[w3c sml] [4637] Construction of EPR Reference Scheme

All,

 

This email will serve to "instigate" discussion of issue 4637 regarding
the EPR Reference Scheme.  See section 3.2.2.  (I fully expect this
"instigation" to raise controversies.)

 

The SML spec attempts to illustrate the creation of another reference
scheme (besides the URI scheme) using EPRs.  The initial version of this
scheme was rejected because of an unacceptable use of the
<wsa:ReferenceParameters> particle in order to contain information
identifying the document segment being referenced.  (The "wsa" namespace
refers to the namespace of the adopted WS-Addressing recommendation
http://www.w3.org/2005/08/addressing.)  While there are defenders of
this kind of use of ReferenceParameters (to contain resource
identification information), it is strongly rejected by W3C as
"inconsistent with the architecture of the Web".  (I have worked and
currently am working in several standards groups which have used
ReferenceParameters in this manner.  One group just decided to avoid all
talk of EPR construction; the other still adopts the identification role
of ReferenceParameters as central to its own architecture.)

 

The issue regarding the use of ReferenceParameters is sometimes
misleading represented as an issue over the opacity of
ReferenceParameters to client.  Making the issue one of opacity vs.
transparency of the ReferenceParameters misses the main issue.  The main
issue is, What is the referencing mechanism under the Web?  Where/How
should the resource identification information be incorporated into a
Web reference?  The architectural requirement of the Web is that all
such information be placed in the URL (essentially the <wsa:Address>
particle of the EPR), not spread out in other elements of the EPR.  It
follows that incorporating the identification information into the EPR
cannot be achieved by placing it in a "less" opaque element such as
<wsa:Metadata> or even an extension element.

 

To have an acceptable EPR scheme, therefore, the resource identification
information must be contained in the URL that is the value of the
<wsa:Address> element.  A case might be made for using EPRs under this
condition if the reference is to root element of the document (the URL
would essentially be the URL of the document itself).  However, we
encounter a problem in the case of referencing a segment, or fragment,
of a document.  As I understand it (although I haven't found anything
that explicitly says this), the use of fragment identifiers is
illegitimate in EPRs because they are handled by the client-servers
providing resources do not handle fragment identifiers in the
identification of the service being addressed.  On the other hand, could
the query component of the URI be used?  This seems dubious to me, since
a query must contain non-hierarchical data (which a document fragment
certainly is not).  Modifying the URL to reference a document fragment
appears to be the only solution to sustaining a robust EPR reference
scheme that is consistent with the semantics of EPRs; however neither
the query component nor fragment identifiers seems to be a possible
course to address achieving this scheme.  (Ok, I'm "instigating" here
:-).)

 

Indeed, I'm wondering what is the use-case behind the creation of a
document reference scheme using EPRs.  Typically, EPRs are provided by
services to clients so that those clients have a means (and possibly
other relevant information) to establish an active connection with those
services.  I suspect the use-case for the EPR reference scheme involves
a SML consumer that is actively going out to an information resource (in
real time), which is addressed by the EPR, to obtain some information
contained in the document that the information resource makes available.
There are protocols to accomplish this exchange, e.g.,
WS-ResourceProperties (OASIS), fragment-level "get" in WS-Management
(DMTF), and WS-ResourceTransfer (unaligned).  This exchange cannot be
accomplished simply through the EPR.  What must the EPR scheme look like
to satisfy this use-case?  We would need the EPR in order to address the
resource, but we would also need to know the message exchange protocol
and the structure of the SOAP Body (or in one case, an element of the
SOAP header - !) that specifies the query for the desired fragment of
the document.  (The query language can range from referencing a single
child of the document root to a full XPath expression depending on the
protocol.)

 

If the conclusions reached here are plausible, then I would suggest that
the minimum course is to remove the EPR reference scheme from SML and
let it be defined by a separate effort.  (Now I'm trying to be
provocative. :-).)

 

Kirk Wilson, Ph.D.
CA Inc.
Research Staff Member, CA Labs
Intellectual Property and Standards
Council of Technical Excellence
W3C Advisory Committee Representative

Tele: + 1 603 823-7146
Fax:   + 1 603 823-7148
<mailto:kirk.wilson@ca.com>