Perhaps there is a discussion here for
later, but with hidden elements I don’t think there’s much of an
issue here.
My take on this is that we are using the
XSD to do more than it is supposed to, we are not just using it to describe
valid XML we are using it constructively to describe how to build an XML
instance. We are establishing a semantics for how the DFDL parser constructs
this XML document from the XSD and the data.
Now, it is easy to imagine cases where you
have a particular xml model you want to populate which is similar to the data
but is perhaps a subset with slightly different ordering. Hidden elements make
this easy to handle. You put the stuff that is to be omitted in a hidden
element. Stuff that is out of order can be put in hidden elements and then
referenced by correctly ordered elements for value.
In this framework attributes are just
another node for data population. If it happens that you can arrange the XML
Schema so that the order in which attributes are laid out in the file corresponds
to the ordering of the data, well then its no different from elements. If there
are constraints that prevent that ordering then you are in a situation just
like the one I describe above and you solve it with hidden elements.
So I don’t really disagree with your
points below it just seems to me that there’s nothing much to worry about
here. I can’t see why we would put it off – there’s no extra
work to do?
Cheers,
Martin
From: Mike Beckerle
[mailto:beckerle@us.ibm.com]
Sent: Thursday, January 19, 2006
12:27 PM
To: Westhead, Martin (Martin)
Cc:
Subject: Re: Attributes - (was
Ambiguous XPaths to hidden elements)
Here's some of the issues with attributes:
1)
every element can have two kinds of children now, attributes and sub-elements.
Actual DFDL-described data will typically have just one kind of sub-fields of
data, but some want to be mapped to attributes, others to elements. How do you
specify how the attributes are separated from the elements. Do the attributes
have to be first, or last, or can they be interleaved with the sub-elements'
physical data?
2)
in XSD syntax, if you define a element with both sub-elements and attributes,
the attributes textually come after the sub-elements. This means you may or may
not have them in the right order for DFDL annotation to be convenient. E.g.,
suppose you had 5 children the first 2 you want to be attributes the remaining
3 sub-elements in a sequence. How can you annotate this conveniently?
3)The
attributes are always unordered w.r.t. the XSD concept, but the sub-elements
can be either ordered or un-ordered.
These
all seemed like avoidable issues. That is, I'm not saying there is no good
solution here, I'm just saying it's probably uninteresting.
One
solution to the whole issue is to say that attributes can't be in the
bottom-most layer. I.e., you create them by referencing element values in
hidden or non-hidden layers. This eliminates all the above problems.It is
equivalent to using a stylesheet to create the attributes you want from
elements-only data. However, by use of the layers you can embed it all right
into the DFDL schema.
I
also believe attributes are to some extent beside the point for DFDL. What we
needed from XSD is a standardized hierarchical type system that we can hang
represetnation properties on. We get that by using the elements-only subset of
XSD. Attributes are only needed if we broaden our agenda to include
"creating the XML I want out of my data", which layering can do.
Mike
Beckerle
STSM, Architect, Scalable Computing
IBM Software Group
Information Integration Solutions
voice and FAX 508-599-7148
home/mobile office 508-915-4767
|
"Westhead, Martin
\(Martin\)" <westhead@avaya.com> 01/19/2006 03:05 PM |
|
I think (hope) that with the emerging operational
semantics, that there will not be any problems putting annotations into
attributes and having them evaluated just like an element. It is not obvious to
me that they are any different.
Martin
From: Mike Beckerle
[mailto:beckerle@us.ibm.com]
Sent: Thursday, January 19, 2006 12:03 PM
To: Westhead, Martin (Martin)
Cc:
Subject: RE: [dfdl-wg] Ambiguous XPaths to hidden elements
I think we did not agree formally to this. It was one of those things where
we're trying to find something we can cut out with low risk of later
complications.
Mike Beckerle
STSM, Architect, Scalable Computing
IBM Software Group
Information Integration Solutions
voice and FAX 508-599-7148
home/mobile office 508-915-4767
|
"Westhead, Martin
(Martin)" <westhead@avaya.com> 01/19/2006 01:59 PM |
|
Why are we not allowing attributes?
Martin
From: owner-
Sent: Thursday, January 19, 2006 10:57 AM
To: Steve Hanson
Cc: Mike Beckerle;
Subject: Fw: [dfdl-wg] Ambiguous XPaths to hidden elements
The main problem will be performance and excessively long validation times and
either asking the user to change their schema or model it different way. These
are all undesirable. Attributes I hope will be supported in the future
release . Redefine construct is hardly used in the practical
applications; at least I haven't come across any customer that uses this
construct ..
Suman Kalia
IBM Toronto Lab
WebSphere Business Integration Application Connectivity Tools
Tel : 905-413-3923 T/L 969-3923
Fax : 905-413-4850
Internet ID : kalia@ca.ibm.com
----- Forwarded by Suman Kalia/Toronto/IBM on 01/19/2006 01:49 PM -----
|
Steve Hanson
<smh@uk.ibm.com> 01/19/2006 01:15 PM |
|
We are already putting constraints on user-defined schema, by saying that
we don't support redefines and attributes for example. I don't see an issue
with further constraints if they make DFDL easier to understand and/or
easier to create a DFDL parser.
I don't have a problem with saying that an XPath must return a single
unambiguous node else it is an error.
I don't have a problem with saying the XPaths can't reference hidden
elements, and that context must be used instead.
Regards, Steve
Steve Hanson
WebSphere Message Brokers,
IBM
Internet: smh@uk.ibm.com
Phone (+44)/(0) 1962-815848
Suman Kalia
<kalia@ca.ibm.com
>
To
Sent by:
Mike Beckerle <beckerle@us.ibm.com>
owner-dfdl-wg@ggf
cc
.org
owner-
Subject
19/01/2006 18:02
Fw: [dfdl-wg] Ambiguous XPaths to
hidden elements
Well if we go with global complex type approach (which I described option 1
in previous append) then it is not issue.. XPATH work and there are no
conflicts with user defined schemas ..
Suman Kalia
IBM Toronto Lab
WebSphere Business Integration Application Connectivity Tools
Tel : 905-413-3923 T/L 969-3923
Fax : 905-413-4850
Internet ID : kalia@ca.ibm.com
----- Forwarded by Suman Kalia/Toronto/IBM on 01/19/2006 12:59 PM -----
Mike
Beckerle/Worcester/IBM@IBMUS
To
01/19/2006 12:59 PM
Suman Kalia/Toronto/IBM@IBMCA
cc
owner-
Subject
Re: Fw:
[dfdl-wg] Ambiguous
XPaths to
hidden elementsLink
So we have a quandry here:
on one hand we don't want to change the XPath syntax to include a device
that would let us be clear that we're navigating a hidden layer
on the other hand we don't want to constrain what can be included so that
we wouldn't need such a device.
...mikeb
Mike Beckerle
STSM, Architect, Scalable Computing
IBM Software Group
Information Integration Solutions
voice and FAX 508-599-7148
home/mobile office 508-915-4767
Suman Kalia/Toronto/IBM@IBMCA
01/19/2006 11:52 AM
To
Mike
Beckerle/Worcester/IBM@IBMUS
cc
owner-
Subject
Fw: [dfdl-wg]
Ambiguous
XPaths to
hidden elements
As a design point , We should strive not to put limitations on the user
defined schemas - it just works out better in the long run.
Note the xsd:groups can be nested and they could be many levels deep and
this problem is not restricted to groups included from noTarget namespace
, it could be from any namespace. As per schema rules, all local elements
defined in groups or complex types belong to noTarget namespace unless
elementFormDefault is explicitly set to "qualified" at schema level
or on
the specific element.
Detecting such conflicts could be quite expensive particularly when you
have very large schemas. Industry standard ACORD messaging schema is a
good example it is about 1.5 M and it takes awfully long (hours) to
validate it. Putting additional constraints like this will further slow
down validation.
Suman Kalia
IBM Toronto Lab
WebSphere Business Integration Application Connectivity Tools
Tel : 905-413-3923 T/L 969-3923
Fax : 905-413-4850
Internet ID : kalia@ca.ibm.com
----- Forwarded by Suman Kalia/Toronto/IBM on 01/19/2006 11:39 AM -----
Mike Beckerle
<beckerle@us.ibm.com>
Sent by: owner-
To
"Robert E. McGrath"
01/19/2006 10:48 AM
<mcgrath@ncsa.uiuc.edu>
cc
owner-
Subject
Re:
Fw: [dfdl-wg] Ambiguous
XPaths to hidden elements
One idea that hasn't been advanced yet is ruling out the problematic case.
Let me illustrate. Here's the example, modified to have a model group
reference which can introduce the name conflict:
<xs:element name="root">
<xs:complexType>
<xs:sequence>
<xs:annotation><xs:appinfo
source=”http://dataformat.org” />
<hidden>
<xs:element name="repeats" type="xs:integer"/>
</hidden>
</xs:appinfo></xs:annotation
>
<xs:element name="testElement"
type="xs:integer " minOccurs=”0”
maxOccurs=”unbounded”
dfdl:repeatCount=”../repeats”>
<xs:group ref="groupFromOtherSchemaFile"/>
<!-- what if this has an element decl named "repeats"? -->
</xs:complexType>
</xs:element>
So, what hasn't been suggested yet is this: What if we just say DFDL
doesn't allow this? It's an error which must be detected. This DFDL schema
is broken because the path "../repeats" cannot be analyzed along with
the
DFDL schema to return only a single node.
I beleive name conflicts like this are what namespace management is for.
XSD has truly great namespace managment. You can solve the problem that
way.
Furthermore, when you define a reusable named group like the definer of the
"groupFromOtherSchemaFile" above, and you put it in no target
namespace,
that's the situation where this conflict can arise. Expecting that your
names are never going to conflict with anything in that case is just naive.
It's equivalent to having global variables in a C program module and
expecting you can never link it to something else that uses the same names.
Those name conflicts can occur, and someone has to change the conflicting
name. In XSD we can do that by including the group in a schema which puts
it into a target namespace so that after that the namespaces can be used to
disambiguate.
The approach above is consistent with the path "../repeats" still
being
officialy an "XPath", it just adds the semantic restriction that it
must be
an XPath that identifies a single node unambiguously, independent of what
data is being processed. This is one of these "data independent"
notions
(what I had previously been calling "static"), as we discussed
yesterday.
...mikeb
"Robert E. McGrath"
<mcgrath@ncsa.uiuc.edu>
Sent by: owner-
To
01/19/2006 10:00 AM
cc
Subject
Re: Fw: [dfdl-wg]
Ambiguous XPaths to hidden
elements
I would want to change XPath only as a last resort. (Any of the
options is OK by me, assuming we have to mess with the Xpath
at all.)
Can we deal with this some other way?
Can we document the problematic cases, and suggest best practices that
will minimize the problem?
On Thursday 19 January 2006 08:45, Suman Kalia wrote:
> I fully agree with Steve - let's not invent another XPATH like syntax ..
>
> Suman Kalia
> IBM Toronto Lab
> WebSphere Business Integration Application Connectivity Tools
> Tel : 905-413-3923 T/L 969-3923
> Fax : 905-413-4850
> Internet ID : kalia@ca.ibm.com
> ----- Forwarded by Suman Kalia/Toronto/IBM on 01/19/2006 09:43 AM -----
>
> Steve Hanson <smh@uk.ibm.com>
> Sent by: owner-
> 01/19/2006 04:43 AM
>
> To
> "Westhead, Martin (Martin)" <westhead@avaya.com>
> cc
>
> Subject
> Re: [dfdl-wg] Ambiguous XPaths to hidden elements
>
>
>
>
>
>
> As a DFDL parser implementor I do not want modifications to the XPath
> syntax. I want to be able to reuse existing XPath implementations. It's
> also something else for the user to have to learn. So 2a/b/c are not
> attractive.
>
> Regards, Steve
>
> Steve Hanson
> WebSphere Message Brokers,
> IBM Hursley,
> Internet: smh@uk.ibm.com
> Phone (+44)/(0) 1962-815848
>
>
>
> "Westhead, Martin
> (Martin)"
> <westhead@avaya.c
To
>
> om>
<
> Sent by:
cc
>
> owner-dfdl-wg@ggf
> .org
Subject
>
>
[dfdl-wg]
Ambiguous XPaths to
>
hidden
elements
> 18/01/2006 20:24
>
>
>
>
>
>
>
>
>
> Hi folks,
>
> This is to try to pick up on the issue identified by Suman in today?s
> call.
>
> The Issue
> Consider the following example:
>
> <xs:element name="root">
> <xs:complexType>
>
<xs:sequence>
>
<xs:annotation><xs:appinfo
> source=?http://dataformat.org? />
>
<hidden>
> <xs:element name="repeats" type="xs:integer"/>
>
</hidden>
>
> </xs:appinfo></xs:annotation >
>
<xs:element
name="testElement"
> type="xs:integer " minOccurs=?0? maxOccurs=?unbounded?
>
dfdl:repeatCount=?../repeats?>
>
</xs:complexType>
> </xs:element>
>
> The problem is that the path ?../repeats? can be broken by modifications
> to
> the logical model due to name clashes on ?repeats? and there are cases
> that
> can be constructed where this would not be obvious to a user.
>
> Possible Solutions
> Possible fixes to this include:
> 1. Disallow XPath references to hidden
elements the user is forced
> to
> place the material into the global context to refer
to it.
> 2. Provide a special XPath operator
to indicate we are referencing
> a
> hidden element, possibilities include:
> a. ?../hidden(repeats)?
> b. ?hidden(../repeats)?
> c. ?../dfdl:hidden/repeats?
> 3. Only allow hidden elements to be present in top
level global
> complex
> types. These can then be included where needed. (This
is the
> solution
> that Suman was pushing but
I don?t fully understand it ?
> in
> particular I don?t see how it resolves the ambiguity
issue.)
>
>
> I believe my preference here is 2a or 2b followed by 1.
>
> Comments/suggestions/opinions?
>
> Thanks,
>
> Martin
--
---
Robert E. McGrath, Ph.D.
1205
(217)-333-6549
mcgrath@ncsa.uiuc.edu