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RDF Twig Accessing RDF Graphs in XSLT Norman Walsh http://www.sun.com/ Extreme Markup Languages 04 - 08 August, 2003 Version 1.0 Introduction There's nothing as practical as a good theory http://www.sun.com/ 2 / 29 Introduction

SLIDE 1

RDF Twig Accessing RDF Graphs in XSLT

Version 1.0

http://www.sun.com/

Norman Walsh

Extreme Markup Languages 04 - 08 August, 2003

SLIDE 2

“There's nothing as practical as a good theory”

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Introduction

SLIDE 3

“There's nothing as practical as a good theory.” “There's nothing as theoretical as good practice.”

3 / 29 http://www.sun.com/

Introduction

SLIDE 4

“There's nothing as practical as a good theory.” “There's nothing as theoretical as good practice.” “Sometimes a practical solution is good enough.”

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Introduction

SLIDE 5

RDF is a useful way to store and process information that

fits into the RDF paradigm.

Lots of information does fit into that paradigm.
RDF can be serialized in XML.
XSLT is a useful way to process XML.

But...

Processing RDF with XSLT is difficult and tedious.

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Observations

SLIDE 6

XSLT (and XPath) are designed to operate on XML documents.

XML documents are trees.

A collection of RDF statements is a directed graph, but it is

not generally a tree.

Templates designed to transform RDF often stumble over

this missmatch at the data model level.

But RDF has an XML serialization, doesn't it?

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The Problem

SLIDE 7

Nodes in a tree have only one parent.
Nodes in a graph may have several “parents”.
If node identity is to be preserved:
Nodes must be treated in two different ways.
It boils down to: instantiate once.
Reference elsewhere.

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What about RDF Serialization?

SLIDE 8

Consider this small graph: And how it might be serialized: <A> <B> <C> ??? What do you do about A?

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Serialization Example

SLIDE 9

<A node="n1"> <B node="n2"> <C node="n3"> <A node="n1"/> <E node="n4"> ...

Graph: B/C/A/* = B
Tree: B/C/A/* = empty node set

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Serialization Example (Continued)

SLIDE 10

Know your serialization tool. There are several flavors and

recent RDF changes introduce at least one more.

Use keys and conditional logic in your templates to identify

and correctly process nodes that are inline and nodes that are referenced.

In the general case, you need a choose statement for each

node, one to test for @rdf:resource and one to test for @rdf:about. Difficult and tedious.

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Working With Serialized RDF

SLIDE 11

There's no single, right way to do the serialization.

Any node could be the “root” of the tree.
Nodes must be instantiated exactly once.
Which nodes are “new” and which are “duplicates” depends
n where you start and how you build the tree.

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There's More Than One Way To Do It

SLIDE 12

Let's you start at any node in the RDF graph.
Builds a serialized representation of that part of the graph

(with a few user-tuneable parameters).

Returns the tree as a document so that you can apply XSLT

to it. In short, RDF Twig lets you serialize interesting parts of the graph on the fly.

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RDF Twig

SLIDE 13

RDF Twig is implemented as a set of (Java) XSLT extension

functions and elements.

The current implementation is built on top of the Jena RDF

toolkit.

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RDF Twig Implementation

SLIDE 14

Consider this graph: How can this be serialized (starting at A)?

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How to Serialize

SLIDE 15

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Serialize Breadth First

SLIDE 16

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Serialize Depth First

SLIDE 17

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Serialize Breadth First Deep

SLIDE 18

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Serialize a Leaf

SLIDE 19

twig A shallow breadth-first tree. dftwig A shallow depth-first tree. branch A deep tree. leaf A “tree” with no instantiated children.

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RDF Twig Terminology

SLIDE 20

Load the model: <xsl:variable name="model" select="rt:load('diagrams/bgraph.rdf')"/> Grab a node: <xsl:variable name="A" select="rt:resource('http://uri/for/A')"/> Turn the results into a tree: <xsl:variable name="tree" select="rt:twig($A)"/>

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RDF Twig in Action

SLIDE 21

At this point, $tree contains an XML document that can be queried and transformed with XSLT like any other input docu- ment.

RDF Twig in Action (Continued)

SLIDE 22

Construct a property: <xsl:variable name="label" select="rt:property('http://example.com/graph#', 'label')"/> Find some nodes: <xsl:variable name="findResults" select="rt:find($label, 'D')"/> Turn the results into a tree: <xsl:variable name="tree" select="rt:twig($findResults)/twig:result"/>

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RDF Twig in Action

SLIDE 23

load() (and store()) RDF graphs.
resource() gets (or creates) a single resource.
property() gets (or creates) a property.
twig(), dftwig(), branch(), leaf() get parts of a

graph.

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RDF Twig Functions

SLIDE 24

find() finds resources (that have a property).
get() finds resources (that are a property).
filter(), filterNot() trim a set of resources.
union(), intersection(), difference() perform

the obvious boolean operations on sets of resources.

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RDF Twig Functions (Continued)

SLIDE 25

RDF Twig now supports RDQL: <xsl:variable name="a"> <rq:rdql return="a"> SELECT ?a, ?b WHERE (?a, <http://somewhere/pred1>, ?b) AND ?b < 5 </rq:rdql> </xsl:variable> This is a result tree, so you need a node-set extension to ac- cess it.

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RDQL Support

SLIDE 26

Wouldn't it be better to extend XPath (XSLT?) to operate over graphs? Yes, probably. But RDF Twig satisfies an immediate need: to access RDF graphs in XSLT stylesheets today.

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Isn't There a Better Way?

SLIDE 27

<xsl:variable name="contactType" select="rt:resource('http://nwalsh.com/rdf/palm#Contact')"/> <xsl:variable name="allContacts" select="rt:twig(rt:find($rdf:type, $contactType),1)/twig:result"/> ... <xsl:for-each select="$allContacts"> <xsl:apply-templates select="rt:leaf(string(@rdf:about))" mode="Contact"/> </xsl:for-each>

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A “Real” Example

SLIDE 28

Deep trees can be prohibitively large.
“Serialize on the fly” is conceptually different.
Trying to build trees that are “just big enough” sometimes

introduces the inline/reference problem again.

Function dispatch oddness in the current implementation.

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Warts

SLIDE 29

RDF Twig: http://rdftwig.sourceforge.net/
Saxon: http://saxon.sourceforge.net/
Xalan Java: http://xml.apache.org/xalan-j/
Jena: http://www.hpl.hp.com/semweb/jena

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RDF Twig Accessing RDF Graphs in XSLT

Norman Walsh

“There's nothing as practical as a good theory”

Introduction

“There's nothing as practical as a good theory.” “There's nothing as theoretical as good practice.”

Introduction

“There's nothing as practical as a good theory.” “There's nothing as theoretical as good practice.” “Sometimes a practical solution is good enough.”

Introduction

fits into the RDF paradigm.

But...

Observations

XML documents are trees.

not generally a tree.

this missmatch at the data model level.

The Problem

What about RDF Serialization?

Consider this small graph: And how it might be serialized: <A> <B> <C> ??? What do you do about A?

Serialization Example

<A node="n1"> <B node="n2"> <C node="n3"> <A node="n1"/> <E node="n4"> ...

Serialization Example (Continued)

recent RDF changes introduce at least one more.

and correctly process nodes that are inline and nodes that are referenced.

node, one to test for @rdf:resource and one to test for @rdf:about. Difficult and tedious.

Working With Serialized RDF

There's no single, right way to do the serialization.

There's More Than One Way To Do It

(with a few user-tuneable parameters).

to it. In short, RDF Twig lets you serialize interesting parts of the graph on the fly.

RDF Twig

functions and elements.

toolkit.

RDF Twig Implementation

Consider this graph: How can this be serialized (starting at A)?

How to Serialize

Serialize Breadth First

Serialize Depth First

Serialize Breadth First Deep

Serialize a Leaf

twig A shallow breadth-first tree. dftwig A shallow depth-first tree. branch A deep tree. leaf A “tree” with no instantiated children.

RDF Twig Terminology

Load the model: <xsl:variable name="model" select="rt:load('diagrams/bgraph.rdf')"/> Grab a node: <xsl:variable name="A" select="rt:resource('http://uri/for/A')"/> Turn the results into a tree: <xsl:variable name="tree" select="rt:twig($A)"/>

RDF Twig in Action

At this point, $tree contains an XML document that can be queried and transformed with XSLT like any other input docu- ment.

RDF Twig in Action (Continued)

Construct a property: <xsl:variable name="label" select="rt:property('http://example.com/graph#', 'label')"/> Find some nodes: <xsl:variable name="findResults" select="rt:find($label, 'D')"/> Turn the results into a tree: <xsl:variable name="tree" select="rt:twig($findResults)/twig:result"/>

RDF Twig in Action

graph.

RDF Twig Functions

the obvious boolean operations on sets of resources.

RDF Twig Functions (Continued)

RDF Twig now supports RDQL: <xsl:variable name="a"> <rq:rdql return="a"> SELECT ?a, ?b WHERE (?a, &lt;http://somewhere/pred1&gt;, ?b) AND ?b < 5 </rq:rdql> </xsl:variable> This is a result tree, so you need a node-set extension to ac- cess it.

RDQL Support

Wouldn't it be better to extend XPath (XSLT?) to operate over graphs? Yes, probably. But RDF Twig satisfies an immediate need: to access RDF graphs in XSLT stylesheets today.

Isn't There a Better Way?

A “Real” Example

introduces the inline/reference problem again.

Warts

References

RDF Twig now supports RDQL: <xsl:variable name="a"> <rq:rdql return="a"> SELECT ?a, ?b WHERE (?a, <http://somewhere/pred1>, ?b) AND ?b < 5 </rq:rdql> </xsl:variable> This is a result tree, so you need a node-set extension to ac- cess it.