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RE: Xpath transform changes and questions

From: Jonathan Marsh <jmarsh@microsoft.com>
Date: Fri, 17 Mar 2000 15:05:38 -0800
Message-ID: <5F68209F7E4BD111A5F500805FFE35B91D3FDE74@RED-MSG-54>
To: "'John Boyer'" <jboyer@PureEdge.com>, TAMURA Kent <kent@trl.ibm.co.jp>, IETF/W3C XML-DSig WG <w3c-ietf-xmldsig@w3.org>
Cc: "Martin J. Duerst" <duerst@w3.org>, "Christopher R. Maden" <crism@exemplary.net>, James Clark <jjc@jclark.com>

> -----Original Message-----
> From: John Boyer [mailto:jboyer@PureEdge.com]
> Sent: Friday, March 17, 2000 12:48 PM
> Cc: Jonathan Marsh; Martin J. Duerst; Christopher R. Maden; 
> James Clark
> Subject: Xpath transform changes and questions
> Hi,
> I have a few concerns that I think can be worked out, so I am 
> requesting
> feedback on the information below.  If everything works out, 
> then yes, we
> could remove parse() and exact order.
> 1) On parse(),
> I would be in favor of dumping parse() if we can solve ALL of the
> implementation problems it solves.  It  solved some 
> interesting, but not too
> important problems for XPath transform users, but it also 
> specified how
> implementers were to solve certain problems.
> Please have a closer look at the output expectations of 
> serialize().  The
> serialize() function cannot operate without several features 
> of parse().  In
> particular,
> i) Serialization of the root node requires that we output the 
> byte order
> mark and xmldecl read by parse() on input.  If parse() is not 
> under our
> control, we cannot specify that it retains this information.

It seems useful that the BOM and encoding are preserved through
re-serializing the document.  If these are inputs to the serialization
mechanism, this is added incentive not to expose the serialization mechanism
as a function.  Otherwise these would have to be passed through as variables
(as you are doing) and users must not forget to use them, and must use them
appropriately (e.g. not change them), if they want correct results.

If serialization is not exposed as a function, but is performed
automatically, these problems are avoided.  Since it seems that this
capability already exists (nodesets are automatically serialized), at least
the simple cases are already handled.  Thus an explicit serialize() and BOM
and encoding variables seems to be an advanced feature.  It's necessity in
solving important problems should be weighed against its potential for
abuse.  I haven't seen you justify your design yet.  Mainly I'm curious
here, not trying to kill serialize().  Of course, if you can't justify it,
drop it.

> This would
> seem to suggest that root node serialization should result in 
> the empty
> string, which in turn suggests that serialize should output in UTF-8
> regardless of the input encoding.  That would be OK with me.
> ii) Attribute and namespace serialization require a namespace 
> prefix.  Based
> on a new read of XPath I believe this information must be 
> available, but I
> want to be sure.

Note in XPath that Namepace Nodes are different in quantity and position
than the attributes used to declare namespaces.  Your current serialization
does not take this into account.

On the other hand, serialization does not necessarily have to be limited to
the XPath Data Model.  This model of a document is used when locating the
nodes, but given a document and a set of locations, your serializer can
describe what to do on it's own terms.  Specifically, any namespace
attributes in the source are copied through unchanged, and namepace
attributes are added to elements taken out of scope (e.g. parents trimmed)
to represent the namespace nodes of that element.  Retain the prefixes in
all cases - otherwise QNames in content (e.g. XPaths) will break under the
transformation.  Maybe the experts will have some comments on this idea...

> iii) If everything else checks out, we can get rid of exact 
> order and just
> use lex order provided that lex ordering in UTF-16 results in 
> the same order
> as lex ordering in UTF-8 (which is Christopher Maden's claim).
> Also, parse() has an additional feature that would need to be 
> dealt with in
> some other way:
> iv) If the parser used to implement parse() is 
> non-validating, then parse()
> is required to throw an exception if it encounters an 
> external reference
> that would cause it to interpret the document differently 
> than a validating
> parser.  This exception is necessary since an unverifiable 
> signature is
> different than an invalid signature.

I don't see that moving parsing out of parse() changes this.  The
restriction still applies, and needs to be stated.  An implementation would
either need to initialize their parser to provide such an exception during
parsing, or do some post-parsing pre-filtering checks.

> 2) On eliminating exprBOM and exprEncoding.
> Sounds fine.  Sounds like any difference of encoding between the Xpath
> expression and the transform input will be handled implicitly 
> by the XPath
> transform implementation.
> 3) On automatic serialization,
> There was some concern that serialization should be automatic 
> ("why call
> serialize() when that's always what we want to do").  Please 
> see the first
> paragraph of section, which already includes this feature.

The question perhaps is better stated as "why do we need an explicit way to
serialize, instead of always relying on the default that nodesets are

For one thing, this design deeply entwines serialization with pruning of the
tree, which makes it difficult to use off-the-shelf serialization components
in an implementation.

For another, I would not in general expect XPath implementations to handle
large strings such serialize() generates particularly efficiently, since
such strings are at this point pretty rare.  For example, I can't really
imagine returning strings from XPath asynchronously, which I would expect
from a serializing component.

> Thus if we remove parse(), then there is no *need* to start 
> expressions with
> a function call.
> 4) On providing an initial namespace context
> We can provide an initial namespace context as is done in XPointer.
> I was reviewing how XPath handles namespaces, and realized that it is
> different than what I had previously understood, and it seems 
> broken (so
> there must be some good reason why it is done the way its 
> done, or maybe I'm
> just misreading the spec).
> Section 2.3 says "A QName in the node test is expanded into 
> an expanded-name
> using the namespace declarations from the expression context. 
> This is the
> same way expansion is done for element type names in start 
> and end-tags
> except that the default namespace declared with xmlns is not 
> used: if the
> QName does not have a prefix, then the namespace URI is null 
> (this is the
> same way attribute names are expanded). It is an error if the 
> QName has a
> prefix for which there is no namespace declaration in the expression
> context".
> This seems to indicate that the input XML document's 
> namespace declarations
> are ignored and the expression context's namespace 
> declarations are used
> solely.

Yes.  Prefixes are scoped, and can change throughout the document, so it is
not possible to use these declarations in a global context such as an XPath.
Also, the namespace rec implies that prefixes can be changed without
changing the underlying names.  Since XPath has it's own namespace
declarations, it is unaffected by prefix changes in the source document.

> When XPath claims to be XML namespace compliant, I thought 
> that meant it
> would interpret a node's namespace in the context of the namespace
> declarations in the document, but that appears not to be 
> true.  To clear
> this up, suppose I have an element x:E, and the document 
> containing this
> element associates x with www.w3.org, but the expression 
> context associates
> x with www.ietf.org, then which value will the 
> namespace-uri() function
> applied to x:E return?

namespace-uri(x:E) will either return the namespace declared in the context
(www.ietf.org) or an empty string if no element {www.ietf.org : E) exists in
the document, which appears to be the case in your question.  In short,
XPath cannot talk about an element without knowing it's full name, including
the namespace uri.  This is an essential component to it's namespace
> If it returns www.ieft.org, then although it seems weird to me that we
> aren't using the namespace declarations in the document, it 
> would at least
> be good in the sense that it implies XPath implementations have the
> namespace prefix kicking around for serialize().
> It also would mean that Jonathan Marsh is correct in 
> requiring an initial
> namespace context since we could not do ANY namespace 
> comparisons without it
> (the XPath seems to say that it is an error to use a QName 
> containing a
> namespace prefix in an expression if that namespace prefix is 
> not defined in
> the expression context).

Yep.  This really isn't a problem for XPaths appearing in XML documents,
since there is a ready set of namespace declarations (and an existing syntax
for declaring them) to pass into XPath.  We couldn't actually make this part
of XPath because certain uses of XPath do not appear in an XML document
context - namely XPointers embedded in URIs.

I don't veiw this issue as a conceptual mistake, but as a cheap fix with
large author simplicity benefits.  It's virtually free (no new syntax
needed) - just add one line saying that the namespaces are initialized to
the namespaces in scope on the <xpath> element.
> I would appreciate your feedback, esp. from those who have 
> sent prior emails
> and therefore seem to be most interested in how this turns 
> out.  If you
> would please give this some extra priority, I will prepare an 
> alternative
> document for consideration before or during the meeting in Adelaide (I
> regret that I will not be at that meeting, but I will be at 
> the following
> one in Victoria ;-).
> John Boyer
> Software Development Manager
> PureEdge Solutions, Inc. (formerly UWI.Com)
> jboyer@PureEdge.com
> -----Original Message-----
> From: w3c-ietf-xmldsig-request@w3.org
> [mailto:w3c-ietf-xmldsig-request@w3.org]On Behalf Of TAMURA Kent
> Sent: Friday, March 17, 2000 12:59 AM
> Subject: Re: XSL WG comments on XML Signatures
> > <John>
> > XPath filtering will not be substantially rewritten.  Based 
> on Clark's
> > feedback, we can remove the parse function and instead 
> simply assert that
> > the transform input is parsed and provided to XPath as a 
> node set.  The
> > notions of lex and exact order will be removed (since we 
> cannot directly
> > specify the parse).
> That's good!  It would be easy to understand, easy to implement,
> easy to use.
> --
> TAMURA Kent @ Tokyo Research Laboratory, IBM
Received on Friday, 17 March 2000 19:43:36 UTC

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