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Abandon the (mistaken) belief that XML attributes provide"metadata" and

• From: "Costello, Roger L." <costello@mitre.org>
• To: "xml-dev@lists.xml.org" <xml-dev@lists.xml.org>
• Date: Sat, 19 Mar 2011 10:54:54 -0400

Hi Folks,

I just did a search of the XML specification for the word "metadata" 

Number of occurrences: 0


Consider this XML snippet:

  <altitude units="feet">12000</altitude>

I have heard it stated:

   The units attribute provides data about the
   element's data. That is, units provides
   metadata.

However, the XML specification makes no such claim about attributes providing "metadata". This is what the XML specification says:

  Attributes are used to associate name-value pairs with elements.

And then the specification goes on to describe the specific syntax for attributes.

Thus, attributes don't provide data about an element's content. Attributes don't provide metadata.  

Attributes are purely a syntactic construct. 

If you abandon the belief that attributes provide metadata then it frees you to create XML designs based on the capabilities of the applications you use.

Example: In XML Schema 1.1 attributes (but not elements) can be declared "inheritable". There are benefits to having an inheritable item. Namely, descendent elements can make decisions (using the new <assert> element) on inherited attributes. Thus, if it is important to enable the descendants of <altitude> to make decisions based on units and value, then design the XML using exclusively attributes:

   <altitude units="feet" value="12000">
      <!-- descendant elements -->
   </altitude>

In the XML Schema the attributes are declared inheritable:

   <attribute name="units" inheritable="true" />
   <attribute name="value" inheritable="true" />

Note that your decision to make them attributes was not based on whether their values are providing metadata (whatever that means), but rather, based on the ability to exploit inheritance in XML Schema 1.1.

Comments?

/Roger

• Follow-Ups:
  • Re: Abandon the (mistaken) belief that XML attributes provide"metadata" and set yourself free to explore capability-based designs
    • From: Michael Kay <mike@saxonica.com>
  • Re: Abandon the (mistaken) belief that XML attributesprovide "metadata" and set yourself free to explore capability-based designs
    • From: James Fuller <james.fuller.2007@gmail.com>