A Really Brief Crash Course in Semantic Web Technologies

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A Really Brief Crash Course in Semantic Web
Technologies

• Rocky Dunlap
• Spencer Rugaber
• Georgia Tech

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Languages you may encounter...

• XML (eXtensible Markup Language)
• XML Schema
• XPath (navigate an XML document)
• XQuery (query an XML document)
• XSLT (Extensible Stylesheet Language
  Transformations)
• RDF (Resource Description Framework)
• RDF Schema
• OWL (Web Ontology Language)
• SPARQL (Query language for RDF triples)
• SQL (Structured Query Language for RDBMS)
• UML (Unified Modeling Language conceptual)
• SKOS (Simple Knowledge Organization System)
  glossary

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Links to language specs
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XML

• General purpose markup language
• Mechanism for structured data exchange between
  heterogeneous systems
• Basically elements (tags) and attributes
• Not really for human consumption, although it is
  easy for us to read and write in small amounts
• An XML file is often called an instance document

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XML Schema

• Defines the allowed structure of a set of
  instance documents
• Defines a set of types -- valid chunks of XML
• Typically the schema is defined up front and
  applications are written to process valid or
  schema-conforming instance documents
• The schema is a way to achieve standardization
  like a contract
• If you provide a valid document, well provide
  you with tools that do X, Y, and Z.

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RDF

• A knowledge representation language
• Conceptual in nature
• It really has nothing to do with XML
• But, there happens to be an XML representation
• A way to make statements about pretty much
  anything you want
• The Curator meeting is at GFDL.
• The Curator meeting is Oct 18-19.
• Balaji works at GFDL.

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RDF Statements
The Curator meeting is at GFDL.
Curator meeting
hasLocation
GFDL
subject
predicate
object
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RDF Statements
The Curator meeting is Oct 18-19.
Curator meeting
hasLocation
resource
GFDL
starts
ends
18 Oct 2007
literal
19 Oct 2007
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RDF Statements
Balaji works at GFDL.
Balaji
worksAt
Curator meeting
hasLocation
GFDL
starts
ends
18 Oct 2007
19 Oct 2007
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RDF XML Representation
ltrdfRDF xmlnsrdf"http//www.w3.org/1999/02/22-r
df-syntax-ns" xmlnsesc"http//www.earthsystemcu
rator.org"gt ltrdfDescription rdfabouthttp//.
...OctCuratorMeeting"gt lteschasLocation
rdfresourcehttp//....GFDL/gt ltescstartsgt18
Oct 2007lt/escstartsgt ltescendsgt19 Oct
2007lt/escendsgt lt/rdfDescriptiongt ltrdfDescrip
tion rdfabouthttp//....Balaji"gt ltescworksAt
rdfresourcehttp//....GFDL/gt lt/rdfDescripti
ongt lt/rdfRDFgt
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RDF Schema

• Define a domain specific data model for RDF
• Includes classes and properties (along with
  subclasses and subproperties)
• Properties are first class (they are not defined
  as part of a particular class)

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RDF Schema
Classes
Properties
hasLocation domain Event range
Place starts domain Event range date
ends domain Event range date worksAt
domain Person range Place
Event
Meeting
Flight
Person
Place
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OWL (Web Ontology Language)

• Builds on RDF by adding increased expressivity
• Every OWL file is RDF (but not necessarily the
  reverse)

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RDF vs. OWL
OWL
Property constraints -allValuesFrom
-someValuesFrom -hasValue
RDF
Classes Subclasses Properties Subproperties Indivi
duals
Cardinality constraints on properties
-cardinality (exact) -minCardinality
-maxCardinality
Class definitions -intersection -union
-complement -equivalentClass -disjointWith
-oneOf (enum)
Transitive Properties Symmetric Properties
Individuals -sameAs -differentFrom
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Things you can NOT say in RDF, but can say in OWL

• The class TriangularUnstructuredGrid is at the
  intersection of TriangularGrid and
  UnstructuredGrid
• UnstructuredGrid is the complement of
  StructuredGrid
• A Dataset is generated by exactly one Model
• A Model is made up of at least one Component
• An AtmosphereComponent is a Component with
  ScienceType equal to Atmosphere
• X subComponent Y, Y subComponent Z ? X
  subComponent Z

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Things you can NOT say in RDF, but can say in OWL

• The class Model is equivalent to ConfiguredModel
• ScienceType is the exact enumeration Atmosphere,
  Ocean, Ice, and Land
• ObservationDataset is disjoint from
  SimulationDataset
• Dataset123 is the same object as DatasetXYZ

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SPARQL

• A language for querying RDF/OWL triples
• Example query

PREFIX foaf lthttp//xmlns.com/foaf/0.1/gt SELECT
?x ?name WHERE ?x foafname ?name
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Curators Current Strategy

• Curator data model written in XML Schema
• Models and Datasets (Resources) annotated with
  conforming XML instance documents
• Portions of XML translated into RDF and exposed
  by CDP-Curator faceted search
• This means
• Low level details remain in XML instance
• Higher level concepts pulled out into the RDF
• Can we confirm this strategy?

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Technical Challenges

• XML to RDF translation
• Hierarchical, low level ? graph-based, conceptual
• Is there a need to go from RDF back to XML?
• What stays in XML? What goes to RDF?
• Automation of translation
• Schema level (e.g., schema evolution)
• Instance level (e.g., submission of new resource
  to CDP-Curator)