RDF increasingly becomes the preferred model for metadata on the
Web. It thus provoked me to do a bit of research into the RDF
and RDF/XML compatibility of the Metalink XML format.
Regarding the previous discussion of using RDF (and Dublin Core)
I was able to locate [1], my understanding is that the point
isn't about making Metalink XML compatible with RDF/XML, but
rather about defining a mapping, which can be used to “import”
Metalink metadata into RDF-based models.
As shown below, the most valuable elements of Metalink XML
cannot be parsed with an RDF/XML parser, which is somewhat
unfortunate.
Dublin Core is less of a problem. Apparently, the “best current
practice” for an RDF-based representation is to define its own
vocabulary of predicates, while also specifying how these
predicates relate to the similar ones in other vocabularies,
including Dublin Core. Such relations may be described in OWL.
Now, into the technicalities…
The subsection 2.13 of the RDF/XML specification [2] reads:
It is common for RDF graphs to have rdf:type predicates from subject
nodes. These are conventionally called typed nodes in the graph, or
typed node elements in the RDF/XML. RDF/XML allows this triple to
be expressed more concisely. by replacing the rdf:Description node
element name with the namespaced-element corresponding to the RDF
URI reference of the value of the type relationship. There may, of
course, be multiple rdf:type predicates but only one can be used in
this way, the others must remain as property elements or property
attributes.
This way, some Metalink documents may be read as RDF/XML
documents, provided that the following two superficial changes
are made:
• the top-level ‘metalink’ element be replaced with ‘rdf:RDF’;
• all the attributes are given namespace prefixes.
Consider, e. g.:
$ cat < example.metalink
<?xml version="1.0" ?>
<rdf:RDF
xmlns="urn:ietf:params:xml:ns:metalink"
xmlns:mtl="urn:ietf:params:xml:ns:metalink"
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";>
<file mtl:name="example.ext">
<size>14471447</size>
<url>ftp://ftp.example.com/example.ext</url>
<url>http://example.com/example.ext</url>
</file>
</rdf:RDF>
$ rdfproc example-metalink \
parse example.metalink rdfxml
rdfproc: Parsing URI file:... with rdfxml parser
$ rdfproc example-metalink \
serialize turtle
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
[]
a <urn:ietf:params:xml:ns:metalinkfile> ;
<urn:ietf:params:xml:ns:metalinkname> "example.ext" ;
<urn:ietf:params:xml:ns:metalinksize> "14471447" ;
<urn:ietf:params:xml:ns:metalinkurl> "ftp://ftp.example.com/example.ext";,
"http://example.com/example.ext"; .
$
Note, however, that (apart from the changes shown above) RDF/XML
representation is incompatible with Metalink in that the
“objects” (RDF graph “nodes”) must be interspersed with
“properties” (RDF “arcs”), which is incompatible with, e. g.:
<file ...>
...
<metaurl
mtl:mediatype="torrent"
>http://example.com/example.ext.torrent</metaurl>
...
</file>
There, the ‘metaurl’ element is directly contained within
‘file’, and thus is considered a “proprety” by the RDF/XML
parser, while it should be a “node” in order to contain the
‘mtl:mediatype’ attribute (which would then be considered a
property by RDF/XML.)
Unfortunately, most of the valuable elements of Metalink are
specified as both having attributes, and being direct
descendants of the ‘file’ element, which makes them incompatible
with RDF/XML. In particular, these are: ‘metalink:pieces’,
‘metalink:hash’, ‘metalink:metaurl’, ‘metalink:origin’,
‘metalink:publisher’, ‘metalink:signature’, ‘metalink:url’.
While this particular issue could be solved by providing a
separate Metalink XML parser to the RDF tool in question (or by
providing an XSLT transformation), it isn't clear as to whether
the elements above should name (typed) RDF nodes or arcs (or
both?)
[1] http://article.gmane.org/gmane.comp.web.metalink.devel/318
[2] http://www.w3.org/TR/REC-rdf-syntax/#section-Syntax-typed-nodes
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