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<title>RDF Research notebook: On edge-labelled graphs in XML</title>
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<h1>Research notebook: On edge-labelled graphs in XML</h1>

<p>This document serves as an informal survey of XML applications
that adopt an edge-labelled graph data model similar to that used
in W3C's Resource Description Framework (<a href="/RDF/">RDF</a>).
It also points to discussion and proposals regarding improvements
to the RDF XML syntax.
Hopefully these pointers will prove useful even while the document is
incomplete.
</p>


<p>Status: very very sketchy, disorganised collection of URLs, excerpts
and commentary. This is a pile of stuff for some (possibly
non-existent) future version to flesh out. Suggestions for other related
materials welcomed (W3 folk, feel free to edit directly).
</p>

<p>Author: <a href="mailto:danbri@w3.org">dan brickley</a></p>

<h2>Overview</h2>

<p>There seems to be some consensus around the claim that RDF has a
useful data model but a problematic XML syntax. This document is an
attempt to gather together the various discussion documents and proposals
that relate this topic to the broader context of XML-based graph
serialization systems.
</p>

<p></p>

<h2>XML Graph resources</h2>

<p>@@in-progress. This is a dump of some resources / references to
collect and summarise.</p>

<p>RDF Syntax proposals are tracked in the  <a
href="/RDF/Interest/#docs">discussion documents</a> section of
the RDF Interest Group home page.
</p>


<h3>Layman et al</h3>

<p><a href="http://www.w3.org/TR/1998/NOTE-XML-data-0105/">XML-Data</a>
<a href="http://www.w3.org/Submission/1998/01/">submission</a>. Subsequent publications
in same tradition: QL'98 position paper from Layman, "<a href="
http://www.w3.org/TandS/QL/QL98/pp/microsoft-serializing.html">XML Syntax
Recommendation for Serializing Graphs of Data</a> (Dec 2nd 1998).
</p>

<p><a href="http://www.biztalk.org/Resources/canonical.asp">BizTalk white
papers: Serializing Graphs of Data in XML, Adam Bosworth, Andrew Layman,
Michael Rys</a>:
</p>
<blockquote>XML is evolving as the standard format of exchanging data
among
heterogeneous, distributed computer systems and as such is used to
represent
data of various origins in a common format. Often, this data possesses
rich structure and represents relationship among various entities. These
relationships form graphs, where the relations are directed from one
entity
to another (and may have inverses) and where there may be multiple paths
to an entity. Thus, an important goal of the encoding of this data is to
preserve the exact graph structure in the serialization to XML. The aim
of this paper is to describe a specific way to use XML to serialize graphs
of data (such as database tables and relations or nodes and edges from
directed labeled graphs) in such a way that the graph structure is
preserved
and can be reconstructed.
<address>
<a href="http://www.biztalk.org/Resources/canonical.asp">Biztalk.org
Resources:
Canonical Reference</a> Thu, 01 Jan 1970 00:00:00 GMT</address>
</blockquote>


<blockquote>The elements and attributes in an XML document often are a
representation of objects from a specific data model such as
Directed-Labelled-Graph
or Database Relations. We can annotate a schema so that a reader can
determine
the mapping from an XML document instance to an instance of the other data
model. Mapping information for a specific data model is expressed using
attributes from a namespace specific to the mapping. Each mapping system
will have its own rules. For example, mapping from XML instances to
Directed-Labelled-Graph
instances has the rule that all attributes and all elements whose names
differ from their type represent edges. However, elements without a name
distinct from the type may represent either nodes or edges, and we must
indicate which by using a role attribute in the type declaration in the
schema.
<address>
<a
href="http://www.lindamann.com/xml/XML%20Schemas%20NG%20Guide%20HTML.htm">XML
Schemas NG Guide</a> Mon, 14 Jun 1999 05:54:24 GMT</address>
</blockquote>


<h3>XML-DEV discussion</h3>

<p>xml-dev threads: <a
href="http://www.lists.ic.ac.uk/hypermail/xml-dev/xml-dev-Dec-1999/0128.html">object-oriented
serialization</a>, dec 1999.
</p>

<blockquote>I honestly feel that XML provides all the tools to do what
RDF is trying to do, without an additional syntactic layer. What is
missing
from the picture is a mechanism for modelling object structures according
to object-oriented principles, and this is why an OO schema language is
necessary. The only other thing the RDF brings to the game is that it
turns
relationships into first-class objects that can be referenced as well
<address>
<a
href="http://www.lists.ic.ac.uk/hypermail/xml-dev/xml-dev-Dec-1999/0128.html">xml-dev-Dec-1999:
Object-oriented serialization (Was Re: Some q</a> Sat, 11 Dec 1999
00:14:28
GMT</address>
</blockquote>

<blockquote>If you're interested in a collection of objects in the first
place, why should you have to see or know about XML elements and
attributes
at all? Or to put it a different way, why should people constantly have
to redo the work of extracting objects from XML, when they're all trying
to do the same thing? I think that reasonable people can argue that RDF
is not the best solution to the problem of object exchange in XML, but
I am somewhat surprised to hear people deny that the problem even exists:
there is an enormous demand for exchanging objects in XML (businesses
exchange
a lot of structured data), and it's hard work to have to figure out over
and over how to construct objects from a SAX stream or a DOM tree
especially
when programmers with XML knowledge are scarce and expensive. I have no
doubt that we need an abstract object layer on top of XML. Right now, RDF
is the best solution currently available (XMI also has its advocates),
but I'm ready to listen about anything better.
<address>
<a
href="http://www.lists.ic.ac.uk/hypermail/xml-dev/xml-dev-Dec-1999/0130.html">xml-dev-Dec-1999:
Re: Object-oriented serialization (Was Re: So</a> Fri, 03 Dec 1999
14:25:07
GMT</address>
</blockquote>


<blockquote>As a recap: There are, broadly, two approaches to serializing
a graph in XML. One is to invent a meta-grammar, a set of canonicalization
rules. That is what RDF syntax did, and what the attribute-centric and
element-centric canonical format papers do, what SOAP section eight does.
I think of this as "tunnelling the graph through XML." The other is to
allow XML documents to follow any pattern described in a schema, and
augmenting
the schema with a set of mapping rules. There appears to be significant
value to each approach. (In particular, however, I disagree with the
sometimes-asserted
claim that graphs capture the semantics of a communication while grammars
do not. Graphs are just another grammar. This makes me reluctant to
deprecate
grammars.)
<address>
<a
href="http://www.lists.ic.ac.uk/hypermail/xml-dev/xml-dev-Dec-1999/0363.html">xml-dev-Dec-1999:
RE: Object-oriented serialization (Was Re: So</a> Fri, 10 Dec 1999
23:07:19
GMT</address>
</blockquote>

<blockquote>In this vein, schematron-rdf at
http://www.ascc.net/xml/resource/schematron/schematron.html
generates RDF documents (currently with bogus XLinks, but you can
customize
it easily) based on Schematron schemas. In this case, the schema is not
converted to RDF, rather the RDF shows which assertions in the schema
apply
to each element in the instance. This is a rather different use for
schemas:
as programs for automated annotation. The thing that became immediately
clear from working on it was that RDF is good for arcs (relationships)
but grammar-based schemas largely hide these relationships (between
elements,
attributes, data) behind a few generic but superficial types: containment,
sequence, repetition. Schematron assertions now allow a "role" attribute,
for labelling classes of arcs. I think developers of other schema
languages
might also consider this kind of thing too: that the connectors between
particles of patterns (e.g., compositors in the content models in a
grammar-based
schema language) should have some role attribute (and documentation?) for
labelling their significance. For example, if element A must be follwed
by element B, to say why. The nodes that conventional schemas define (e.g.
elements and attributes) are interesting, but the arcs between them can
also be very interesting for automatic annotation using RDF.
<address>
<a
href="http://www.lists.ic.ac.uk/hypermail/xml-dev/xml-dev-Dec-1999/0296.html">xml-dev-Dec-1999:
Re: Object-oriented serialization (Was Re: So</a> Sun, 05 Dec 1999
17:58:02
GMT</address>
</blockquote>


<p>
Presentations by Michael Rys WWW8 (@@URL?)
</p>

<p>Topic-maps syntax effort -- @@TODO: egroups URL, charter, example RDF
mappingetc., emiller's msg...</p>

<p> Henrik Neilsen,  WWW9 presentation on on
RDF/SOAP. SOAP as an RDF serialization syntax:</p>

<blockquote>At the WWW9 Conference, I was exited to give a presentation
on dev day as part of the <a
href="http://www9.org/w9-devsemantic.html">Semantic
Web track</a> on SOAP serialization.
Dan has been so kind to <a
href="http://www.ilrt.bristol.ac.uk/discovery/2000/08/www9-slides/henrik/">make
the slides available</a>. <br /> <br />
The purpose
of the presentation was to explain the model behind the <a
href="http://www.w3.org/TR/SOAP/">SOAP</a> serialization
 as well as how it might be used to serialize RDF graphs. Similarly,
SOAP may be used to serialize object graphs etc. For more information on
the SOAP specification, see the W3C Note which was submitted May 8
by 11 W3C Member organizations.
<br /> <br />

There are also a set of <a
href="http://www.ilrt.bristol.ac.uk/discovery/2000/08/www9-slides/henrik/soaprdf.html">more
specific examples</a>.
<br /> <br />
 As is stated in the slides, take this as input rather than
anything else.<br /><br />
Henrik Frystyk Nielsen mailto:frystyk@microsoft.com
<address>
<a
href="http://lists.w3.org/Archives/Public/www-rdf-interest/2000Aug/0082.html">www-rdf-interest@w3.org
from August 2000: Slides from WWW9 pres</a> Tue, 22 Aug 2000 07:32:46
GMT</address>
</blockquote>


<p>Papers from Lore(l) group at stanford. Also Pensylvania work and
some of the XML query proposals. Point to discussion point in XMLQ
data model work.</p>

<p>Other XML specs: XML Schema 'edge-labelled graph' mention. XML
Infoset RDF appendix (current status?). XML-Linking RDF model (Ron
Daniel's Note draft). Context: XArc proposal, (X)HTML typed links.
Web architecture stuff.</p>

<p>RDF Syntax proposals: Sergey's strawman (and Java parser).
TimBL's strawman. EricP's syntax (and Perl parser).</p>

<p>RDF dump syntax proposal(s) on www-rdf-interest. Issues (raise
one in <a href="http://www.w3.org/2000/03/rdf-tracking/">rdf issue
list</a> on the dependencies between model + syntax.</p>


<p>Dan Connolly notes on Jigsaw's Java serialization system:
</p>
<blockquote>ntuition: RDF, SOAP, WebDAV, and Java Beans share a data model,
and should be able to share many implementation details
<address>
<a href="http://www.w3.org/2000/08/jr456">A review of Jigsaw</a> Tue, 29
Aug 2000 07:44:36 GMT</address>
</blockquote>

<p>XSLT-based screenscraping approach. Cambridge communique, online
demos, DanC stuff.</p>

<p>Annotated DTDs / schemata. Do we have any implementation
experience of this? Henry Thompson had a good presentation on this
topic (@@url??). Point into extensibility mechanism in XML Schema.
Also issue (who owns this problem?) that XML Schema constructs need
URIs.</p>
<h3>Recent Changes (CVS Log)</h3>

<pre>
$Log: Overview.html,v $
Revision 1.5  2000/09/06 19:38:22  danbri
minor tidyup, added H3s, XHTML valid.
linked from /RDF/Interest/

Revision 1.4  2000/09/06 14:14:15  danbri
added danc notes on jigsaw / rdf serialisation

Revision 1.3  2000/09/06 11:18:19  danbri
Added a bunch of excerpts from XML-DEV thread and related discussion, papers etc.

Revision 1.2  2000/09/05 18:53:45  danbri
added few links

</pre>


<address>maintained: <a href="mailto:danbri@w3.org">dan
brickley</a></address>
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