targetNamespace/import conflict

[Charles Fineman]

I've been asked to look over an integration toolkit that has a bunch of schemas to specify message format. There are a couple of strange things I noticed right off the bat and I wanted to get others' thoughts on it (disclaimer, obviously I'm no XML expert so forgive me if these are basic).

--------Exhibit A: The start of a response message schema-----------

<xsd:schema targetNamespace="http://www.QQQ.com" xmlns:QQQ="http://www.QQQ.com/datatypes" xmlns="http://www.QQQ.com" xmlns:xsd="http://www.w3.org/2001/XMLSchema" elementFormDefault="unqualified">

<xsd:import namespace="http://www.QQQ.com/datatypes" schemaLocation="../general/datatypes.xsd"/>

<xsd:import namespace="http://www.QQQ.com"
schemaLocation="../general/datatypes.xsd"/>

--------Exhibit B: The start of general/datatypes.xsd --------------

<xsd:schema targetNamespace="http://www.QQQ.com/datatypes" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://www.QQQ.com/datatypes" elementFormDefault="unqualified" attributeFormDefault="unqualified">

#1) Exhibit A seems to be following poor form by using such a generic targetNamespace (the same one is used for all their top-level schema). The convention I've seen is that the targetNamespace should be the public URL of the document.

#2) Exhibit A defines both the default and QQQ namespace to point to the same schema (namely, Exhibit B). I guess I don't have a problem with it technically (sans #3) but it does seem bizarre. Seems like a really bad idea in fact. Would you do this so you could normally refer to a name X without qualification unless there were some ambiguity with an X in the current document (and so you could refer to it as QQQ:X)?

#3) The namespace used in the 2nd import of Exhibit A does not match the targetNamespace of Exhibit B. Why would someone do this?

#4) Please confirm/dispute my interpretation of the elementForDefault and the attributeFormDefault: all these say is that I do not have to qualify elements in the document but they are still implicitly part of the default namespace for that document (and hence would be subject to the appropriate validations).

If this is all just bad form, so be it (and I may have a chance to get it fixed) but if there are plausible reasons for why one would do things this way I sure would like to hear about them.

Thanks,

Charlie Fineman

 

[C. M. Sperberg-McQueen]

--------Exhibit A: The start of a response message schema-----------

<xsd:schema targetNamespace="http://www.QQQ.com" xmlns:QQQ="http://www.QQQ.com/datatypes" xmlns="http://www.QQQ.com" xmlns:xsd="http://www.w3.org/2001/XMLSchema" elementFormDefault="unqualified">

<xsd:import namespace="http://www.QQQ.com/datatypes" schemaLocation="../general/datatypes.xsd"/>

<xsd:import namespace="http://www.QQQ.com"
schemaLocation="../general/datatypes.xsd"/>

--------Exhibit B: The start of general/datatypes.xsd --------------

<xsd:schema targetNamespace="http://www.QQQ.com/datatypes" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://www.QQQ.com/datatypes" elementFormDefault="unqualified" attributeFormDefault="unqualified">

#1) Exhibit A seems to be following poor form by using such a generic targetNamespace (the same one is used for all their top-level schema). The convention I've seen is that the targetNamespace should be the public URL of the document.

Odd, perhaps, but it doesn't seem likely to make anything harder, now or later. On the question of whether a namespace name should be dereferenceable, large numbers of electrons have been spilt; suffice to say that both sides of the question can be argued, and are.

#2) Exhibit A defines both the default and QQQ namespace to point to the same schema (namely, Exhibit B).

Well, not as you have shown it. Exhibit A and Exhibit B each define their target namespace as their default namespace. Exhibit A binds the prefix 'QQQ' to the URL "http://www.QQQ.com/datatypes", not to the default namespace (which is "http://www.QQQ.com").

And (pedantry alert) Exhibit B is, I assume, a document available via some URI. A namespace is an abstraction named by a URI, and should not be confused with a document.

#3) The namespace used in the 2nd import of Exhibit A does not match the targetNamespace of Exhibit B. Why would someone do this?

Good question.

Perhaps they want to test the conformance of the schema processor: as far as I can tell, the second import statement violates constraint src-import.1.1 (" If the namespace [attribute] is present [on the import element], then its actual value must not match the actual value of the enclosing <schema>'s targetNamespace [attribute]").

As it happens, Xerces J agrees with me: its error message is

[Error] a.xsd:11:38: src-import.1.1: The namespace attribute 'http://www.QQQ.com' of an <import> element information item must not be the same as the targetNamespace of the schema it exists in.

To my surprise, XSV doesn't seem to catch this problem and accepts the schema documents.

#4) Please confirm/dispute my interpretation of the
elementForDefault and the attributeFormDefault: all these say is that I do not have to qualify elements in the document but they are still implicitly part of the default namespace for that document (and hence would be subject to the appropriate validations).

Well, I think you're close but I'm not sure I understand exactly what you mean.

If the elementFormDefault and attributeFormDefault attributes are both set to "unqualified", it means that local elements and local attributes are defined without any namespace affiliation. They will be referred to in a document instance by unqualified names (i.e. unprefixed names in an environment without a default namespace), and occurrences of them in the document instance in the relevant contexts will be validated against the relevant declarations or definitions in the schema -- so far, I think this is what you mean by "I do not have to qualify elements in the document" and by being "subject to the appropriate validations".

But they are not implicitly, explicitly, or in any other way "in", or "part of", the target namespace of the schema document. (Or, to be painfully precise, such a conclusion is not licensed by either the namespaces Rec or the XML Schema spec.) Their only link to the target namespace is that they are declared as local to a type defined in the target namespace.

I hope this helps.

-C. M. Sperberg-McQueen

 

[Charles Fineman]

Thanks for the response... only just now noticed it

Well, not as you have shown it. Exhibit A and Exhibit B each define their target namespace as their default namespace. Exhibit A binds the prefix 'QQQ' to the URL "http://www.QQQ.com/datatypes", not to the default namespace (which is "http://www.QQQ.com").

And (pedantry alert) Exhibit B is, I assume, a document available via some URI. A namespace is an abstraction named by a URI, and should not be confused with a document.

Ah but this was the source of my confusion. In this case, they specify that the document in exhibit B as the definition of both the default and QQQ namespace. I just don't see the value in doing this.

Well, I think you're close but I'm not sure I understand exactly what you mean.

If the elementFormDefault and attributeFormDefault attributes are both set to "unqualified", it means that local elements and local attributes are defined without any namespace affiliation. They will be referred to in a document instance by unqualified names (i.e. unprefixed names in an environment without a default namespace), and occurrences of them in the document instance in the relevant contexts will be validated against the relevant declarations or definitions in the schema -- so far, I think this is what you mean by "I do not have to qualify elements in the document" and by being "subject to the appropriate validations".

But they are not implicitly, explicitly, or in any other way "in", or "part of", the target namespace of the schema document. (Or, to be painfully precise, such a conclusion is not licensed by either the namespaces Rec or the XML Schema spec.) Their only link to the target namespace is that they are declared as local to a type defined in the target namespace.

Thanks... the distinction is clear and makes sense. I have questions about ambiguity rolling around in my head though... what happens in complex types when sub-elements come from different name spaces. For example, can we ever run into problems with things like:

<a:A>
<b:B>
<c:C>
<value> ... </value>
</c:C>
</b:B>
</a:A>

If value is declared in multiple namespaces?

Thanks again for the response.