Title: Schema composition questions




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Hello,



I hope that it is appropriate to post a link to a thread in xmlschema-dev, where I could not get a response (other than that of Paul Kiel, who tried kind to answer from a schema author's point of view).



I have some questions which are related to schema composition. I read the XML Schema recommendation and was not able to understand the desired correct behavior in some cases.



I hope that some of the experts here can help me by shedding some light on this. I need the perspective from an implementation of an XML processor rather than the one of a schema author (although that perspective can surely give some insights as well): I'm trying to understand how it is expected to work, not design patterns that work around incompatibility issues.



The original questions were asked here: http://lists.w3.org/Archives/Public/xmlschema-dev/2007Jul/0001.html



Some more examples for "what I mean" are listed in my response to Paul Kiel below.



Thanks for your help



Shlomo.



-----Original Message-----

From: xmlschema-dev-request@w... on behalf of Shlomo Yona

Sent: Sun 7/8/2007 10:41 AM

To: Paul Kiel; xmlschema-dev@w...

Subject: RE: schema composition questions







Hello,





[This is related to my 1st question]

>>Paul: this is certainly legal and useful. It is called "namespace

>>coercion" because you are coercing the schema B into the namespace A.  But

>>if the nesting gets more complicated it sometimes is not exactly

>>interoperable across implementations.  We ran into trouble using this some

>>time ago.



I think that I understand what happens in a trivial case such as this:

A (tns "A") includes B (no tns)



All the top level names in B are now in the namespace that A declares as its target namespace (tns "A").



What I don't understand is the case where

A (tns "A") includes B (no tns) which imports C (tns "C").

Is this a legal situation?

What is the fully qualified name of top level names from C in the composed schema document?

I would argue that the following possibilities should be considered (I am not sure that I listed all possibilities and am not sure at all which is the correct or desired behavior):

* This is illegal and the whole schema processing fails

* A include B import C fails (not all top level names in the composed B and C can be coerced into the target namespace of A) and the result is only A include B (ignoring the import of C into B)

* The composed schema is only A itself due to failure of coercing all the names in the composed B and C schema documents

* The composed schema document includes top level names in A under the target namespace "A" + top level names in B under the target namespace "A" + top level names in C under the target namespace "C".



So... what is the correct or desired behavior?







[this is related to my 2nd question]

>>Paul:  The ordering is not significant here.



Is it always the case that the order of processing xsd:import and xsd:include in a schema document is insignificant? Or are there examples where order matters?





[This is related to my 3rd question]

>>Paul: This is where interoperability can be a problem.

>>I would suggest that you keep it to one no-ns schema being

>>included into one ns schema.

>>If you have multiple levels of includes and multiple levels

>>of "coercion" then tools can interpret that differently.

>>To be frank, we ran into problems with namespace coercion

>> and decided to abandon it altogether. 



This is exactly my question. I'm asking this from an XML processor implementation point of view and not from a schema author "best practice" point of view. I want to implement the "right thing" and the correct behavior. My problem is that I do not understand from the recommendation what is exactly the correct behavior.



[This is related to my 4th question]

>>Paul: There is not a problem with circular includes per se.

>>Namespaces aside, the spec says that duplicative includes/imports

>>should be ignored.

>>So just being circular is not an error.

>>Now with namespaces, you are better off avoiding this kind of

>> behaviour because tools may interpret it differently.



Again, I want to implement the correct behavior into my XML processor. I'm not asking this as a schema author. Can you point out the proper way to handle such cases? Where in the recommendation is this issue being discussed and how is it suggested to process a set of XML schema documents that have circular dependencies in the general case?



[This is related to my 5th question]

>>Paul: not sure I understand here.

>>If it is about coercion of the namespace of the any, then I refer

>>to earlier comments.

>>If it is about what other options are available for namespace

>>declaration of any, then there are options such as "##other"

>>for specifying a different namespace, "##any" for any namespace,

>>and there are some others too.  The spec lists them.



I was probably not clear. Let me try with an example:



A (tns "A") includes B (no tns) which includes C (no tns) which imports A.

C contains: <xsd:any namespace="A"/>.

To which top level names does this wildcard refer to?

Only those listed in the A schema document?

Perhaps those listed in B too?

Perhaps those in A, B and C?



I can give other examples where this is complicated for me to understand, for example:



A (tns "A") includes AA (tns "A") and also includes B (no tns) which includes C (no tns).

C contains: <xsd:any namespace="A"/>.

But to which top level names does this wildcard refer to?

What if AA contains <xsd:any namespace="A"/>?

To which top level names does this wildcard refer to?



Of course, there are plenty more other examples to cook up which are, at least for me, similarly unclear.







I hope that you and the other experts here can help me out.



Thanks.



Shlomo.