Saturday, October 23, 2010

XSD: schema type definition for empty XML content models

I'm inclined to write a little post, suggesting a correction (perhaps a better schema design) to an XML schema document I wrote in the blog post, [1].

In this post [1], I suggested the following XML schema type definition for empty content models (I assume there would not be any attributes on an element):
  <xs:complexType name="EMPTY"> 
        <xs:restriction base="xs:anyType" /> 

Instead of the above schema type definition, I find the following (which is simpler I believe) schema type definition [2] (intending to constrain an XML element) to be better instead:
  <xs:element name="X">

The element definition [2] above intends to validate an XML fragment like following:

In the above example, I intend to suggest that there must not be any child nodes (and neither any XML attributes on an element) within element "X". Interestingly (nothing new really for people knowing XML schema language :) the XML Schema language, only allows constraining XML element and attribute nodes (and optionally these being XML namespace aware) and it doesn't bother about other XML infoset components like comments, processing-instructions and so on (which are present in XPath data model for example) [A] -- this means that any other kinds of nodes, than XML elements and attributes are ignored by XML Schema language and a compliant XML schema validator. This nature [A] of XML schema language is OK as I've learnt (there have been some nice discussions about all of this at XML-DEV list in recent past).

2010-10-26: Here's another variant for definition of empty XML content models.
  <xs:simpleType name="EMPTY">
     <xs:restriction base="xs:string">
        <xs:maxLength value="0"/>

This defines an XML schema 'simpleType' -- and enforces content emptiness with the schema 'maxLength' facet on type xs:string, instead of a complex type as defined in the previous example. I'm more inclined to define element emptiness by an simpleType like above, since intent (and semantics) of schema simple types is never to define XML attributes, but those of complexType are.

I hope the corrections I've shared in this post is appreciated by folks who've read my earlier post cited above [1].

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