What is the relationship between XML namespaces and the XML 1.0 recommendation?
Answer:
Although the XML 1.0 recommendation anticipated the need for XML namespaces by
noting that element type and attribute names should not include colons, it did not actually
support XML namespaces. Thus, XML namespaces are layered on top of XML 1.0. In
particular, any XML document that uses XML namespaces is a legal XML 1.0 document
and can be interpreted as such in the absence of XML namespaces. For example, consider
the following document:
<google:A xmlns:google="http://www.google.org/">
<google:B google:C="bar"/>
</google:A>
If this document is processed by a namespace-unaware processor, that processor will see
two elements whose names are google:A and google:B. The google:A element has an
attribute named xmlns:google and the google:B element has an attribute named google:C.
On the other hand, a namespace-aware processor will see two elements with universal
names {http://www.google.org}A and {http://www.google.org}B. The
{http://www.google.org}A does not have any attributes; instead, it has a namespace
declaration that maps the google prefix to the URI http://www.google.org. The
{http://www.google.org}B element has an attribute named {http://www.google.org}C.
Needless to say, this has led to a certain amount of confusion. One area of confusion is
the relationship between XML namespaces and validating XML documents against
DTDs. This occurs because the XML namespaces recommendation did not describe how
to use XML namespaces with DTDs. Fortunately, a similar situation does not occur with
XML schema languages, as all of these support XML namespaces.
The other main area of confusion is in recommendations and specifications such as DOM
and SAX whose first version predates the XML namespaces recommendation. Although
these have since been updated to include XML namespace support, the solutions have not
always been pretty due to backwards compatibility requirements. All recommendations in
the XML family now support XML namespaces.
noting that element type and attribute names should not include colons, it did not actually
support XML namespaces. Thus, XML namespaces are layered on top of XML 1.0. In
particular, any XML document that uses XML namespaces is a legal XML 1.0 document
and can be interpreted as such in the absence of XML namespaces. For example, consider
the following document:
<google:A xmlns:google="http://www.google.org/">
<google:B google:C="bar"/>
</google:A>
If this document is processed by a namespace-unaware processor, that processor will see
two elements whose names are google:A and google:B. The google:A element has an
attribute named xmlns:google and the google:B element has an attribute named google:C.
On the other hand, a namespace-aware processor will see two elements with universal
names {http://www.google.org}A and {http://www.google.org}B. The
{http://www.google.org}A does not have any attributes; instead, it has a namespace
declaration that maps the google prefix to the URI http://www.google.org. The
{http://www.google.org}B element has an attribute named {http://www.google.org}C.
Needless to say, this has led to a certain amount of confusion. One area of confusion is
the relationship between XML namespaces and validating XML documents against
DTDs. This occurs because the XML namespaces recommendation did not describe how
to use XML namespaces with DTDs. Fortunately, a similar situation does not occur with
XML schema languages, as all of these support XML namespaces.
The other main area of confusion is in recommendations and specifications such as DOM
and SAX whose first version predates the XML namespaces recommendation. Although
these have since been updated to include XML namespace support, the solutions have not
always been pretty due to backwards compatibility requirements. All recommendations in
the XML family now support XML namespaces.
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