Re: I-D: How Roy would Implement URNs and URCs Today

Daniel LaLiberte (liberte@ncsa.uiuc.edu)
Sun, 9 Jul 95 20:07:47 CDT


Date: Sun, 9 Jul 95 20:07:47 CDT
From: liberte@ncsa.uiuc.edu (Daniel LaLiberte)
Message-Id: <9507100107.AA13546@void.ncsa.uiuc.edu>
To: Roy Fielding <fielding@beach.w3.org>
Cc: liberte@ncsa.uiuc.edu (Daniel LaLiberte), uri@bunyip.com
Subject: Re: I-D: How Roy would Implement URNs and URCs Today 
In-Reply-To: <199507092151.RAA14543@beach.w3.org>
 <199507092151.RAA14543@beach.w3.org>

Roy and I are in violent agreement except on one point.

Roy Fielding writes:
 > >Second, although it is valuable to consider a URC as a document (or an
 > >object, or a resource that is interacted with), it is important to
 > >acknowledge that the URC is different from the document that it is
 > >"about".  It is different not necessarily in type but in the sense of
 > >data vs metadata.  Metadata is data too, but *as metadata*, it is not
 > >the data we are referring to.  If we don't make this distinction, how
 > >can we refer to the URC itself as data?
 > 
 > By providing a toggle on the client that says "show me the metadata",
 > or a shift-alt-click thingy that prevents auto-indirection.  This is
 > only an issue for browsers.

It is also an issue for authors, as explained below.

 > >So we need to either know in advance (based on the protocol or how it
 > >is used) that what is coming back from a resolution is the data itself
 > >or that it is the metadata, or we need to be able to identify which it
 > >is similar to how MIME types are used to identify the type of the
 > >document.
 > 
 > The latter is the only (IMHO) practical way to do it, which is why
 > a urc/* major type is needed.

Are you suggesting that if you get back a MIME type of urc/*, then
what you have must be metadata?  If so, how does an author of a
document point a user at some URC as the data to be looked at
without giving the user instructions like "do the shift-alt-click
thingy on this link otherwise you won't see the URC"?  

How do you get the metadata for the URC itself, supposing there is any?

Perhaps we should have an explicit method call in the anchor
indicating that what is being referred to is the result of, say,
getting the metadata for the object.  Maybe something like:

  <A METHOD="HEAD" HREF="http://www.ncsa.uiuc.edu/">
or
  <A METHOD="META" HREF="path:/A/B/C/doc.html">

But this won't help us get the metadata for the URC unless the URC has
its own URL.  If it does, fine, but a more general scheme would let us
refer to an object by an arbitrary expression including all parameters
to method calls.  Such an identifier might be usable to cache the
data.  Here are some examples of what I am thinking.  Ignore the
syntax if you can.

  <A HREF="ilu:get(URL:http://www.ncsa.uiuc.edu/)">
  <A HREF="ilu:post(URL:http://www.ncsa.uiuc.edu/cgi-bin/search,subject=CGI)">
  <A HREF="ilu:meta(meta(URL:http://www.ncsa.uiuc.edu/))">

Daniel LaLiberte (liberte@ncsa.uiuc.edu)
National Center for Supercomputing Applications
http://union.ncsa.uiuc.edu/~liberte/