[jsdl-wg] Naming elements

[Karl Czajkowski]

On Apr 07, Yuri Demchenko loaded a tape reading:
> Sorry, I can not understand most of comments and statements here.
> 
> Maybe explanation how most of these changes can improve understanding
> or schema definition would help.
> 

I am trying to answer and reiterate some of the many questions below
w/ inline comments...


> Michel Drescher wrote:
...
> >    A JobDescription does not necessarily has to be referencable, since 
> >it is usually a unique association between other elements that, in this 
> >configuration, serve a unique purpose. It is adequatically specified 
> >through the document's namespace.
> >    Similar jobs (who have a different namespace) may want to use 
> >exactly the same set of resources or the same application etc. Most
> 
> Sorry for my ignorance, why and when this may happen that "Similar
> jobs (who have a different namespace)"?
> 
> Completely different? Can it be solved by using imported namespaces
> and extension mechanism?

Michel is not talking about namespaces for schema, but for instance
documents.  He intends to mark up elements in an instance document
with NCName attributes and refer to them in a global context by
qualifying those names w/ the target namespace of the instance
document within which the name is introduced.

Michel means that an instance document may want to reference
subdocuments such as application, data, etc., from another instance
document. These documents may exist in different target namespaces.

The terminology around XML and XSD is unfortunate for people often
being ambiguous as to whether they mean "some XML element instance" or
"an XSD element declaration" when they say the word "element"...


> What does it mean referenceable?
> 
> IMHO, you can reference with ID/Id attribute or with XPath or with
> xpointer any legitimate element in the XML document.
> 
> Do you mean referencing schema elements/types?
> 

As I hope I clarified above, Michel means "named by an instance-unique
NCName in a well-defined target namespace" when he says
"referenceable".

I agree that the motivation for this complicated discussion seems
weak, as there are existing structural reference mechanisms like XPath
which work without special support in the instance schema.


> >- JobDescription defines local elements(!) with the same name (for
> >  applications, resource sets and data staging) and each gets an
> >  optional attribute named "ref" of type "xsd:QName" to refer to
> >  already defined elements (either in the same document, or externally).
> >- These locally defined elements allow exactly the same appropriate
> >  groups of elements as the elements sibling to JobDescription.
> >- Elements specified in JobDescription's local elements override
> >  elements with the same name in referred elements.
> 
> Sorry I can not understand how/why some elements are qualified as
> local and what is the incentive of using global vs local elements?
> 
> If you are talking about definitions like
> 
> <xsd:element name="Description" type="jsdl:description" minOccurs="0"/>
> 
> this I can understand.
> 

I think Michel made an unfortunate choice of words... I _think_ he is
resurrecting the Profile mechanism which was removed at GGF-13, by
suggesting how an instance document would reference another (using the
naming mechanism described above) and then "override" some of its
content by providing its own content with the same names.  In essence,
he is trying to define a semantics for JSDL documents based on rewrite
rules for merging the referenced document and the referencing document
to yield a new result document that validates to the JSDL schema but
no longer has the external references.


> >The following proposals are of more concern to schema hackers and are 
> >related to technical issues of design and "taste". They can easily be 
> >skipped by others although they may touch higher level concerns:
> >
> >- I further propose to follow one of the two scenarios:
> >  a) define exactly one global element ("JobDefinition")
> >     so that any valid JSDL document always has "JobDefinition"
> >     as root element
> >  b) define a limited number of global elements, i.e.
> >     JobDefinition, JobDescription, Resource (rename to "ResourceSet"?)
> >     Application and DataStaging so that any valid JSDL document
> >     may have any of these globally defined elements as root element
> >
> How above is related to the schema attributes elementFormDefault and
> AttributeFormDefault?
> 
> What is the reason of limiting number of global elements? What is the
> benefits of having local elements?
> 
> Does it help implementation/programming?
> 

I also question the goal of limiting the number of global elements. I
think exposing most of the JSDL concepts (the job ontology) as global
elements is a good thing, to encourage others to reuse the syntax
instead of copying their own when they decide we were silly for trying
to control the use of our definitions.  I think the only justification
for local element declarations in a standard like this is when the
elements are not meaningful outside the context of their defined use.
For example, it makes sense to me if the children of a JSDL range type
would not be global elements, because the range boundary is not really
meaningdul outside the context of the range.


> >- Any other elements and types should be locally defined. Or do we
> >  want to allow i.e. "jsdl:NetworkBandwidth" to be root element?
> >

I don't see any reason not to make any of our resource terms global
elements.  I think we must purge from our minds the idea that there is
a "JSDL document".  There is a JSDL Specification which defines a
family of documents such as jsdl:JobDefinition.  If you want to talk
about a JSDL document, you need to define a "JSDL" element!  Every XML
element is a document, once you embrace the universe as an
infoset. :-)


karl

> >- Use xsd:group to group elements together and refer to these groups
> >  instead of repeating element definitions
> >
> There should be some reason to group some elements except referencing
> them together. How many these cases of repeated definitions you have?
> 
> Sorry I am really confused.
> 
> Yuri
> 

-- 
Karl Czajkowski
karlcz at univa.com