Resource Identity and Semantic Extensions:

Making Sense of AmbiguityDavid Booth, Ph.D.

Cleveland Clinic (contractor)

Semantic Technology Conference

25-June-2010

Latest version of these slides:http://dbooth.org/2010/ambiguity/

Also available: Companion paper

Outline

• Part 0: Myths about resource identity

• Part 1: RDF semantics and ambiguity– Interpretations

– Interpretations of a URI

• Part 2: Constraining ambiguity through URI

declarations– Bounding ambiguity

– Ambiguity and owl:sameAs

• Part 3: Determining resource identity– Semantic extensions

PART 0:

Myths About Resource Identity

This section makes some observations that will be further

explained in later sections.

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URIs as names for resources

URI

http://example/#apple

Resource

• In the semantic web, URIs are used as names for

resources– Separate from a URI's use as a locator

• “Resource” == “Thing” == Universal class– E.g., people, proteins, medications, concepts, etc.

• But which resource does a URI name?

?

Resource identity

• Which resource does a given URI denote?

• Because a URI can denote any resource, this

question is central to RDF semantics

• This is the question of resource identity – the

determination of which resource a URI denotes

Myth 1: A URI denotes only one resource

Myth:

• “By design a URI identifies one resource”

– W3C Architecture of the World Wide Web

Reality:

• True as an ideal

• True for one interpretation of one RDF graph, but . . .

• Different interpretations of the same graph may map

the same URI to different resources

• Different graphs may permit different interpretations

Myth 2: RDF semantics are global

Myth (a variation of #1):

• There is only one giant graph, with global semantics

• E.g., “owl:sameAs makes a very strong statement”– The implication is that it must hold universally

Reality:

• RDF semantics are defined for a given graph

• There are many graphs

• The “meaning” of a URI depends on the graph– A URI may denote different resources in different

graphs

Myth 3: Resource ambiguity is due to sloppiness

Myth:

• A URI's resource identity can be uniquely defined if

you are precise enough

Reality:

• Ambiguity is unavoidable– . . . with vanishingly few exceptions

• Always possible to make ever finer resource

distinctions

• See examples in In Defense of Ambiguity by Pat

Hayes and Harry Halpin

Myth 4: Truth is absolute

Myth:

• “If your RDF models the world as flat, then it is

wrong”

Reality:

• “Truth” is irrelevant; what matters is usefulness

• Different apps have different needs– Flat world model may be best for street navigation:

Precise enough, and simpler than round world model

• Different apps need different models

PART 1:

RDF Semantics and Ambiguity

This section examines some consequences of standard

RDF semantics.

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“Interpretations” in RDF Semantics

• An interpretation maps URIs to resources

http://example/#plum

http://example/#apple

http://example/#pear

http://example/#banana

http://example/#orange

InterpretationURIs Resources

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An interpretation applied to a single URI

• An interpretation maps that URI to one resource– Associates the name with a particular resource

http://example/#apple

InterpretationURI Resource

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Multiple interpretations

• RDF semantics does not constrain a graph to a

unique interpretation

• Different interpretations may map the same URI to

different resources

http://example/#apple

InterpretationsURIs Resources

i3

i2

i1

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Many interpretations

• There may be many interpretations– Potentially infinite

http://example/#apple

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RDF semantics constrains the possible interpretations for a given graph

• For a given graph, RDF semantics constrains the

possible interpretations

http://example/#apple

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Adding assertions reduces the set of possible interpretations

• By merging RDF graphs, constraints of both graphs

must be satisfied

http://example/#apple

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“Interpretations of a URI”

• For a given graph, “Interpretations of a URI” == The set of resources from applying all possible interpretations to that URI

http://example/#apple

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Resource ambiguity

• For a given RDF graph, a URI's resource is ambiguous if there exists

more than one possible interpretation for that URI– I.e., the possible interpretations map that URI to more than one resource

• Referent of a URI is almost always ambiguous!– But that's okay – it's just life

http://example/#apple

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Interpretations of different URIs may overlap

• URIs X and Y may map to some of the same

resources

Interpretations of X Interpretations of Y

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Effect of owl:sameAs

• X owl:sameAs Y

• Limits the interpretations for X and Y to the intersection

X owl:sameAs Y

Interpretations of X Interpretations of Y

PART 2:

Constraining Ambiguity through

URI Declarations

This section proposes a standard way to constrain

resource ambiguity.

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URI Declarations

• A URI declaration provides a definition for a

resource denoted by a URI– See “URI Declaration in Semantic Web Architecture”

• Definition is provided by a set of core assertions

• Core assertions constrain the possible

interpretations for the URI

• URI declaration should be provided via the URI's

follow-your-nose location– See “Cool URIs for the Semantic Web”

Why URI Declarations?

• Easy to know what definition to use– Dereference the URI to find its URI declaration (usually)

• Permits all users of the URI share the same

definition– Stablizes meaning / Avoids semantic drift

• Resource ambiguity is precisely bounded– Interpretations can still vary within bounds

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Bounding the interpretations of a URI X

• URI declaration bounds the interpretations of URI X

• Use of X in graph A further limits the possible

interpretations

Interp. of Xin graph A

Interpretations of X consistentwith X's URI declaration

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Interpretations of a URI X in different graphs

• Same URI may have different possible interpretations in

different graphs– E.g., URI X is used in graphs A and B

• All are within the bounds of the X's URI declaration

• When graphs are merged, the possible interpretations for

X are limited to the intersection

In graph A In graph BIn A+B

In URI declaration

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In X's URI declaration

Inconsistent combined graphs

• URI X is used in graphs A, B and C– Graph A+B is consistent

– Graph B+C is consistent

• Graph A+C (or A+B+C) is inconsistent: no possible

interpretations

In AIn B

In C

In A+B In B+C

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In X's URI declaration

Splitting identities

• Use of A+C (or A+B+C) together requires splitting X's identity,

e.g.:– Mint new URI Xab to replace X in graph A to make A'

– Mint new URI Xbc to replace X in graph C to make C'

– Then merge graph A' with graph C'

• See http://dbooth.org/2007/splitting/

X in AX in B X in C

Xabin A+B

Xbcin B+C

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Trade-off: precision versus reusability

• Broader URI declaration:– Permits the URI to be used in more applications

– Causes more down-stream contradictions, when the

URI is re-used in other graphs and those graphs are

later combined

• Narrower URI declaration:– Restricts the URI to few applications

– Reduces likelihood of downstream contradictions

• Recommendation:– Choose the degree of precision that will best attract the

community of applications that you wish to attract

– See also discussion of “clumping” in

http://dbooth.org/2007/uri-decl/20100615.htm#clumping

PART 3:

Determining Resource Identity

This section proposes a standard process for determining

resource identity.

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Determining resource identity

1. Select assertions – what graph?

1.a. Recursively merge ontologies and URI declarations– Ontologies and URI declarations should be cached!

2. Apply RDF semantics

• Constrains the possible interpretations for each URI

3. Select an interpretation

RDF Semantics only defines step 2!

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3. Select aninterpretation

Resource identity with RDF semantics

1. Selectassertions

2. ApplyRDF semantics

Available assertions

Possible interpretations

e.g. <http://example#apple> ...

Informalassertions

e.g. rdf:comment " ... " .

Formalassertions

1.a. Get ontologies& URI declarations

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Semantic extensions

• Define additional entailment rules and constraints– E.g., OWL or FruitOnt

• Must be monotonic– All previous entailments still hold

• Further limit the set of possible interpretations

• Typically triggered by a predicate URI

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Resource identity under semantic extensions

1. Select assertions – what graph?

1.a. Recursively merge ontologies and URI declarations– Ontologies and URI declarations should be cached!

2. Apply RDF semantics + semantic extensions

• Predicate URI triggers the use of semantic extensions:– Opaque plug-in, or

– Set of rules

3. Select an interpretation

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Resource identity under semantic extensions

1. Selectassertions

3. Select aninterpretation

2. ApplyRDF+extensionsemantics

Available assertions

Possible interpretations

e.g. <http://example#apple> ...

Informalassertions

e.g. rdf:comment " ... " .

Formalassertions

Semantic extensionse.g. OWL, FruitOnt

1.a. Get ontologies& URI declarations

Summary

• Part 0: Myths about resource identity

• Part 1: RDF semantics and ambiguity– Interpretations

– Interpretations of a URI

• Part 2: Constraining ambiguity through URI

declarations– Bounding ambiguity

– Ambiguity and owl:sameAs

• Part 3: Determining resource identity– Semantic extensions

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Questions?

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BACKUP SLIDES

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Splitting URI X resource identity

• What if you really want to combine graphs A+B+C?

• URI X may be split into two URIs, e.g.:

• In graph AB = A+B, change all X to X1

• In graph BC = B+C, change all X to X2

In graph A In graph B In graph C

In A+B In B+C

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X owl:sameAs Y

Interpretations of X Interpretations of Y

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X owl:sameAs Y

Interpretations of X Interpretations of Y

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42

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Effect of owl:sameAs

• X owl:sameAs Y

• Each URI has a set of possible interpretations

• owl:sameAs limits this set to the intersection

Interpretations for X Interpretations for Y

Brief Description

• This presentation shows how ambiguity fits within

standard RDF semantics, explains how it relates to

owl:sameAs, and proposes a standard operational

sequence for determining the referent of a URI.

Abstract

• What does a URI denote? How should its referent be determined, even in

the presence of semantic extensions that affect the interpretation of an RDF

graph? How should ambiguity be viewed?

• One view is that a given URI has no fixed referent, but may denote different

things in different contexts. Another is that each URI should have a URI

declaration that precisely delimits its interpretation. Some suggest reusing

existing URIs in new contexts, while others prefer to mint new URIs and

then allow owl:sameAs assertions to indicate that two URIs denote the same

thing.

• This presentation sheds light on these issues by explaining how ambiguity

of a URI's referent fits within standard RDF semantics, how this ambiguity

applies to the use of owl:sameAs, and proposes a standard operational

sequence for determining the intended referent of a URI, even in the the

presence of semantic extensions.