uk ontology network

The Semantic WebLinked Data

Digging Deeper into Linked DataFinal Words

Linked DataLike It or Loathe It?

Peter F. Patel-Schneider

April 11, 2012

Peter F. Patel-Schneider Linked Data Like It or Loathe It?

Abstract

Linked Data was supposed to be “the Semantic Web done right”,at least according to Tim Berners-Lee. But just what is LinkedData? How is Linked Data different from the Semantic Web? IsLinked Data really a better way to add semantics to the web, or isit just a way to throw data out on the Web in a way that can beused in hacks? There are some signs that some order is coming toLinked Data, but is this just doing (or redoing) the work thatremains to be done (has been done) in the Semantic Web?

Warning: This is a personal view of Linked Data.

Abstract

Linked Data was supposed to be “the Semantic Web done right”,at least according to Tim Berners-Lee. But just what is LinkedData? How is Linked Data different from the Semantic Web? IsLinked Data really a better way to add semantics to the web, or isit just a way to throw data out on the Web in a way that can beused in hacks? There are some signs that some order is coming toLinked Data, but is this just doing (or redoing) the work thatremains to be done (has been done) in the Semantic Web?

Warning: This is a personal view of Linked Data.

Outline

1 The Semantic Web

2 Linked Data

3 Digging Deeper into Linked Data

4 Final Words

Table of Contents

1 The Semantic Web

2 Linked Data

4 Final Words

The Semantic Web

Tim Berners-Lee’s initial vision of the web was much morethan just browsers, and included a notion of a web of datawith a well-defined meaning

This vision as come to be known as the Semantic Web, butthe “Visual” Web took off much faster and is much betterknown

The Semantic Web is supposed to be a web of informationthat can be combined and processed by machines with aslittle direct human intervention as possible

Making the Semantic Web Work—Data

Data in the Semantic Web is written as RDF triples

An RDF triple consists of a subject, a predicate, and an object

e.g., pfps:Peter foaf:knows ox:Iane.g., pfps:Peter foaf:age "57"^^xsd:inte.g., pfps:Peter rdf:type foaf:PersonSubjects, predicates, and objects are names (URIs oranonymous names) or literals (i.e., data values)

URIs are global names, which can be used anywhere,emphasizing connections within the Semantic Web

RDF triples are available in Web documents, emphasizingaccessibility in the Semantic Web

Data Alone is Insufficient

Given that Alex has heart disease

ex :Alex hasDisease ex :d1ex :d1 rdf :type HeartDisease

Who has circulatory disease?

?who hasDisease ?disease?disease rdf :type CirculatoryDisease

No one can be determined to have a circulatory disease

. . . because there is no connection in the data between heartdisease and circulatory disease

Data Alone is Insufficient

Given that Alex has heart disease

ex :Alex hasDisease ex :d1ex :d1 rdf :type HeartDisease

Who has circulatory disease?

?who hasDisease ?disease?disease rdf :type CirculatoryDisease

No one can be determined to have a circulatory disease

. . . because there is no connection in the data between heartdisease and circulatory disease

What’s Missing: Ontologies

Ontologies

Model some portion of the world

e.g., anatomy, aircraft engines, cell phones

Introduce a vocabulary

e.g., organs, muscles, heart, diseases

Specify meaning of vocabulary

e.g., a heart is a muscular organ in the circulatory sytem

All in a formal language (a logic)

Sample Ontology Fragments

The heart is a muscular part of the circulatory system

Heart v MuscularOrgan u ∃isPartOf .CirculatorySystem

Heart disease is a disease of the heart

HeartDisease ≡ Disease u ∃affects.Heart

Circulatory disease is a disease of a part of the circulatory sys.

CirculatoryDisease ≡Disease u ∃affects.(∃isPartOf .CirculatorySystem)

Note that the syntax here is not the syntax used in the SemanticWeb.

Sample Ontology Fragments

The heart is a muscular part of the circulatory system

Heart v MuscularOrgan u ∃isPartOf .CirculatorySystem

Heart disease is a disease of the heart

HeartDisease ≡ Disease u ∃affects.Heart

Circulatory disease is a disease of a part of the circulatory sys.

CirculatoryDisease ≡Disease u ∃affects.(∃isPartOf .CirculatorySystem)

Note that the syntax here is not the syntax used in the SemanticWeb.

Using Ontologies

Given that Alex has heart diseaseex :Alex hasDisease d1d1 ∈ HeartDisease

And ontology information about anatomy and diseases,containing

Heart v MuscularOrgan u ∃isPartOf .CirculatorySystemHeartDisease ≡ Disease u ∃affects.HeartCirculatoryDisease ≡

Disease u ∃affects.(∃isPartOf .CirculatorySystem)

Who has circulatory disease?Find ?who hasDisease ?disease

?disease ∈ CirculatoryDiseaseAnswer: ex :Alex has circulatory disease

Is heart disease a specialization of circulatory disease?Is HeartDisease v CirculatoryDiseaseAnswer: Yes

Using Ontologies

Making the Semantic Web Work—Ontologies

OWL (and RDFS) provide ontologies for the Semantic Web

Ontologies provide machine-processable meanings for (thenames in) the data, particularly the categories andrelationships

Ontologies admit deduction of implicit information

Humans also do this (but perhaps in very different ways)

Distributed and decentralized nature of (Semantic) Webmeans that data sources often have

differing amounts of information in different sourcesvocabulary shiftsvarying specificity

Ontologies help overcome problems in all of these

Semantic Web History

Semantic Web recommendations from W3C:

1999 RDF (abstract data version)—data

2000 RDFS—very simple ontologies

2004 RDF and RDFS (logic version)

2004 OWL—general ontologies

2008 SPARQL—query language

2009 OWL 2

2012 SPARQL 1.1

2012? RDF extension

The Semantic Web (recap)

RDF is used to write data, in the form of triples

OWL (and RDFS) provide ontology support

Global names provide connections

Semantic Web data and ontologies are accessible as webdocuments

Applications access and load documents into reasoners, whichdetermine implicit consequences

The Semantic Web (recap)

RDF is used to write data, in the form of triples

OWL (and RDFS) provide ontology support

Global names provide connections

Semantic Web data and ontologies are accessible as webdocuments

Applications access and load documents into reasoners, whichdetermine implicit consequences

Table of Contents

1 The Semantic Web

2 Linked Data

4 Final Words

Any Role for Linked Data?

We appear to have managed to build a viable artifact withoutexplicitly appealing to “Linked Data”

Where is the need for anything more?

Maybe there is a need for tying Semantic Web documentstogether, beyond that provided by global names

Missing from the Semantic Web?

Some early versions of 2004 RDF documents had a section onsocial meaning, including

Defining Information

The defining information for an RDF identifier, e.g.,http://www.whitehouse.gov/pres43, should available as an(RDF) document retrievable from that URI (fragment)

Follow Your Nose

Using an RDF identifier, e.g.,http://www.whitehouse.gov/pres43, commits you to retrievingand using its defining information

Softened and then removed from RDF specifications after outcry

What if document includes irrelevant information?

What if document includes controversial information?

How can you dispute defining information?

Missing from the Semantic Web?

Some early versions of 2004 RDF documents had a section onsocial meaning, including

Defining Information

The defining information for an RDF identifier, e.g.,http://www.whitehouse.gov/pres43, should available as an(RDF) document retrievable from that URI (fragment)

Follow Your Nose

Using an RDF identifier, e.g.,http://www.whitehouse.gov/pres43, commits you to retrievingand using its defining information

Softened and then removed from RDF specifications after outcry

What if document includes irrelevant information?

What if document includes controversial information?

How can you dispute defining information?

RDFS Links

RDFS has two properties that might be used to linkdocuments together

rdfs:isDefinedBy, e.g., http://www.whitehouse.gov/pres43rdfs:isDefinedBy http://www.whitehouse.gov/pres41.rdf

a link to something that provides a definition

rdfs:seeAlso, e.g., http://www.whitehouse.gov/pres43rdfs:seeAlso http://www.whitehouse.gov/pres41.rdf

a link to something that provides additional information

This provides a way of providing defining information that isnot necessarily coupled with retrieval using the RDF identifieritself

Also, there is no requirement to using the defining oradditional information

OWL Imports, or explicit following

When using OWL, you very often want to combineinformation from several documents

One or more documents with base facts (in RDF)Documents that contain the ontologies needed to givemeaning to the identifiers used

Even ontologies often should be written down in multipledocuments, e.g., ontology for anatomy and ontology fordiseases in different documents

OWL imports is a construct that explicitly says to useinformation from another (web) document, e.g, the diseaseontology imports the anatomy ontology

Was rather controversial(!), but was adopted into OWLrecommendations

OWL imports can be used to replace “follow your nose”, butis more general

Instead of (blindly) following your nose, use OWL importsonly when following links is appropriate

OWL imports can be used to import from any otherdocument

Applications no longer have to explicitly load up all requireddocuments, but can use OWL imports to gather togethercollections of documents that form an entire ontology (or justcontain relevant data)

Linking in the Semantic Web

Even without RDF social meaning, there are good ways oflinking between Semantic Web documents

In OWL, imports provides linking when the use of the linkedinformation is required

RDFS provides two kinds of linking in cases where use is notmandated

There is no bar to using follow-your-nose when it is possibleand reasonable

The Emergence of Linked Data

Linked Open Data

“Semantic web done right”“Paradigm shift all over again”(www.w3.org/2008/Talks/0617-lod-tbl)

Many documents provide quick introductions to Linked Data

Provide a view of what its originators believe is important inLinked Data

. . . as opposed to what is actually available as Linked Data

What is Linked Data?

From www.w3.org/DesignIssues/LinkedData

The four rules of linked data1 Use URIs as names for things2 Use HTTP URIs so that people can look up those names3 When someone looks up a URI, provide useful information4 Include links to other URIs, so that they can discover more

things

Five Star Linked Open Data

? Available on the web with an open licence?? Available as machine-readable structured data

? ? ? Available in a non-proprietary format? ? ?? Use open standards from W3C (RDF and SPARQL) to identify

things, so that people can point at your stuff? ? ? ? ? Link your data to other people’s data to provide context

things

What’s New in Linked Data?

Not using URIs as names—that’s required by RDF

Not using HTTTP URIs as names—that was normal

Not interlinking—that was normal

Not machine-readable—that’s required by RDF

Not non-proprietary—that’s RDF

Not open standards—that’s RDF

Perhaps

Open licenceEmphasis on interlinkingProviding useful information via name lookup

Is Linked Data just a way of recovering informal meaning forRDF?

Instead of requiring follow-your-nose, just put it out there

Liking Linked Data—Initial View

Linked Data requires

combining RDF data from various (web) sources and

agreeing to use non-local identifiers

which are both laudable aspects of the Semantic Web, but perhapswere underemphasized.

Linked Data

encourages, but does not require, follow your nose

combines (simple) data from multiple sources

so that simple applications can be quickly built.

Linked Data is a hacker’s version of the Semantic Web supportingquick and dirty data mash-ups.

Loathing Linked Data—Initial View

Linked Data ignores

distinctions between data sources and ontology sources

issues with trust and consistency

which should both be aspects of the Semantic Web.

Linked Data

follows your nose, but when and where to?

doesn’t specify which language is to be used

requiring close control when building applications.

Linked Data is a hacker’s version of the Semantic Web supportingquick and dirty data mash-ups.

Linked Data—Initial View

There doesn’t appear to be much to Linked Data at all, only

a slight change in emphasis from the Semantic Web

a restating of follow your nose

Maybe there isn’t much Linked Data, so we shouldn’t care about it

There certainly is enough noise about Linked Data, and evenworkshops on it (Linked Open Data Workshops)

Linked Data—Initial View

There doesn’t appear to be much to Linked Data at all, only

a slight change in emphasis from the Semantic Web

a restating of follow your nose

Maybe there isn’t much Linked Data, so we shouldn’t care about it

There certainly is enough noise about Linked Data, and evenworkshops on it (Linked Open Data Workshops)

How Big is Linked Data?

Linking Open Data Cloud lod-cloud.net

Map of major linked open RDF data sources

Currently (September 2011) 295 sources

Appears to have attracted considerable attention

What just is this measuring?

Major RDF data sources that tightly link together

How Big is Linked Data?

Linking Open Data Cloud lod-cloud.net

Map of major linked open RDF data sources

Currently (September 2011) 295 sources

Appears to have attracted considerable attention

What just is this measuring?

Major RDF data sources that tightly link together

Linking Open Data Cloud

Linking Open Data cloud diagram of 2007-05-01, by RichardCyganiak and Anja Jentzsch. http://lod-cloud.net/

Linking Open Data Cloud

Linking Open Data cloud diagram of 2011-09-19, ibid

Linked Data Exists

Considerable self-identified Linked Data exists

Considerable other attention has been paid to Linked Data

Lots of web sites on linked data, tutorials, talks, books, etc.Continuing workshop series on linked datahttp://events.linkeddata.org/ldow2012/

Can’t just dismiss Linked Data

Table of Contents

1 The Semantic Web

2 Linked Data

4 Final Words

Linked Data is Just Data

Linked Data is Merely More Data(Jain, Hitzler, Yeh, Verma, Sheth, AAAI SSS 2010)

Analysis of actual Linked Data (a few years ago)

Different data sources use different vocabularies

Different data sources use different modeling decisions

Linked Data does not describe the different data sources

Few constructs to describe the data and tie it together

Effects:

Finding data requires human control

Combining data from different sources requires human control

Useful inferences cannot be made because of missing ontologyinformation

Ontologies in the Linked Data Story

Ontologies do get mentioned

“Ontology bits for . . . ”(www.w3.org/2008/Talks/0617-lod-tbl)Use RDFS to get lightweight ontologies(linkeddatabook.com/book)Add in equality, equivalency, and inverses from OWL (ibid)Other sources suggest different bits

. . . but not very prominently

Precise ontology language is left un(der)specified

How can this be if there is single Linked Open Data Cloud?Which ontology reasoner to use?

The Problem with Linked Data

It is not that linking data is not useful—it is, but the linked datastory is not addressing the big problems with the Semantic Web,such as

lack of ontologies

need for better ontology tools

need for even better reasoners

lack of good methods for selecting data

lack of good methods for accessing non-RDF data

Some work that is labelled as linked data does address a few of thebig problems, but nothing in the analysis or solution is specific tolinked data.

lack of ontologies

Why is Linked Data being used?

Semantic Web requires reasoning, so it’s too hard

Semantic Web requires thought, so it’s too hard

Semantic Web requires global consistency, so it’s impossible

Semantic Web requires explicit importing of used information,so it’s too hard

Where is the Theory of Linked Data

How to select data sources?

When to follow your nose?

Where to get ontologies?

How to recover from contradictions?

Who to trust?

All need to be addressed to make Linked Data really usablewithout direct human control.

Where is Reasoning in Linked Data?

Need more than RDFS reasoning plus owl:sameAs

but even RDFS reasoning is hard,Can’t just say that parallelism solves the problem,need to think about what needs to be done.

Full ontology Reasoning is even harder,

but so is communication, DBs, . . .

Reasoning can be slow

but so can be communication, DBs, . . .

No reason to avoid reasoning

just need to consider its potential costs

Reasoning for Linked Data

Maybe full ontology reasoning is just too hard at Linked Datascale.

Perhaps look at what ontology constructs are used in LinkedData and construct a reasoner for these constructs.

OWL: Yet to arrive on the Web of Data?Glimm, Hogan, Kroetzsch, Polleres LDOW 2012

Examining 2011 Billion Triple Challenge data found some useof OWL constructs

Mostly single-triple constructs

Define new OWL profile (OWL LD) with only the constructswith significant use—a subset of OWL RL

Design very fast reasoners for this profile

Combining Linked Data and the Semantic Web

Good parts of Linked Data . . .

publish data nicelyuse other’s identifiers

. . . plus good parts of Semantic Web

use RDF and OWLontologies to describe dataontologies to bridge between vocabulariesOWL imports helps control which documents to use

But, but, but, . . . , that’s just the Semantic Web, isn’t it?

Combining Linked Data and the Semantic Web

Good parts of Linked Data . . .

publish data nicelyuse other’s identifiers

. . . plus good parts of Semantic Web

use RDF and OWLontologies to describe dataontologies to bridge between vocabulariesOWL imports helps control which documents to use

But, but, but, . . . , that’s just the Semantic Web, isn’t it?

Table of Contents

1 The Semantic Web

2 Linked Data

4 Final Words

Like or Loathe?

Perhaps the effect of Linked Data is to move attention awayfrom semantic issues towards data issues

Loathe (a lot)

Perhaps Linked Data is just something for hackers to hangtheir hat on, since the Semantic Web has gotten semantics

Loathe

Perhaps Linked Data is just a marketing slogan, and it isreally just the Semantic Web itself, in disguise

Loathe

Perhaps Linked Data is a buzzword of the past and has beenovertaken by big data (whatever that is)

Like ,Perhaps Linked data is an honest attempt to get people topay attention to a neglected part of the Semantic Web

Like, but where is the evidence that this part was actuallyneglected?

Like or Loathe?

Loathe (a lot)

Loathe

Like or Loathe?

Loathe (a lot)

Loathe

Like or Loathe?

Loathe (a lot)

Loathe

Like or Loathe?

Loathe (a lot)

Loathe

Like or Loathe?

Loathe (a lot)

Loathe

Like or Loathe?

Loathe (a lot)

Loathe

Like or Loathe?

Loathe (a lot)

Loathe

Future Work

Analysis of all the data in the Linked Data Cloud (or all thecrawlable RDF in the Semantic Web)

What constructs are used?What ontologies are there?

Should there be a language (and reasoner) for the SemanticWeb or for Linked Data, as they currently exist?

If so, what should the language look like?If so, how should the reasoner behave?

Questions?

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Documents

linked data different

semantic web workdatadata

betterknownthe semantic

semantic web2

semantic webpeter

personal view of linked

tolinked data

islinked data