I agree with the sentiment here of not putting limitations on the user-defined schema. However, I believe the example we are looking at does not add any problem that does not already exist in XML Schema when XPath expressions are referring to it. For example, consider this XSD Fragment. Same as before without any DFDL annotations or "hidden stuff" <xs:element name="root"> <xs:complexType> <xs:sequence> <xs:element name="repeats" type="xs:integer"/> <xs:element name="testElement" type="xs:integer " minOccurs=?0? maxOccurs=?unbounded?/> <xs:group ref="groupFromOtherSchemaFile"/> </xs:complexType> </xs:element> Now consider an XPath for manipulating this document "/root/repeats". Perhaps I am using it in an XSLT stylesheet. Now, what happens in my stylesheet depends on what the definition of the group "groupFromOtherSchemaFile" is. That is, this XSD is legal regardless of whether that group contains an element named "repeats". But my stylesheet could be broken if it was expecting there to be only one such element and more than one is returned. My point is this: anything that couples an XSD with XPaths can have this kind of dependency where the meaning of the paths might not be respected. DFDL is not the only thing that bundles XPaths with XSD. The new XSLT (2.0 ?) specification, reportedly allows the XSD for the input and the XSD for the output to be bundled with the stylesheet. If the XSD were to include an externally defined group as above, then the stylesheet can be broken even though the XSDs are bundled. This is why I think it is simply ok for our hidden layer example to simply fail. Because anything with Xpaths and XSD coupled can have this class of failure. ...mikeb Mike Beckerle STSM, Architect, Scalable Computing IBM Software Group Information Integration Solutions Westborough, MA 01581 voice and FAX 508-599-7148 home/mobile office 508-915-4767 Suman Kalia <kalia@ca.ibm.com> Sent by: owner-dfdl-wg@ggf.org 01/19/2006 11:52 AM To Mike Beckerle/Worcester/IBM@IBMUS cc dfdl-wg@ggf.org, owner-dfdl-wg@ggf.org 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-dfdl-wg@ggf.org 01/19/2006 10:48 AM To "Robert E. McGrath" <mcgrath@ncsa.uiuc.edu> cc dfdl-wg@ggf.org, owner-dfdl-wg@ggf.org 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-dfdl-wg@ggf.org 01/19/2006 10:00 AM To dfdl-wg@ggf.org 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-dfdl-wg@ggf.org 01/19/2006 04:43 AM
To "Westhead, Martin (Martin)" <westhead@avaya.com> cc dfdl-wg@ggf.org, owner-dfdl-wg@ggf.org 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, England Internet: smh@uk.ibm.com Phone (+44)/(0) 1962-815848
"Westhead, Martin (Martin)" <westhead@avaya.c To
om> <dfdl-wg@ggf.org> 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. National Center for Supercomputing Applications University of Illinois, Urbana-Champaign 1205 West Clark Urbana, Illinois 61801 (217)-333-6549 mcgrath@ncsa.uiuc.edu