[XML-SIG] <url> checked and response attributes

[Fred L. Drake]

Marc van Grootel writes:
 > I suggested them in the very beginning because I thought they
 > would be useful (not because it is tracked by MSIE - maybe it does
 > but, does it?).

  Good question; 

 > Attributes for storing the status of a link is still a good idea
 > but I didn't give the choice of attributes for supporting it

  I agree that storing this kind of information is potentially very
useful, especially if there's any software that can use it.  I think a 
link-update monitor could very easily use such information.  I am
concerned about adding several "untested" attributes to a primary
element in the hope that someone will actually write enough software
that uses it (more than one app.).  We might want to drop the
attributes from <url> and create a <metadata> profile for it.

 > BTW what about the contents of the scheme and  language attributes? I
 > don't know the Dublin Core but I understand you got it from there?
 > Could you give an example of an info element?

  Good idea.
  Metadata about objects (including documents) is typically given as a 
set of key/value pairs.  The keys are usually just strings (like RFC
822 headers), and values may be strings (possibly constrained by the
definition of the bit of metadata, i.e., it may be boolean, or numeric, 
or a date), or it may be structured in some way (XML, SGML, or
whatever).
  The catch with metadata is to understand what it means (no AI here,
though).  To "understand" metadata, you need to understand what schema 
it conforms to.  As an example, consider a library's cataloging
system.  To understand what a catalog number means, you need to know
what kind of number was assigned: Dewey Decimal, U.S. Library of
Congress (LOC), or something else.  Since there's no reason not to
assign catalog numbers for both Dewey Decimal and LOC use, an <info>
for a book (with completely made up numbers; I don't remember either
system well enough) might look like this:

	<info>
	  <metadata scheme="Library of Congress">
	    <meta name="catalog number">TR567 A45.1</meta>
	    </metadata>
	  <metadata scheme="Dewey Decimal">
	    <meta name="catalog number">Z567 12</meta>
	    </metadata>
	  </info>

  The Dublin Core is a specific metadata system; particular bits of
data about a resource are defined and given identifying keys.  It is
being used on the Web by some projects and the working group has dealt 
with issues related to embedding in HTML as well as semantics.  An
example of storing Dublin Core data in XBEL:

	<info>
	  <metadata scheme="http://purl.oclc.org/metadata/dublin_core/"
		    lang="en">
	    <meta name="Creator">Fred L. Drake, Jr.
	      and Roger E. Masse</meta>
	    <meta name="Publisher">Corporation for National
	      Research Initiatives</meta>
	    <meta name="Description">Python interface to the
	      Kerberos V5 security package.</meta>
	    <meta name="Identifier">
	      URN:hdl:1895.22/1001</meta>
	    <meta name="Identifier">
	      URL:ftp://ftp.python.org/pub/python/contrib/System/krb5module-0.1.tar.gz</meta>
	    </metadata>
	  </info>

  Adding the scheme and lang attributes to <metadata> seems to make
the most sense; typically, several metadata items from a single scheme 
will be used, with natural language text in a single language.
  More information on the Dublin Core is available at
<http://purl.oclc.org/metadata/dublin_core/>.  I'll try to include
some useful examples and links in the XBEL documentation.


  -Fred

--
Fred L. Drake, Jr.	     <fdrake@acm.org>
Corporation for National Research Initiatives
1895 Preston White Dr.	    Reston, VA  20191