cc6202-1 l a w eb de d atos p rimavera 2015 lecture 2: rdf model & syntax aidan hogan...
TRANSCRIPT
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LAST TIME …
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The “Semantic Web”
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Google does not change the fact that …
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What if we could “structure” everything …
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One symbol, one meaning …
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One symbol, one meaning …
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One (simple) way to say one thing …
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(1) Data, (2) Query, (3) Rules/Ontologies
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STRUCTURING DATA WITH RDF: RESOURCE DESCRIPTION FRAMEWORK
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(1) Data, (2) Query, (3) Rules/Ontologies
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RDF: Resource Description Framework
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Modelling the world with triples
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Concatenate to “integrate” new data
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RDF often drawn as a (directed, labelled) graph
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Set of triples thus called an “RDF Graph”
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But why triples?
What is the benefit of triples?
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NAMING THINGS IN RDF: IRIS
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One symbol, one meaning …
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Need unambiguous symbols/identifiers
• Since we’re on the Web … use Web identifiers
• URL: Uniform Resource Location– The location of a resource on the Web– http://ex.org/Dubl%C3%ADn.html
• URI: Uniform Resource Identifier (RDF 1.0)– Need not be a location, can also be a name– http://ex.org/Dubl%C3%ADn
• IRI: Internationalised Resource Identifier (RDF 1.1)– A URI that allows Unicode characters– http://ex.org/Dublín
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We will use IRIs with prefixes
• http://ex.org/Dublín ↔ ex:Dublín
• “ex:” denotes a prefix for http://ex.org/• “Dublín” is the local name
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Frequently used prefixes
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From strings …
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… to IRIs …
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NAMING THINGS IN RDF: LITERALS
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What about numbers?
Should we assign IRIs to numbers, etc.?
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RDF allows “literals” in object position
• Literals are for datatype values, like strings, numbers, booleans, dates, times
• Only allowed in object position
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Datatype literals
• “lexical-value”^^ex:datatype– “200”^^xsd:int– “2014-12-13”^^xsd:date– “true”^^xsd:boolean– “this is a string”^^xsd:string
• If the datatype is omitted, it’s a string– “this is a string”– “200” is a string, not a number!
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Many datatypes borrowed from XML Schema
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Boolean datatype
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Numeric datatypes
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Temporal datatypes
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Text/string datatypes
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Language-Tagged Strings
• Specify that a string is in a given language• “string”@lang-tag• No datatype!
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(NOT) NAMING THINGS IN RDF:BLANK NODES
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Having to name everything is hard work
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For this reason, RDF gives blank nodes
• Syntax: _:blankNode• Represents existence of something– Often used to avoid giving an IRI (e.g., shortcuts)
• Can only appear in subject or object position
• (More later)
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RDF TERMS: SUMMARY
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A Summary of RDF Terms
1. IRIs (Internationalised Resource Identifiers)– Used to name generic things
2. Literals– Used to refer to datatype values– Strings may have a language tag
3. Blank Nodes– Used to avoid naming things– A little mysterious right now
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MODELLING DATA IN RDF
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Let’s model something in RDF …Model the following in RDF:
“Sharknado is the first movie of the Sharknado series. It first aired on July 11, 2013. The movie stars Tara Reid
and Ian Ziering. The movie was followed by ‘Sharknado 2: The Second One’.
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RDF Properties
• RDF Terms used as predicate• rdf:type, ex:firstMovie, ex:stars,
…
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RDF Classes
• Used to conceptually group resources• The predicate rdf:type is used to relate
resources to their classes
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Modelling in RDF not always so simple
Model the following in RDF:“Sharknado stars Tara Reid in the role of ‘April Wexler’.
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Modelling in RDF not always so simple
Model the following in RDF:“The first movie in the Sharknado series is ‘Sharknado’.
The second movie is ‘Sharknado 2: The Second One’.The third movie is ‘Sharknado 3: Oh Hell No!’.
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RDF Collections: Model Ordered Lists
• Standard way to model linked lists in RDF• Use rdf:rest to link to rest of list• Use rdf:first to link to current member• Use rdf:nil to end the list
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RDF Collections: Generic Modelling
• Not just for Sharknado series
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RDF SYNTAXES: WRITING RDF DOWN
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N-Triples
• Line delimited format• No shortcuts
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RDF/XML
• Legacy format• Just horrible
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RDFa
• Embed RDF into HTML• Not so intuitive
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JSON-LD
• Embed RDF into JSON• Not completely aligned with RDF
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Turtle
• Readable format
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Turtle: Collections Shortcut
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BLANK NODES ADD COMPLEXITY
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Blank nodes names aren’t important …
(Isomorphic)
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Blank nodes are local identifiers
How should we combine these two RDF graphs?
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Need to perform an RDF merge
How should we combine these two RDF graphs?
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Are two RDF graphs the “same”?
(Isomorphic)
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Are two RDF graphs the “same”?
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RECAP
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RDF: Resource Description Framework
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RDF = Resource Description Framework
• Structure data on the Web!
• RDF based on triples:– subject, predicate, object– A set of triples is called an RDF graph
• Three types of RDF terms:– IRIs (any position)– Literals (object only; can have datatype or language)– Blank nodes (subject or object)
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RDF = Resource Description Framework
• Modelling in RDF:– Describing resources– Classes and properties form core of model– Try to break up higher-arity relations– Collections: standard way to model order/lists
• Syntaxes:– N-Triples: simple, line-delimited format– RDF/XML: legacy format, horrible– RDFa: embed RDF into HTML pages– JSON-LD: embed RDF into JSON– Turtle: designed to be human friendly
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RDF = Resource Description Framework
• Two operations on RDF graphs:– Merging: keep blank nodes in source graphs apart– Are they the “same” modulo blank node labels:
isomorphism check!
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Questions?