Schema Component Designator - particle requirement

This is a request submitted on behalf of the Open Grid Forum (OGF)  
Data Format Description Language (DFDL) work group.

The purpose of a DFDL-annotated XML schema is to describe the logical  
structure and physical format of non-XML data.  A subset of XML  
Schema is used to describe the logical structure of the data, and  
DFDL-specific xsd:appinfo annotations on schema components carry  
properties that describe the physical format of the data. The result  
of parsing a stream of DFDL described data using a conformant DFDL  
parser is a DFDL info set, which is structurally similar to an XML  
info set; specifically it consists of a 'tree' of info items which  
correspond to elements in the DFDL described data.

There is a requirement to be able to link from a DFDL info item back  
to the DFDL annotated XML schema component that describes it, for the  
purposes of accessing the DFDL properties of that component.  SCD  
appears to be an ideal way of specifying that link, except for one  
problem. DFDL annotations can be carried on various schema  
components, including element references and group references.

However, the SCD specification  at http://www.w3.org/TR/xmlschema- 
ref/ states:
a) 2.1. "It is not a requirement to designate particle components as  
distinct from terms"
b) 4.2.1.2. "Particle schema components are skipped over in the  
traversal through the graph, do not contribute a step, and have no  
reflection in the path syntax. "

Because of this treatment of particles it does not appear that an  
element reference or group reference can be directly accessed using  
an SCD path. Therefore it is not possible to link from a DFDL info  
item to a DFDL annotated element reference or group reference and  
hence access its DFDL properties.

Please can you confirm that our interpretation of the SCD  
specification is correct.

If so, we would like to request an enhancement to SCD specification  
so that particles can be directly accessed using an SCD.

-----------------------------
Here's why DFDL annotations on element references and group  
references is important. This is an excerpt from draft 025 of the  
DFDL specification (http://forge.gridforum.org/sf/go/doc14764?nav=1)
Scoping of Element and Group References

The exception to the general case concerns annotations positioned on  
element references and group references. When this occurs, the  
annotations on the reference will take precedence over any top-level  
annotations on the referenced element or group. Consider the  
mechanism of substituting an element reference declaration with the  
referenced elements. If annotations are present on both the element  
reference declaration and the referenced element, they will need to  
be combined in some way. The rules of DFDL dictate that those on the  
element reference take precedence over those on the referenced element.

In the example below, the annotation on the element reference  
specifying a format encoding of ascii takes precedence over the utf-8  
format encoding of the referenced element.

<xs:element name=”title” ref="name">
   <xs:annotation>
     <xs:appinfo source=”http://www.ogf.org/dfdl/”>
       <dfdl:format applies="toScope" encoding="ascii" />
     </xs:appinfo>
   </xs:annotation>
</xs:element>

<xs:element name=”name” type="xs:string">
   <xs:annotation>
     <xs:appinfo source=”http://www.ogf.org/dfdl/”>
       <dfdl:format applies="toScope" encoding="utf-8" />
     </xs:appinfo>
   </xs:annotation>

This mechanism provides a way to establish default properties for an  
element declaration but provide optional overrides to them at the  
point of use.

-------------------------------

Steve Hanson
WebSphere Message Brokers
Hursley, UK
Internet: smh@uk.ibm.com
Phone (+44)/(0) 1962-815848





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