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RE: What is a SOAP Message

From: <noah_mendelsohn@us.ibm.com>
Date: Thu, 13 Feb 2003 10:04:09 -0500
To: "David Orchard" <dorchard@bea.com>
Cc: xml-dist-app@w3.org
Message-ID: <OFA539B5AD.E727CE84-ON85256CCC.00526AB5@lotus.com>

David Orchard quotes an exchange with Roy Fielding:

> DO: Can representations be transferred over protocols
> other than http, say ftp?
> 
> RF:Yes. Any shared memory by message passing system is
> dealing in representations, though most are constrained
> to a particular implementation.

So FTP >can< transfer a representation, at least in
Roy's usage.  Here are the questions I would follow
with:

Q.  Is every piece of data carried by FTP a representation?
A (proposed by Noah):  I think if we answer "yes", then
we are greatly diluting the interesting characteristics
of representation as defined in REST.  So my preferred
answer would be "no"

I believe Roy states in his thesis that REST is not the
only appropriate or useful model for distributed
computation, just an interesting and important one.

Thus,  I think we need two different terms.
One needs to be specific to REST, and to carry all the
connotations that go with that.  If necessary, we can
have a broader one that refers to all load/store
transfers.  Confusing them is dangerous, IMO.

 I think that many SOAP activites will,
for better or worse, fall outside of the REST model.  I
think these convey "representations" only in the trivial
and uninteresting sense that all messages are conveying
some sort of data.  Unless you model the resources and
constrain the protocol, as REST does, I see little
value in adding a fancy name to the general case.  I
find it confusing to use the same name (representation)
that has a rather specific connotation in REST.

So, I would prefer to reserve the term "representation"
to refer to a set of data (typically a MIME-typed 
stream) that conveys some aspect of the state
of a named Resource (I think I'd prefer to 
restrict this specifically to resources identified
by URIs, though clearly similar systems could
be built in other ways), and only in the case
where access to such resources is modeled
using the structures of REST (which is not
necessarily HTTP, but necessarily builds
on a view of GET, PUT, POST of
representations as the means of manipulating
and accessing a resource.)

I find the simultaneous use of the term resource
in this narrow sense and in a broader (e.g.
FTP of a named file) sense to be more
confusing than helpful.

If you buy this narrow definition, then I claim
that SOAP does not in all cases transfer
a representation.  Neither, for that matter,
does a typical HTTP GET message, though
that surely conveys some bits of state 
that represent something or other at the
sender.

------------------------------------------------------------------
Noah Mendelsohn                              Voice: 1-617-693-4036
IBM Corporation                                Fax: 1-617-693-8676
One Rogers Street
Cambridge, MA 02142
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Received on Thursday, 13 February 2003 10:06:14 GMT

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