Xml – What are XML namespaces for

namespacesschemaxmlxml-namespacesxsd

This is something that I always find a bit hard to explain to others:
Why do XML namespaces exist?
When should we use them and when should we not?
What are the common pitfalls when working with namespaces in XML?

Also, how do they relate to XML schemas? Should XSD schemas always be associated with a namespace?

Best Answer

They're for allowing multiple markup languages to be combined, without having to worry about conflicts of element and attribute names.

For example, look at any bit of XSLT code, and then think what would happen if you didn't use namespaces and were trying to write an XSLT where the output has to contain "template", "for-each", etc, elements. Syntax errors, is what.

I'll leave the advice and pitfalls to others with more experience than I.

Related Solutions

XML attribute vs XML element

I use this rule of thumb:

An Attribute is something that is self-contained, i.e., a color, an ID, a name.
An Element is something that does or could have attributes of its own or contain other elements.

So yours is close. I would have done something like:

EDIT: Updated the original example based on feedback below.

  <ITEM serialNumber="something">
      <BARCODE encoding="Code39">something</BARCODE>
      <LOCATION>XYX</LOCATION>
      <TYPE modelNumber="something">
         <VENDOR>YYZ</VENDOR>
      </TYPE>
   </ITEM>

XML namespaces and attributes

Most of the time, attributes will not be in any namespace. The namespace spec says (emphasis mine):

A default namespace declaration applies to all unprefixed element names within its scope. Default namespace declarations do not apply directly to attribute names; the interpretation of unprefixed attributes is determined by the element on which they appear.

There's a reason that most XML vocabularies use non-namespaced attributes:
When your elements have a namespace and those elements have attributes, then there can be no confusion: the attributes belong to your element, which belongs to your namespace. Adding a namespace prefix to the attributes would just make everything more verbose.

So why do namespaced attributes exist?
Because some vocabularies do useful work with mostly attributes, and can do this when mixed in with other vocabularies. The best known example is XLink.

Lastly, W3C XML Schema has an all too easy way (<schema attributeFormDefault="qualified">) of declaring your attributes as being in a namespace, forcing you to prefix them in your documents, even when you use a default namespace.

Best Answer

Related Solutions

XML attribute vs XML element

XML namespaces and attributes

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