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Re: On subsetting XML...

From: <noah_mendelsohn@us.ibm.com>
Date: Thu, 16 Jan 2003 16:34:43 -0500
To: Norman Walsh <Norman.Walsh@sun.com>
Cc: www-tag@w3.org
Message-ID: <OF0560110B.CEA11245-ON85256CB0.0074E399@lotus.com>

Norm Walsh writes regarding PI's inSOAP:

>> It is. But they are not explicitly forbidden, only very strongly 
deprecated.


Actually, I disagree with that.  The Candidate Rec says:  [1] 

      "SOAP provides a distributed processing 
        model that assumes a SOAP message originates 
        at an initial SOAP sender and is sent to an 
        ultimate SOAP receiver via zero or more SOAP 
        intermediaries."

and then as has been previously quoted at [2]:

        "SOAP messages sent by initial SOAP senders 
         MUST NOT contain processing instruction information 
         items. SOAP intermediaries MUST NOT insert processing 
         instruction information items in SOAP messages they 
         relay. SOAP receivers receiving a SOAP message containing 
         a processing instruction information item SHOULD generate 
         a SOAP fault with the Value of Code set to "env:Sender". 
         However, in the case where performance considerations 
         make it impractical for an intermediary to detect 
         processing instruction information items in a message 
         to be relayed, the intermediary MAY leave such processing 
         instruction information items unchanged in the relayed 
         message."

This is followed by an Infoset-level description of a SOAP message, and 
that description does not allow for PIs.

Put together these say:

"SOAP message Infosets do not contain PIs.  All SOAP messages originate at 
an original sender, which MUST NOT put processing instructions in the 
message.  All modifications to messages occur at intermediaries, which 
MUST NOT put processing instructions in the message."

End of story.  PIs are never legal in SOAP messages.  They're disallowed 
by the format description (infoset), and they are specifically disallowed 
at the only places that SOAP messages can originate or be modified.

The last part of the quote may be paraphrased as:

"A receiver may find that it receives an erroneous message containing a 
PI.  Receivers SHOULD report such errors if at all practical, but we 
recognize that such error detection may not in all cases be the best 
tradeoff.  Accordingly, when absolutely necessary the erroneous message 
MAY be propagated down the line."

I don't think it's quite fair to characterize this as saying they are not 
forbidden but only deprecated.

Also: this is obviously controversial, but I don't see where it says in 
the XML recommendation that every application of XML must use every 
feature.  Surely there are applications that use only elements, and will 
fault at the application level if confronted with an attribute.  SOAP is 
such an application.  It makes use of some attributes, but not of PIs. 
SOAP applications MUST NOT put PIs in messages, just as certain 
applications MUST NOT put attributes in their XML.   SOAP is an 
application of XML, not a redefinition of it. 

I think the major practical implication is that SOAP messages are somewhat 
limited in their ability to carry arbitrary XML documents fragments as 
Body or Header data.  XML is sufficiently broken in its ability to nest 
such arbitrary XML (conflicting entity definitions and inability to send 
nested DOCTYPE, for example) that we never could have achieved general 
container support in any case.  As it happens, the semantics of a PI in a 
SOAP body would be very questionable.  Does it apply only to the body, or 
to the SOAP message as a whole?  PI's aren't well scoped to the XML 
tree...they just sort of float in the document.  SOAP processing is tied 
very deeply to the tree structure of XML.  Accordingly, it is somewhat 
difficult to provide stable semantics for PIs floating around in SOAP 
messages.  That was among the reasons that I was one of those who endorsed 
the current design.  As I say:  we're an application of XML, and PIs did 
not meet our needs.  Thank you.


[1] http://www.w3.org/TR/2002/CR-soap12-part1-20021219/#msgexchngmdl
[2] http://www.w3.org/TR/2002/CR-soap12-part1-20021219/#soapenv



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Noah Mendelsohn                              Voice: 1-617-693-4036
IBM Corporation                                Fax: 1-617-693-8676
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Received on Thursday, 16 January 2003 16:35:51 GMT

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