Simple question on XML Schema

[Nimmi Srivastav]

Greetings,

I have two newbie questions.

1) Using XML Schema, how can I describe an XML document in which the
root element is either A or B? Also, using DTD, how can the same be
achieved?

2) In an XML driven messaging system, is it a good design practice to
use XML documents with different root elements to indicate the message
version information? For example, the root element can be Ver1 or
Ver2. If the recipient receives XML content with root element Ver1,
it processes it as a version 1 message and if the recipient receives
XML content with root element Ver2, it processes it as a version 2
message. The structure of Ver1 message can be substantially different
from the structure of the Ver2 message.

Thanks,
Nimmi

 

[Martin Honnen]

1) Using XML Schema, how can I describe an XML document in which the
root element is either A or B? Also, using DTD, how can the same be
achieved?

I don't think DTD can do that.
As for a schema, any top level element you define in your schema can
serve as a root element so when you do e.g.

<xs:schema
xmlns:xs="http://www.w3.org/2001/XMLSchema">

<xs:element name="A">
<xs:complexType>
...
</xs:complexType>
</xs:element>

<xs:element name="B">
<xs:complexType>
...
</xs:complexType>
</xs:element>

</xs:schema>

then you allow 'A' or 'B' as a root element.

 

[Peter Flynn]

I don't think DTD can do that.

In a DTD, it's the same concent as in a Schema, in that any top-level
element type (one that is not in the content model of any other element
type) could be assumed to be a root element type, although the SGML
standard doesn't specify whether you have to make that assumption or
not, because in a DTD, *any* element type can be a root element type, eg

<!DOCTYPE td SYSTEM "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<td></td>

///Peter