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Subject: RDF/XML and XPath
Svante,
I'll go through as many problems as I can point out with thinking that
XPath is feasible with just a few variations on the RDF/XML serializations
of an RDF model.
During the call I heard you mentioned "walking up the tree" looking for the
top most @rdf:about. I just want to point out how that itself wouldn't work
using examples from the RDF/XML specification.
I'm going to assume that to find out the subject of ex:fullName you'd go up
and look for @rdf:about which points to
<http://www.w3.org/TR/rdf-syntax-grammar>
<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
xmlns:dc="http://purl.org/dc/elements/1.1/";
xmlns:ex="http://example.org/stuff/1.0/";>
<rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar";
dc:title="RDF/XML Syntax Specification (Revised)">
<ex:editor ex:fullName="Dave Beckett" />
</rdf:Description>
</rdf:RDF>
In N3:
<http://www.w3.org/TR/rdf-syntax-grammar> dc:title "..." ;
ex:editor [
ex:fullName "Dave Beckett" .
] .
Meaning that ex:fullName "Dave Beckett" is a property on an anonymous node
and NOT on the <http://www.w3.org/TR/rdf-syntax-grammar> itself. There's an
extra node in the graph w/o a name, so you can't assume the "highest"
rdf:about is the one that pertains. RDF/XML is trying to represent a graph,
not a tree, like HTML. RDFa takes advantage of the tree properties of HTML
to define an extraction mechanism but only of triples than then together
will make up a graph.
Let's take another variation on the previous one.
<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
xmlns:dc="http://purl.org/dc/elements/1.1/";
xmlns:ex="http://example.org/stuff/1.0/";
xml:base="http://www.w3.org/TR/";>
<rdf:Description rdf:about="rdf-syntax-grammar"
dc:title="RDF/XML Syntax Specification (Revised)">
<ex:editor ex:fullName="Dave Beckett" />
</rdf:Description>
</rdf:RDF>
Notice how I've added xml:base to the document. Now your XPath would break
because you were matching for the full URI.
Another short variation....
<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
xmlns:dc="http://purl.org/dc/elements/1.1/";
xmlns:ex="http://example.org/stuff/1.0/";
xml:base="http://www.w3.org/";>
<rdf:Description xml:base="TR/" rdf:about="rdf-syntax-grammar"
dc:title="RDF/XML Syntax Specification (Revised)">
<ex:editor ex:fullName="Dave Beckett" />
</rdf:Description>
</rdf:RDF>
Notice yet another xml:base declaration in rdf:Description. Remember, all
of these serializations generate an isomorphic RDF model.
Also note how I have not yet even used RDF/XML vs RDF/ABBREV, completely
throwing out the argument of using a "simplified/constrained" RDF/XML
serialization.
Here's another example that shows the love/hate relationship between XPath
and RDF.
Notice how ex:fullName property is expressed using an attribute and then in
the second version, it's done using an element.
<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
xmlns:dc="http://purl.org/dc/elements/1.1/";
xmlns:ex="http://example.org/stuff/1.0/";>
<rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar";
dc:title="RDF/XML Syntax Specification (Revised)">
<ex:editor>
<rdf:Description ex:fullName="Dave Beckett">
<ex:homePage rdf:resource="http://purl.org/net/dajobe/"; />
</rdf:Description>
</ex:editor>
</rdf:Description>
</rdf:RDF>
<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
xmlns:dc="http://purl.org/dc/elements/1.1/";
xmlns:ex="http://example.org/stuff/1.0/";>
<rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar";
dc:title="RDF/XML Syntax Specification (Revised)">
<ex:editor rdf:parseType="Resource">
<ex:fullName>Dave Beckett</ex:fullName>
<ex:homePage rdf:resource="http://purl.org/net/dajobe/"/>
</ex:editor>
</rdf:Description>
</rdf:RDF>
Here's another one showing a completely different structure. Notice the
channel1 and channel2 references.
<?xml version="1.0"?>
<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
xmlns="http://testuri.org#";>
<tv-guide>
<channels>
<rdf:List>
<rdf:li>
<channel rdf:ID="channel1"/>
</rdf:li>
<rdf:li>
<channel rdf:ID="channel2"/>
</rdf:li>
</rdf:List>
</channels>
</tv-guide>
</rdf:RDF>
<?xml version="1.0"?>
<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
xmlns="http://testuri.org#";>
<tv-guide>
<channels>
<rdf:List>
<rdf:li rdf:resource="#channel1"/>
<rdf:li rdf:resource="#channel2"/>
</rdf:List>
</channels>
</tv-guide>
<channel rdf:ID="channel1"/>
<channel rdf:ID="channel2"/>
</rdf:RDF>
Here's one showing long and abbreviated format examples:
<?xml version="1.0" encoding="iso-8859-1"?>
<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
xmlns:dc="http://purl.org/dc/elements/1.0/";>
<rdf:Description rdf:about="http://www.mcgill.ca/libraries-techserv/";
dc:title="McGill University Libraries Library Technical Services Home
Page"
dc:creator="Karen Jensen"
dc:subject="..."
dc:description="..."
dc:publisher="McGill University Libraries. Library Technical
Services."
dc:contributor="..."
dc:date="2003-03-03"
dc:type="Text"
dc:format="text/html"
dc:identifier="http://www.mcgill.ca/libraries-techserv/";
dc:language="en"/>
</rdf:RDF>
and ... not rdf:Description and again everything went from attributes to
elements.
<rdf:RDF xmlns:dc="http://purl.org/dc/elements/1.0/";
xmlns:log="http://www.w3.org/2000/10/swap/log#";
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";>
<Text xmlns="file:/tmp/"
rdf:about="http://www.mcgill.ca/libraries-techserv/";>
<dc:contributor>...</dc:contributor>
<dc:creator>Karen Jensen</dc:creator>
<dc:date>2003-03-03</dc:date>
<dc:description>...</dc:description>
<dc:format>text/html</dc:format>
<dc:identifier>http://www.mcgill.ca/libraries-techserv/</dc:identifier>
<dc:language>en</dc:language>
<dc:publisher>McGill University Libraries. Library Technical
Services.</dc:publisher>
<dc:subject>...</dc:subject>
<dc:title>McGill University Libraries Library Technical Services
Home Page</dc:title>
</Text>
</rdf:RDF>
I hope these examples help you see why using XPath is just not a good idea
to link between ODF and RDF/XML serializations of our metadata.
-Elias
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