19 Short Questions about Namespaces (With Answers)

by

David Megginson

Monday 8 February 1999 (v.2)

Background

For the full specification of XML 1.0, see [1]; for the full
specification of Namespaces in XML, see [2].

This brief review uses James Clark's notation for writing names that
contain both a URI part and a local part.  For example, if the URI
part of a name were "http://www.foo.com/" and the local part were "a",
the name would be written

  {http://www.foo.com/}a

This is purely a convenience notation for the sake of documentation;
it is not defined by any known specification, and is unlikely to be
recognised by any processor.

Chapter One: The XML 1.0 Perspective

[Example]

  <a b="x" c="y"/>

[Q] What is the name of the element in the example above?
[A] The name is "a".

[Q] What is the name of the first attribute in the example above?
[A] The name is "b".

[Q] What is the name of the second attribute in the example above?
[A] The name is "c".

[Q] What do the names mean?
[A] The application determines the meaning of the names.

[Q] How do you write a DTD declaration describing the structure of
    this element?
[A] <!ELEMENT a EMPTY>
    <!ATTLIST a 
      b CDATA #IMPLIED 
      c CDATA #IMPLIED>

Chapter Two. The Namespaces Perspective

[Example 2a]

  <z:a z:b="x" c="y" xmlns:z="http://www.foo.com/"/>

[Q] What is the name of the element in the example above?
[A] The name is "z:a" from the XML 1.0 perspective, or
    "{http://www.foo.com/}a" from the Namespaces perspective.

[Q] What is the name of the first attribute in the example above?
[A] The name is "z:b" from the XML 1.0 perspective, or
    "{http://www.foo.com/}b" from the Namespaces perspective.

[Q] What is the name of the second attribute in the example above?
[A] The name is "c" from both the XML 1.0 and the Namespaces
    perspectives.

[Q] What is the name of the third attribute in the example above?
[A] The name is "xmlns:z" from the XML 1.0 perspective; from the
    Namespaces perspective, this attribute is a declaration.

[Q] What do the names mean?
[A] The application determines the meaning of the names.

[Q] What does the namespace URI "http://www.foo.com/" mean?
[A] It has no defined meaning.

[Q] How do you write a DTD declaration describing the structure of
    this element?
[A] DTDs use the XML 1.0 perspective:
    <!ELEMENT z:a EMPTY>
    <!ATTLIST z:a 
      z:b CDATA #IMPLIED 
      c CDATA #IMPLIED
      xmlns:z CDATA #FIXED "http://www.foo.com">



[Example 2b]

  <a b="x" c="y" xmlns="http://www.foo.com/"/>

[Q] What is the name of the element in the example above?
[A] The name is "a" from the XML 1.0 perspective, or
    {http://www.foo.com/}a from the Namespaces perspective.

[Q] What is the name of the first attribute in the example above?
[A] The name is "b" from both the XML 1.0 and the Namespaces
    perspectives.

[Q] What is the name of the second attribute in the example above?
[A] The name is "c" from both the XML 1.0 and the Namespaces
    perspectives.

[Q] What is the name of the third attribute in the example above?
[A] The name is "xmlns" from the XML 1.0 perspective; from the
    Namespaces perspective, this attribute is a declaration.

[Q] What do the names mean?
[A] The application determines the meaning of the names.

[Q] What does the namespace URI "http://www.foo.com/" mean?
[A] It has no defined meaning.

[Q] How do you write a DTD declaration describing the structure of
    this element?
[A] DTDs use the XML 1.0 perspective:
    <!ELEMENT a EMPTY>
    <!ATTLIST a 
      b CDATA #IMPLIED 
      c CDATA #IMPLIED
      xmlns CDATA #FIXED "http://www.foo.com">

References

• [1] http://www.w3.org/TR/REC-xml

• [2] http://www.w3.org/TR/REC-xml-names

All the best,
David

David Megginson
Email: david@megginson.com
WWW: http://www.megginson.com/

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Prepared by Robin Cover for the The SGML/XML Web Page archive. For more on namespaces, see "Namespaces in XML."