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- To: Mark Feblowitz <mfeblowitz@frictionless.com>
- Subject: Re: [xml-dev] Schema Namespace name, schemaLocation, and Schema V ersioning
- From: Jeni Tennison <jeni@jenitennison.com>
- Date: Wed, 17 Jul 2002 19:19:56 +0100
- Cc: "'CHIUSANO, Joseph'" <JCHIUSANO@lmi.org>, "'Xml-Dev (E-mail)'" <xml-dev@lists.xml.org>, "Duane Krahn (E-mail)" <duane.krahn@irista.com>, "Satish Ramanathan (E-mail)" <Satish.Ramanathan@mro.com>, "Andrew Warren (E-mail)" <awarren@openapplications.org>, "Kurt A Kanaskie (Kurt) (E-mail)" <kkanaskie@lucent.com>, "Michael Rowell (E-mail)" <mrowell@openapplications.org>
- In-reply-to: <4DBDB4044ABED31183C000508BA0E97F040ABD98@fcpostal.frictionless.com>
- Organization: Jeni Tennison Consulting Ltd
- References: <4DBDB4044ABED31183C000508BA0E97F040ABD98@fcpostal.frictionless.com>
- Reply-to: Jeni Tennison <jeni@jenitennison.com>
Hi Mark,
> I also appreciate your comments on the impacts of markup changes on
> XSL (and, for that matter, any application). That is a potentially
> serious concern when altering the markup - that all XPath
> expressions would have to be reviewed and updated. In a very
> general, very extensible substrate vocabulary, changes to the markup
> are few and far between, and breakage would be rare (XSL for
> manipulating/formatting .xsd files would be very stable for a very
> long time ;-). For the rest of us, markup changes are harder to
> avoid. We try to reduce the likelihood of labor-intensive breakage
> by minimizing wholesale restructuring and keeping primarily to
> additions/extensions. At least in these cases, we can leave much of
> the XPath untouched.
I think you've misunderstood me a little. I was trying to say that
*if* you change the namespace every time you change the version of a
markup language, *and* you want the stylesheet to keep on working with
older versions of the markup language then *as well as* any changes
which might arise because of the changes brought about because of the
changes to the markup language, you would have to change every XPath
in the stylesheet to cater to the change in the namespace.
Let me give you an example. Say you start off with the document:
<doc xmlns="http://www.example.com/doc/1.0";>Hello World!</doc>
and you have the stylesheet:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
xmlns:doc="http://www.example.com/doc/1.0";>
<xsl:template match="/">
<xsl:apply-templates select="doc:doc" />
</xsl:template>
<xsl:template match="doc:doc">
<html>
<head><title>Test</title></head>
<body><xsl:value-of select="." /></body>
</html>
</xsl:template>
</xsl:stylesheet>
Now let's say that our markup language changes its namespace:
<doc xmlns="http://www.example.com/doc/1.1";>Hello World!</doc>
Even without actual changes in the allowed content of the doc element,
to cope with both documents properly, a stylesheet should look like:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
xmlns:doc.1.0="http://www.example.com/doc/1.0";
xmlns:doc.1.1="http://www.example.com/doc/1.1";>
<xsl:template match="/">
<xsl:apply-templates select="doc.1.0:doc | doc.1.1:doc" />
</xsl:template>
<xsl:template match="doc.1.0:doc | doc.1.1:doc">
<html>
<head><title>Test</title></head>
<body><xsl:value-of select="." /></body>
</html>
</xsl:template>
</xsl:stylesheet>
(There are other ways of doing it but they're all uglier.)
These are simple paths. If you had a path like:
/doc:doc/doc:chapter/doc:section
things start getting so horrible that I don't even want to write it
because it would take too long.
So my point was about the effect that changing the namespace of
elements has on XSLT/XPath processing, not about the effect of
changing markup languages on XSLT/XPath processing. Of course when you
change a markup language the applications that process it have to
change; how much or how little depends on the extent of the changes
and the way the application is designed to cope with the unexpected.
But discounting those kinds of changes, making changes to a
*namespace* (and requiring that old versions are still supported) can
have huge and horrible consequences for a stylesheet, or any
namespace-aware application.
> In our case, the "proper version" approach that you suggest would
> likely cause breakage for our users. Any time a new version was
> promoted to the proper version, anyone who didn't know about and
> prepare for the change would experience breakage.
I think that depends on how you specify your markup language and what
rules you lay down about how applications should deal with documents
that are using an old (or new) version of the markup language. For
example, XSLT has a rule similar to "if you encounter an element in
the XSLT namespace, but you don't recognise it, and the current
version (as specified by a version attribute) is greater than the one
that you support, then ignore the element". XSLT 2.0 also has a rule
like "if the version of the document is less than the most recent one
known to the application, then go into a backwards-compatible mode and
treat the document just as you did before".
If you have those kinds of rule for applications that support your
markup language, then an application should treat:
<doc xmlns="http://www.example.com/doc";
version="1.0">Hello World!</doc>
in the same way whatever version an application currently supports,
and whatever version is "current".
There is an issue here to do with the schema to which these documents
point, because if they use a single central schema location and the
schema changes then of course the document could become invalid. I
think that this demonstrates that schema locations should be tied to
markup language versions. For example:
<doc xmlns="http://www.example.com/doc";
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
xsi:schemaLocation="http://www.example.com/doc
http://www.example.com/doc/2.3.1/doc.xsd";
version="2.3.1">Hello World!</doc>
> To recover (without doing a full upgrade), they'd have to point at
> an older version *and* they have to alter their namespace
> references, to reflect that they're using an older version.
As I suggested, a version attribute (or similar) to indicate the
version of the markup language being used in a particular document is
essential if you're not using the namespace name to reflect the
version of the markup language in use. With that scheme, old documents
would not have to be touched -- they already state which version of
the markup language they comply to, and the namespace remains the same
across versions, so there's no need to change that.
It sounds as though my suggestion about having different namespaces
for documents adhering to draft specifications wouldn't be appropriate
in your case because from your description there isn't a cycle of
draft/discuss/accept in the markup language development. If there's no
difference between the "latest" version and the "stable" version, then
I don't think there's a need for different namespaces for different
versions at all.
> And the "art" here is in assessing what part the validating parsers
> play in accepting/rejecting version differences. Remove version
> information from the namespace name and the validator plays no
> direct role, at least when namespace names are compared. Add the
> version information and the validator plays a very direct role,
> rejecting mismatches and requiring interchangers to a priori agree
> on version.
I agree absolutely that you can use validating parsers to ensure that
documents comply to a particular version of a markup language, but I
don't agree that this has to be done through the namespace name. For
example, the schema for:
<doc xmlns="http://www.example.com/doc";
version="2.3.1">Hello World!</doc>
might be:
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema";
targetNamespace="http://www.example.com/doc";>
<xs:element name="doc">
<xs:complexType>
<xs:extension base="xs:string">
<xs:attribute name="version" use="required" fixed="2.3.1" />
</xs:extension>
</xs:complexType>
</xs:element>
</xs:schema>
If you tried validating the document:
<doc xmlns="http://www.example.com/doc";
version="5.2.6">Hello World!</doc>
against this schema, you would get a very specific error telling you
that the version of the document does not match the version of the
schema.
Cheers,
Jeni
---
Jeni Tennison
http://www.jenitennison.com/
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