <mods:creator> Ora</mods:creator>

• </rdf:Description>


Underlying model

• Graph based so this is the representation that is always referred to.


RDF representations

• RDF assertions, and models can be represented in different ways.

• 1 Way is N3.– This is a compact representation.– Simple to understand.– Easy to parse.– Not widely understood though.


N3 Representation of RDF

• <http://www.library.cornell.edu/mayantislavery> <http://purl.org/dc/elements/1.1/subject> "Slavery" .

• <uri:isbn:0152038655> <http://mods.org/title> "Arithmetic" . <uri:isbn:0152038655> <http://mods.org/creator> "Sandburg,Carl" .

• <http://example.com/> <http://mods.org/creator> "Ora" .


Simple MODS Document• <mods xmlns:xlink="http://www.w3.org/TR/xlink"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.loc.gov/mods/" xsi:schemaLocation=" http://www.loc.gov/mods http://www.loc.gov/standards/mods/mods.xsd">

<identifier type=”isbn”>0152038655</identifier>

<title>Arithmetic /</title>

<creator>Sandburg,Carl</creator>

</mods>


RDF XML Representation

• <?xml version="1.0"?>• <rdf:RDF>• <rdf:Description

rdf:about="uri:isbn:0152038655">• <mods:title>Arithmetic</mods:title>• <mods:creator>Sandburg,Carl</mods:creator>• </rdf:Description></rdf:RDF>


XML RDF representation

• The RDF Description Element.• Specifies the resource about which we are talking.• Can use the “about” attribute.• Could be unamed, in which case the subject will

be assigned a dummy name.• XML that are immediate children elements are

predicates, and their content is the object, or value of the predicate.


XML RDF Representation

• This is a shorthand notation.

• Elements that are immediate children are predicates, like title, and creator in the example.

• There are other ways to represent this.

• If the value is only a simple string, you can use attributes on the Description Element.


Alternate RDF XML Shorthand

• <rdf:Description rdf:about="uri:isbn:0152038655"

• mods:title="Arithmetic"

• mods:creator="Sandburg,Carl" />


Types of Containers

• RDF has data types for Containers,• Bag: a group of resources or literals, possibly

including duplicate members, where there is no significance in the order of the members.

• Seq : represents a group, possibly including duplicate members, where the order of the members is significant.

• Alt: resources that are alternatives (typically for a single value of a property.


RDF Bag Syntax

• rdf:Description …

• <dc:subject>• <rdf:Bag>• <rdf:li>library use studies</rdf:li>• <rdf:li>magazines and

newspapers</rdf:li>• </rdf:Bag>• </dc:subject>


RDF Bags

•


RDF Sequence - Example• <rdf:Seq>• <rdf:li rdf:resource="00001.TIF"/>• <rdf:li rdf:resource="00002.TIF"/>• </rdf:Seq>• …. </rdf:Description>• <rdf:Description rdf:about="00001.TIF"><page

seq="1" img="00001.TIF" ocr="00001.TXT" ftr="TPG" n="1"/></rdf:Description>

• <rdf:Description rdf:about="00002.TIF"><page seq="2" img="00002.TIF" ocr="00002.TXT" ftr="PRF" n="2"/></rdf:Description>


RDF Values – Not only literals

• <rdf:Description rdf:about="uri:isbn:0152038655">

• <titleinfo>

• <rdf:Description>

• <dc:title>Arithmetic</dc:title>


• </titleinfo>

• <dc:creator>Sandburg,Carl</dc:creator>

• <dc:date>1993</dc:date>



RDF node values.


Another way to achieve Nodehood.

• <rdf:Description rdf:about="uri:isbn:0152038655"> <titleinfo rdf:parseType="Resource"> <dc:title>Arithmetic</dc:title>

• </titleinfo> <dc:creator>Sandburg,Carl</dc:creator> <dc:date>1993</dc:date> </rdf:Description>


Further Examples of Nodehood.<rdf:Description rdf:about="uri:isbn:0152038655"> <titleinfo rdf:parseType="Resource"> <rdf:Alt rdf:parseType="Resource"> <rdf:li rdf:parseType="Resource"> <dc:title>Arithmetic</dc:title></rdf:li> <rdf:li rdf:parseType="Resource">

<dc:title>l'Arithmetique</dc:title></rdf:li> </rdf:Alt> </titleinfo> <dc:creator>Sandburg,Carl</dc:creator>

<dc:date>1993</dc:date> </rdf:Description>


Further Examples of Nodehood,contd.


RDF Sequence: Graph


RDF Schema and XML Schemas

• The RDF Syntax so far does not have any rules for what vocabulary is acceptable.

• RDF schema is different from XML Schema.• XML Schema describe the structure of a

document.• RDF Schema describe classes of resources and

their properties, no matter how they may appear in a document.


RDF Schemas

• Define Classes.

• Define Properties.

• Define relationship between Classes

• Define relationships between Properties.

• Define possible what a property can be applied to and what values it can take on.


RDF Schema – Example Elements

• Rdfs:Class defines a kind of resource.• Rdf:Property defines that the resource being

talked about is a “property”• Notice in the attached dublin core RDF

Schema that only properties are defined, no classes. The only Rdf:Description only describes the dublin core vocabulary itself.

• Attached schemas for RSS and Dublin Core


An Example Schema: Dublin Core

• The Dublin Core Vocabulary in RDF Schema.• Define for each Dublin property what RDF needs

to know about it.• All the Dublin Core terms are properties that can

be applied to any Resource.• <rdf:Property

rdf:about="http://purl.org/dc/elements/1.1/title">


Dublin Core: - RDF Schema cont’d

• Each property is given an rdfs label, for using to display the property.

• Each property is given a comment, to describe it

• Each property is given a rsfs:isDefinedby “to indicate an RDF vocabulary in which a resource is described.”


Another Example Schema: RSS

• What is RSS

• Rich Site Summary

• Encoded Description of web site using RDF

• Used for News, blogging.

• Here is an example file:

• http://encompass-test.library.cornell.edu:20068/may.rss

http://encompass-test.library.cornell.edu:20068/may.rss

http://encompass-test.library.cornell.edu:20068/may.rss


RSS sample

• One Resource class is "channel"• Defined with rdfs:Class.• <rdfs:Class

rdf:about="http://purl.org/rss/1.0/channel" rdfs:label="Channel" rdfs:comment="An RSS information channel.">

• <rdfs:isDefinedBy rdf:resource="http://purl.org/rss/1.0/"/

• </rdfs:Class>


RSS – RDF Schema Discussion

• Another resource is item, which defines a nugget of information, also defined with rdfs:Class.

• <rdfs:Class rdf:about="http://purl.org/rss/1.0/item" rdfs:label="Item" rdfs:comment="An RSS item.">

<rdfs:isDefinedBy rdf:resource="http://purl.org/rss/1.0/"/>

</rdfs:Class>


RSS Validation

• http://www.redland.opensource.ac.uk/rss/ will validate your RSS and display it.

• UK sources of RSS “feeds” - http://rssxpress.ukoln.ac.uk/

• D-Lib Magazine – for instance. - http://dois.mimas.ac.uk/rss/dlib.xml


RDF Query Languages

RDFDB query language(?--??)

RDF Inkling Squish

RDQL


Querying An RDF database using RDFdb

• Each triple is a fact asserted into the database.

• One such data base demo is:

• http://www.redland.opensource.ac.uk/demo

• Query the whole database like this:

• ?--?-->?


?--[http://purl.org/dc/elements/1.1/subject]-->"Abolitionism"


Query and Result


Inking/Squish Queries

• http://swordfish.rdfweb.org:8085/rdfquery/index.html


Inkling/Squish – Sample Query

• SELECT ?about, ?title FROM http://encompass-test.library.cornell.edu:20068/may.rdf WHERE (dc::title ?about ?title) USING dc for http://purl.org/dc/elements/1.1/


Inkling Squish Results


Multi RDF database query.


Squish Query with Qualifiers


Using RDF with FRBR

• Danish Library Center 2002 – Visual Cat FRBR

• A module to handle FRBR as RDF has been implemented

• The FRBR display formats are based on Library of Congress suggestions.

• Interrelated FRBR RDF entities can be located, fetched and edited directly


RDF Schema for FRBR


RDQL – Query Language for RDF

• Similar to the RDF/Squish language.

• Similar to SQL –

• Variables in the SELECT clause

• WHERE describes a "path" in the graph.

• USING allows abbreviation of name spaces.

• FROM specifies a datasource(s)


RDQL Inference

• RDFS – data

• <Teenager rdf:about="http://example.com/colin">

• <mum rdf:resource="http://example.com/rosy" />

• <age>13</age>

• </Teenager>


RDQL Schema supporting Inference

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" xmlns:xsd="http://www.w3.org/2001/XMLSchema">

<rdf:Description rdf:about="http://example.com/mum"> <rdfs:subPropertyOf rdf:resource="http://example.com/parent"/> </rdf:Description> <rdf:Description rdf:about="http://example.com/parent"> <rdfs:range rdf:resource="http://example.com/Person"/> <rdfs:domain rdf:resource="http://example.com/Person"/> </rdf:Description></rdf:RDF>


RDQL data explanation

• Colin is a teenager whose mum is rose,

• And whose age is 13.

• Being a mum is a subProperty of being a parent.

• Being a parent means x is parent of y, and both x and w are Persons,


RDQL

• Example Query with sub property inferences:

• select ?x where

• ( ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> http://example.com/Person> )

• Find all the Persons


RDQL query results

• : java jena.rdfquery --vocab pschema.rdf --rdfs --data pdata.xml --query pquery2.txt

• ==========================• <http://example.com/colin>• http://example.com/rosy• Both colin and rosy are Persons, because being a mum,

means being a parent, and that means being a person.• Likewise having a mum means being a person, so the

Teenager is also a person.

http://example.com/rosy


RDQL Data

• <rdf:RDF xmlns:mods=" http://www.loc.gov/mods/” xmlns:rdf=http://www.w3.org/1999/02/22-rdf-syntax-ns#

• xmlns:dc="http://purl.org/dc/elements/1.1/">

• <rdf:Description rdf:about="uri:isbn:0152038655">

• <mods:title>Arithmetic</mods:title>

• <dc:creator>Sandburg,Carl</dc:creator>

• <dc:date>1993</dc:date>


• </rdf:RDF>

http://www.loc.gov/mods/

http://www.w3.org/1999/02/22-rdf-syntax-ns


RDQL – Schema

<rdf:Description rdf:about="http://www.loc.gov/mods/title">

<rdfs:subPropertyOf rdf:resource="http://purl.org/dc/elements/1.1/title" />

</rdf:Description>


RDQL Query

• java jena.rdfquery --vocab moschema.rdf --rdfs --data http://encompass-test.library.cornell.edu:20068/sandburg.rdf --query moquery1.txt

• select ?x ?y ?date where• ( ?x <http://purl.org/dc/elements/1.1/title> ?y )• ( ?x <http://purl.org/dc/elements/1.1/date> ?date)


Results of RDQL – with inference query

• : java jena.rdfquery --vocab moschema.rdf --rdfs --data http://encompass-test.li

• brary.cornell.edu:20068/sandburg.rdf --query moquery3.txt

• ---------------------------------------results -------• <uri:isbn:0152038655> | "Arithmetic" | "1993"• SELECT ?x ?y ?da WHERE ( ?x <dc:title> ?y )• ( ?x <dc:date> ?da )• AND ?da > 1847• USING dc FOR <http://purl.org/dc/elements/1.1/>


RDQL – inf cont'd

• <uri:isbn:0152038655> | "Arithmetic" | "1993"• : type moquery3.txt• SELECT ?x ?y ?da WHERE ( ?x <dc:title> ?y )• ( ?x <dc:date> ?da )• AND ?da > 1847• USING dc FOR <http://purl.org/dc/elements/1.1/>


RDFS A Part of Interoperability

• Some of the mechanisms of RDFS can be used to help with interoperability.

• Performance

• Adequacy of RDFS by itself, or even in combination with other tools.


Interoperability – Just some samples

• Combining RDF and XML Schemas to Enhance Interoperability Between Metadata, Hunter and Lagoze 2000

• Advocate using both XML Schemas and RDF Schemas, the one for encoding structural information about documents, data type constraints, content models – the other for class hierarchy, semantic modeling

• A Metadata Registry for the Semantic Web Heery and Wagner DLIB Magazine 2002 –

• Describe using RDF Schemas to describe and navigate the DCM Registry


Interoperability Cont'd• The OCLC metaswitch project• http://www.oclc.org/research/projects/mswitch/ • Godby, Smith, and Childress.

http://www.oclc.org/research/projects/mswitch/godby-dc2003.pdf

• The long path: translation to a common core and use of Xlink and Xpointer to document translations. "We believe that the interoperable core could be … RDF, but our experience has led us to conclude that RDF representations introduce .. processing bottlenecks, .. also reported by Heery and Wagner ...


Where we are

http://www.georgetown.edu/labyrinth/search.html

RDF, RSS and all that

Documents

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