RDF: Resource Description Failures?
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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RDF: Resource Description Failures?
Robert Sanderson [email protected] // [email protected] // @azaroth42
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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Overview
• Graphs • The Wide Open World • Ontologies and Identities • Serializations • Temporal Issues • Summary
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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Graphs
Graphs are very powerful for modeling reality Tree is just a simple Graph
(directed, acyclic, with known root node) Novel information can be automatically inferred More interesting questions can be asked Don’t end up in XML semantic/syntactic hell
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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Graphs
Graphs get complicated Querying is much more complicated
Graph: Structure and Data important, + data currently treated as second class citizen Other: Only Data important, so easier to work with
Serialization and storage is more complicated Serialization has a start and end, unlike a graph Storage can’t look at “documents”, deals with structure
Visualization is difficult to get right … and hard to know when it is right
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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Visualization Done Right
http://www.facebook.com/note.php?note_id=469716398919
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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Not So Right
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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Graphs
Graphs are very powerful Graphs get complicated
Mitigating Factors: • Software libraries can manage complexity
eg RDF Object Mapping • NoSQL solutions improving rapidly • Can have cake: Treat part of the graph as a document
And eat it: Also ingest into TripleStore • Other structures don’t get complicated
because they lack the expressivity of a graph
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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The Open World
A Single Global Graph that everyone contributes to Great for data re-use Richness of data from multiple sources Anyone can make assertions about anything Global identities Distributed: Can incrementally add to others descriptions Fits with the WWW: The Data Web
Technically: If a statement is not asserted, then its truth-value is unknown, rather than false.
Data: Grass is Green. Question: Is Grass Red? Closed World: No Open World: I Don’t Know
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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The Open World: Positive Use Case
Jon publishes an Annotation about part of a web page.
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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The Open World: Positive Use Case
Brewster archives the page … and says where it is.
Without modifying the annotation at all.
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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The Open World: Complexity
Local Constructions are Complicated Despite “Think Globally, Assert Locally”
How do we say that Canvas 2 comes after Canvas 1, and Canvas 3 comes after Canvas 2?
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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The Open World: Lists
Lists
Did you think this? Remember anyone can say anything, and it’s global…
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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The Open World: Lists
Now there are two next links from Canvas 1, and our list is … a graph. Use Case: Manuscript has different page order at different times
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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The Open World: Lists
ORE introduces proxy nodes, as not just order is local. Eg may wish to cite a resource in the context of a set of resources.
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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The Open World: Lists
Shared Canvas uses multiple classes and the rdf:List construction. Serializations hide the list’s anonymous nodes.
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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The Open World: Complexity
Assertions can drastically change meaning
Let’s turn this around a little…
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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The Open World: Complexity
Fred asserts:
Without modifying the annotation at all?!
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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A Single Global Graph that everyone contributes to Local constructions are complex Remote assertions can change meaning
Mitigating Factors: • Local identity for local context is good practice • Harder to take short cuts, forces understanding • Actually some grass is red. Could be both red and green • Trust is required in all data, not just open linked data
The Open World
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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Shared ontologies increases semantic interoperability dc:title is ‘name’ or ‘label’, not property title or Dr. Re-use of semantics makes it easier to build applications Communities can develop own ontologies independently
(as opposed to microdata/schema.org)
Shared Identity makes it possible for graph to merge serendipitously Everyone can mint own IDs using http URIs By reusing ids, graphs will merge, creating new knowledge
Ontologies and Identities
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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“The nice thing about … Ontologies … is that there’s so many to choose from” Far far too many to choose from, hard to find the right one If almost right, do you reuse and hope for the best, or
specialize and create yet another ontology?
Ontologies and Identities
http://xkcd.com/927/
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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“The nice thing about … Identities … is that there’s so many to choose from” Far far too many to choose from, hard to find the right one As anyone can create identity for anything, they do Identity often has a contextual component – does LANL’s
identifier for Oppenheimer differ from DBPedia’s?
Ontologies and Identities
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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Shared Ontologies increase Interoperability Shared Identifiers make the graph merge Multiplicity of Ontologies Multiplicity of Identities
Mitigating Factors: • Assertions of equivalence are not just assertions.
Can apply same parsers, trust mechanisms etc. • There are well-known ontologies and identifier schemes • As the global graph increases, winners will become obvious
Ontologies and Identities
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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… … The new JSON-LD format is actually pretty good?
Serializations
{! "@context": "http://www.openannotation.org/spec/core/context.json", ! "@id": "http://example.org/anno1", ! "@type": "oa:Annotation", ! "annotatedAt": "2012-11-10T09:08:07", ! "annotatedBy": {! "@id": "http://public.lanl.gov/rsanderson#me", ! "@type": "foaf:Person", ! "name": "Rob Sanderson"! }, ! "body": {"chars": "This... is CNN.”}, ! "target": {“@id” : “http://www.cnn.com/”}!}!
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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Too many serialization formats
The recommended RDF/XML is absolutely terrible “RDF/XML was the Semantic Web’s 3 Mile Island incident”
-- Manu Sporny, http://manu.sporny.org/2012/nuclear-rdf/
Multiple formats means multiple identifiers for descriptions (one per format) Content Negotiation is a pain Not everyone implements every format = interop hell
Leaves room for competing models/syntaxes Microdata, Schema.Org etc.
Serializations
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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JSON-LD Everything else
Mitigating Factors: • Software libraries help, but are inconsistent • … and that’s all I’ve got!
Serializations
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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Resources change over time Reality, Data and Ontologies Neither document nor data web has a solution Need to remain in sync in distributed environment New URIs for every version doesn’t work Coherency: does assertion still apply when other dataset
changes?
Mitigating Factors: • Memento • ResourceSync
Temporal Issues
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RDF Advantages, Limitations and Questions Mellon IT Projects Mtg, November 28-29, New York NY, USA
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• Graphs are powerful structures, and the complexity can be managed by tools
• Open World introduces complexity, but enforces best practices. Without it, would be just data on the web
• Trust is required for reuse of any data, not just RDF • Winners will emerge for competing ontologies and
identities and better than the alternative. • JSON-LD is very strong, and parsers exist in all
common languages for all serializations • Good people are working on the Temporal issues ;)
Summary
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Slides: http://www.slideshare.net/…
Open Annotation: http://www.openannotation.org/ http://www.w3.org/community/openannotation/
Shared Canvas: http://www.shared-canvas.org/
Rob Sanderson [email protected] // [email protected] @azaroth42 // +azaroth42 http://public.lanl.gov/rsanderson/
Thank You!