Re: Five mechanical approaches to make an XSD profile without getting bogged by individual issues

One other thing about the profile effort agreed at the workshop.  It was 
>not< to be a subset that certain schema processors would support as an 
alternative to full XSD 1.0.  What was decided, as I recall, was to leave 
the conformance rules for schema validators unchanged, so strongly suggest 
that databinding tools provide at least some support for arbitrary XSD 
schemas, but to identify a profile subset of schema features that the 
databinding community would agree to focus on as the prefered case.  So, 
if your schema happens to use features from the profile, you could expect 
particularly convenient databindings and optimized performance;  if you 
use other features, the databinding should work, but the API for 
applications might be less carefully crafted and performance may not have 
been optimized. 

So, I think the mandate coming out of the workshop was in fact someone 
different than the one you've been suggesting here, Rick.  So, not only 
wasn't "the main issue ignored", I don't think it's as closely aligned 
with your proposal as you're suggesting.


Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142
Sent by:
05/22/2009 12:02 PM
        To:     Rick Jelliffe <>
        cc:, Paul Downey <>, (bcc: 
Noah Mendelsohn/Cambridge/IBM)
        Subject:        Re: Five mechanical approaches to make an XSD 
profile without getting   bogged by individual issues

Rick Jelliffe writes:

> I would say that in essence XSD 1.1 ignores the main issue from
> the workshop.

Well, the W3C as a whole did not ignore that.  As you are probably aware, 
a working group [1] was chartered to identify a profile, and they tried 
[2], having produced a working draft in 2007 that appears not to have been 

updated since.  I think it's fair to say that, during the period the group 

was most active, they struggled to get a critical mass of people to 
contribute.  The Schemas working group provided detailed comments [3] on 
that draft(including a few editorial comments from me personally.) 
Frankly, I think the databindings WG fell into the trap of making a 
taxonomy of lots of little schema features, without successfully zeroing 
in on the layerings that would be meaningful to users and to achieving the 

deeper modularity that I think you're looking for.

My point is: after the workshop, the responsibility for suggesting a 
profile or modularization was given not primarily to the XSD group, but to 

the Databindings group.  It was noted by many that for all the talk about 
the importance of the profile (and I believe that done right it would have 

been very useful to the databinding community), relatively few people were 

willing to commit to do the work.  So, that modularization effort 
foundered, as best I can tell, not due to lack of followup by W3C, but by 
lack of investment by those who would have had to provide resources to do 
the work (though some organizations did invest, and those people work 
hard, for which I think everyone is grateful.)

I think it's also worth noting that in my informal discussions with people 

from the databinding community, it's not at all clear that the profiles 
they wanted were the ones that either you or I would consider interesting 
to make XSD cleaner.  As I recall, the two features that came in for the 
most regular criticism on XSD from that community were:

* mixed content
* <xsd:choice>

<xsd:choice> is, of course, the direct analog of "|" in DTDs, and mixed 
content is actually a feature of XML (Schemas got blamed because it was in 

the schema that the databinding community would become aware of the 
presence of mixed content.)  While there are many possible legitimate 
criticisms of undue complexity in XSD, I doubt that either of us considers 

"choice" or mixed content to be prime examples.

In summary: the workshop conclusion on modularity and profiles was not 
ignored, but was considered sufficiently important to merit a working 
group, which was chartered.  As David has indicated, the XSD group 
proceeded with its responsibility to add the other enhancements that were 
requested by attendees.  I don't think all of that history provides very 
good justification for delaying XSD 1.1 now.


P.S. There's a real risk that I've misrepresented some details of the 
databingins history, so I'm cc:'ing Databindings WG chair Paul Downey. 
Paul, this email is in the context of a long and somewhat fraught email 
thread resulting from a proposal from Rick Jelliffe that XSD 1.1 be held 
at the Candidate Recommendation stage while an attempt is made to factor 
the specification in a more modular way.   Before commenting, you should 
probably read the whole thread on www-tag :-(.  As a start, Rick's 
original request is at [4] and a clarification from him is at [5].  Many 
of us have also commented in that thread and perhaps others.






Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142

Rick Jelliffe <>
Sent by:
05/21/2009 12:37 AM
        cc:     (bcc: Noah Mendelsohn/Cambridge/IBM)
        Subject:        Five mechanical approaches to make an XSD profile 
without getting  bogged by individual issues

David Exellwrote:

  In essence, XML Schema 1.1 addresses the issues from the workshop.

I would say that in essence XSD 1.1 ignores the main issue from the 

When I look at the Chair's report (linked to by David) I read:

 > There was significant support for the idea of a written ‘profile’ of 
XML Schema
 > which would document the sweet spot for purposes of data binding, or 
for other
 > specific domains. The word /profile/ is problematic; what was meant 
was not a
 > language subset, but only a definition of the sweet spot in existing 
 > which would allow schema authors to get better results and better 
user experience
 > when data binding tools are used, and which would tell implementors 
in the
 > relevant domain which parts of schema users are most likely to expect 
them to
 > support well.

 > There was strong sentiment against publishing any profiles which 
would restrict
 > or reduce the XML Schema 1.0 specification, impacting existing 
 > or vocabularies.

My request for a profile does not reduce or restrict or otherwise define 
what is in
full XML Schema 1.n.

So what happened to this "significant support"?

The key is in the next paragraph:

 > There appeared to be no obvious way to split the XML Schema 
 > into layers or sub-languages, as with OWL Lite, DL and Full or SVG 
 > Basic and Full. Accordingly, there was no support for trying to 
define profiles
 > of XML Schema as part of the schema language itself. However, many 
 > saw value in application or domain specific 'profiles', in particular 
 > a set of schema patterns to provide a 'good user experience' when using
 > XML Schema 1.0 to bind XML to code or data models.

And how long was this discussion that decided that there was "no obvious 
Well, the formal discussion on this seems to have occupied 15 minutes of 
which ran out before discussions had finished. Indeed, as far as I can 
see, no
straw proposals were asked for, raised, considered or dispatched.

For my rather immoderate response to that event, see my blog item from 
the time:
Snow Season in Schemaland

So what are obvious ways? Here are five:

1) Exchange model

One of the biggest early success stories in vendor-cooperative standards 
was the OASIS CALS Exchange Table Model: now it is part of history though 
has influenced all subsequent table models since. Michael, David, Norm 
and the
other old-timers will certainly remember it. The military CALS table model
was based on going through all the tables in the archive and making a 
schema (DTD)
that could cope with them all. It supported lots of fancy things (tables 
on call-out
pages with different page size, etc). Most vendors could only support a 

So they got together, and rather than dispute each feature, they agreed 
on an algorithm:
where almost all vendors supported a feature, it would be kept and the 
would agree to support it; otherwise it would be dropped.

There are now several profiles out: the W3C databinding minimum and 
the WS-I profile, the UN profile, etc. An algorithmic approach like the 
approach could be used.

2) Modularity model

Chop the 250 page Structures plus the datatypes specs into different 
severable parts:

1) Grammars and particles
1a) Additional constraints
2) Key and uniqueness
3) Assertions
4) Built-in Datatypes
5) Schema location and assembly
6) Complex type derivation and assembly
7) Simple derivation
8) Dynamic schema constructs: xsi:nil, xsi:type, version selection

and encourage implementators to implement fully each part that they 

3) Set-based selection

1) Start with a private syntax for ISO/OASIS RELAX NG using 
XSD-namespace elements.
(Call it RELAXSD) This gives a solid theoretical basis and proven 
capabilities with little work.
2) Create an extra layer of syntax and semantic checking on RELAXSD 
(Call it XSD Lite
and Tite) to implement the appropriate rules of XSD 1.n and remove 
patterns specified
in the maximum W3C databinding note.
3) Adjust RELAXSD to remove any syntax that is removed by XSD Lite and 
Tite if necessary.
(Call this XSD Lite)

The result:
* all XSD Lite documents can be trivially converted to RELAX NG
* all XSD Lite and Tite documents are conforming XSD 1.n documents
* all XSD Lite and Tite documents are usable by XSD Lite systems.

XSD Lite would meet the needs of those for whom ambiguity is not an issue.
XSD Lite and Tite would meet the needs for those for whom ambiguity was 
an issue.
Both would be fairly equivalent to DTDs with simple types.

Neither would use the bogus complex type *derivation* apparatus, though 
they certainly
could be declared as a name binding to a complexType, and they could be 
and used in a full XSD 1.n system that had complex typing.

4) Resolved schemas

Many of the features of XSD are syntactic sugar. They may be useful for 
modelling, but they
do not actually add any expressive power. And they come at a heavy cost.

A resolved schema would be one in which a full XSD 1.n schema had been 
re-written to
remove syntactic sugar (such as element substitution and complex type 
derivation by extension,)
and modeling items (such as complex type derivation by restriction and 
abstract elements.)

In fact, this is how the RELAX NG specification is written: first a 
transform to resolve
the sugar and then formal description of the remaining core. It is also 
how I implemented
my XSD validator, which converts to Schematron.

5) Schema versus Instance validation

When implementing XSD it becomes obvious that there are two very 
different kinds of constraints
involved. They can be seen starkly in the test suite: some tests require 
an instance, some do not.

The specification could be refactored into two parts:

1) Validation that the XSD schema is correct
2) Validation that an instance is correct against the XSD schema.

For example, my implementation largely assumes that the schema is 
correct. This represents a major
simplification in the work involved.

For example, I suggest that of the implementers who require UPA, there 
are many who would
prefer (and perhaps don't) check the schema for UPA and just rely on 
runtime violations if any.


6) Implementation caused

Create a profile which removes any features that have been shown to have 
implementation problems. The W3C databinding profiles are relevant, 
though the
metric is not "what has been implemented?" but "what has been implemented
badly/with difficulty/wrongly/abandoned? I.e. some features may be 
because of mismatch with an underlying model: this is no reason to ditch 
under this method. But a feature that perhaps was needed and missed the 
could be ditched. (This is more like my original suggestion in my 
submission to
the W3C Workshop.)

I think this method is now superceded by events and information and does 
need to be considered.

Each of these 5 methods would allow a mechanical 
of the standard.

They obviously each have their pros and cons (I would be happy with any 
of them). And the
shape of the final profile would be pretty much determined and knowable 
upfront by the
mechanism chosen: the judgements are not matters of expertise or 

I would like to note that I do not believe the XSD WG has ever called for
submissions on how to refactor or profile the spec. So I don't believe 
that they
have indeed addressed a main issue from the W3C Workshop. In fact, the
birthing baby has gone straight into the too-hard basket without even
a slap, to mix metaphors.

Rick Jelliffe

Received on Friday, 22 May 2009 17:25:32 UTC