XML namespace

Definition

XML Namespaces allow to fully/uniquely qualify XML element and attribute names to prevent from confusing two elements that have the same name but mean different things.

Purpose and history of XML namespace

Various XML vocabularies (languages/applications) can be mixed. But then we can run into naming conflicts, i.e. different vocabularies (DTDs) can use the same names for elements !

Since DTDs are not mandatory for most W3C languages, the W3C requires namespace definitions within root elements, e.g. XHTML must have one:

<html xmlns="http://www.w3.org/1999/xhtml">

Namespaces are a W3C standard since Jan 1999. That means that namespaces have been introduced after XML has been invented.

Standards

The Namespace standard:

http://www.w3.org/TR/REC-xml-names/

Namespaces are identified with URIs

http://www.rfc-editor.org/rfc/rfc3986.txt

A URI is either a URL or a URN

http://www.rfc-editor.org/rfc/rfc2141.txt

Compound W3C documents

http://www.w3.org/2004/CDF/

Namespaces

An example demonstrating the need for namespaces

Here is an XML fragment using title within a bibliography element:

<book>
<title>A true story</title>
 <description>A real cool publication</description>
 </book>

A 2nd XML fragment uses title in a employees record:

<record>
<name>Miller</name>  <title>Dr. </title>
<publications> ...  </publications>
</record>

If these two XML fragments were added together, there would be a name conflict because both contain a <title> element with different meaning. But we can solve this like this:

<employees:record>
<employees:name>Miller</employees:name>
<employees:title>Dr.  </employees:title>
<employees:publications>
<biblio:book>
<biblio:title>A true story</biblio:title>
<biblio:description>A real cool publication</biblio:description>
</biblio:book>
</employees:publications>
</employees:record>

Declaring namespaces

Formally speaking, an XML namespace is simply a collection of names (elements and attributes) of a markup vocabulary that can be uniquely identified

Procedure

Create or identify a namespace identifier you wish to use:
An XML namespace is identified by a unique URI reference, usually a URL
The URL need not point to anything on the Internet. It is just used as a unique string, i.e. a name !
However, most namespace identifiers actually point to a real web page that either provides an explanation or at least informations about the organization. This means that if you plan to create your own namespaces, you should use the name of any webpage over which you have control.
Make a namespace declaration within the element that belongs to this namespace, i.e.
map a prefix of your choice to a unique URI.

There are two major declaration variants (1) declaring a namespace that will require insertion of prefixes

<prefix:element xmlns:prefix="URI"> <html:html xmlns:html=’http://www.w3.org/1999/xhtml’> .....

(2) declaring a default namespace (element + children belong to this namespace by default)

<element xmlns="URI"> ....

<html xmlns=’http://www.w3.org/1999/xhtml’> .....

Scoping

"Scoping" here means "where does a declaration apply ?"
The scope of an XML namespace declaration extends from the beginning of the start-tag in which it appears to the end of the corresponding end-tag, including all children
Most frequently, namespaces are declared in the document root.

XHMTL fragment example

<?xml version="1.0"?>
<html:html xmlns:html=’http://www.w3.org/1999/xhtml’>
<html:head><html:title>Frobnostication</html:title></html:head>
<html:body><html:p>Moved to
<html:a href=’http://frob.example.com’>here.</html:a></html:p>
</html:body>
</html:html>

Example with 2 namespaces (both use URNs instead of URLs)

<?xml version="1.0"?>
<!-- both namespace prefixes are available throughout -->
<bk:book xmlns:bk=’urn:loc.gov:books’ xmlns:isbn=’urn:ISBN:0-395-36341-6’>
<bk:title>Cheaper by the Dozen</bk:title>
<isbn:number>1568491379</isbn:number>
</bk:book>

Default namespaces

If most elements in an XML document belonged to the same namespace, it would be ugly to prefix each element name. Instead define a default namespace that applies to all non-prefixed elements and attributes.

<element xmlns="URI"> ....

The default namespace applies to the element on which it was defined and all descendants of that element. But if a descendants has another default namespace defined on it, this new namespace definition overrides the previous one and becomes the default for that element and its descendants. Note that namespaces does not apply to attribute names ! This is a likely source of trouble for XSLT ...

XHTML default namespace example

<?xml version="1.0"?>
<!-- elements are in the HTML namespace by default -->
 <html xmlns=’http://www.w3.org/1999/xhtml’>
 <head><title>Frobnostication</title></head>
 <body><p>Moved to
 <a href=’http://frob.example.com’>here</a>.</p></body>
</html>

Default namespace for a book vocabulary plus a namespace for ISBN example:

<?xml version="1.0"?>
<!-- unprefixed element types are from "books" -->
<book  xmlns=’urn:loc.gov:books’
xmlns:isbn=’urn:ISBN:0-395-36341-6’>
<title>Cheaper by the Dozen</title>
<isbn:number>1568491379</isbn:number>
</book>

A larger example of namespace scoping

It includes some XHTML whose namespace is declared within the "p" element.

<?xml version="1.0"?>

<book xmlns=’urn:loc.gov:books’

xmlns:isbn=’urn:ISBN:0-395-36341-6’>

<title>Cheaper by the Dozen</title>

<isbn:number>1568491379</isbn:number>

<notes>

This is a <i>funny</i> book!

</p>

</notes>

</book>

XSLT namespace example

XSLT instructions are prefixed with "xsl" here
Output is namespace-less here (see the XSLT module for explanations ...)

<?xml version="1.0"?>

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:template match="page">

.....

</html>

</xsl:template>

<xsl:template match="title">

</xsl:template>

<xsl:template match="content">

</xsl:template>

<xsl:template match="comment">

</xsl:template>

</xsl:stylesheet>

Validation of composite documents

Validating documents that contain different namespaces are called composite or compound documents and validation is not always easy since combined DTDs may not exist
A validation standard for composite W3C vocabularies is currently under preparation ...
Usually, combined documents are produced by server-side programs for delivery and they are just well-formed (not attached to DTDs or other schemas)
You may write DTDs that validate compound documents.

A DTD rule with an extra namespace example

In this example, the "a" element and the "description" attribute belongs to the default namespace, but the href and type attributes belong to the xlink namespace
We declare the namespace in the root element, but also could have done it just for the "el" element...

<!ELEMENT link_list (el+)>
<!ATTLIST link_list xmlns:xlink CDATA  FIXED "http://www.w3.org/1999/xlink">
<!ELEMENT el ( PCDATA)>
<!ATTLIST el description CDATA  IMPLIED xlink:href CDATA  REQUIRED xlink:type CDATA  FIXED "simple">

An XML Example file

<?xml version="1.0" encoding="ISO-8859-1" ?> <!DOCTYPE link_list SYSTEM "link_list.dtd"> <?xml-stylesheet href="link_list.css" type="text/css" ?> <link_list xmlns:xlink="http://www.w3.org/1999/xlink"> <el description="verified" xlink:type="simple" xlink:href="http://www.webster.ch/">Webster Geneva</el>

<el xlink:href="http://www.webster.edu/" xlink:type="simple">Webster</el>
</link_list>

Client-side XML languages and namespaces overview

The W3C defines several Internet languages and for which namespace declarations are mandatory (even if you use them "standalone", i.e. not in composite documents.

Some W3C languages

Name	Namespace URL	Explanation
XSLT	http://www.w3.org/1999/XSL/Transform	XSL - Transformations
XSL-FO	http://www.w3.org/1999/XSL/Format	XSL - Formatting
XML Schema	http://www.w3.org/2001/XMLSchema
SVG	http://www.w3.org/2000/svg	Scalable Vector Graphics
RDF	http://www.w3.org/1999/02/22-rdf-syntax-ns	Resource Description Format
SMIL 2.0	http://www.w3.org/2001/SMIL20/	Synchronized Multimedia Integration Lang.
VoiceXML 2.0	http://www.w3.org/2001/vxml	Synthesized voice
XLink	xmlns:xlink="http://www.w3.org/1999/xlink"	XML Linking Language
XForms	http://www.w3.org/2002/xforms	Next generation of HTML forms.
XHTML	http://www.w3.org/1999/xhtml
MathML	http://www.w3.org/1998/Math/MathML	Mathematical markup language

Some of these languages (e.g. SMIL, MathML, SVG, X3D) need special clients for display. Others are transducers or helpers, i.e. are used to define, style, transform and process XML contents

Compound documents

Combining various content delivery formats is the future (also on cell phones). For example, XHTML-formatted content can be augmented by SVG objects or mathematical formula.

Examples of possible Compound Document profiles

XHTML + SVG + MathML
XHTML + SMIL
XHTML + XForms
XHTML + VoiceML

The W3C is working on a generic Compound Document by Reference Framework (CDRF) that defines a language-independent processing model for combining arbitrary document formats ...

Web browser caveats

IE doesn't support natively most of these languages, Firefox supports more but not all.

Workarounds:

Use a browser that is more advanced (e.g. Firefox) or specialized players (e.g. Realplayer for SMIL)
Install plugins for your web browser and use a more indirect method to assemble contents, i.e. <object> or <iframe> This will work with most browsers if you have the right plugin (SVG in this case) installed.

<iframe src="hello-svg.svg" height="300" width="80%" frameborder="0">

... Sorry you need an SVG plugin ...

</iframe>

XHTML / MathML example

See the MathML article. It contains some mixed XHTML/MathML code.

Xlink example

XLink was supposed to replace all linking constructs in the W3C languages. It is adopted by SVG, X3D, but not by XHTML 2 which introduces its own linking constructs. XML-capable browsers are also supposed to implement this (Firefox, but not IE)

Example: Story grammar that implements XLink for an A element

<?xml version="1.0" encoding="ISO-8859-1" ?>
<STORY xmlns:xlink="http://www.w3.org/1999/xlink">

<Title>The Webmaster</Title>

……

<INFOS> <Date>30 octobre 2003 - </Date>

<A xlink:href=http://jigsaw.w3.org/css-validator/check/referer

xlink:type="simple">CSS Validator</A>

</INFOS>

</STORY>

SVG example

XHTML with embedded SVG tags (tested with Firefox)

<?xml version="1.0" encoding="ISO-8859-1" ?>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd" >

<html xmlns="http://www.w3.org/1999/xhtml"

    xmlns:svg="http://www.w3.org/2000/svg">

<head> <title>SVG within XHTML Demo</title> </head>

<body>

.... The SVG part starts below <hr />

  <!-- a small rectangle -->

  <rect x="50" y="50" rx="5" ry="5" width="300" height="100"

        style="fill: CCCCFF;stroke: 000099"/>

     <!-- a text in the same place -->

  <text x="55" y="90" style="stroke: 000099;fill: 000099;font-size:24;">

   HELLO dear reader </text>

 </svg>

<hr />

The SVG part ended above

</body>

</html>

Combining your XML with XHTML

It sometimes can be practical to combine your own XML contents with XHTML, because HTML offers easy-to-use functionality like image inclusions or links. Note however that in practice this feature is not often used since:

XSLT stylesheets allow transformations of XML data into XHTML (i.e. the stylesheet will contain the extra HTML and the result displayed usually will be all HTML and easier to manage with CSS)
the way browsers implement this is not really stable

XHTML with some XML included and CSS example

Tested with Firefox 2.x (works) and IE 7 (CSS doesn’t work, I don’t know why ...)

Procedure:

Define a namespace for the included XML content
Attach a CSS stylesheet with an XML processor instruction
Your file must be understood as "XML", e.g. call it something.xml or something.xhtml but not something.html !

XHTML with some cooking XML included (file cooking.xhtml):

<?xml version="1.0" encoding="iso-8859-1"?>

<?xml-stylesheet type="text/css" href="cooking.css"?>

<html xmlns="http://www.w3.org/1999/xhtml"

xmlns:re="http://webster.unige.ch/coap21/dolores">

<head>

<title>A Compound XHTML-XML Document</title>

</head>

<body>

<h1>A Compound XHTML-XML Document</h1>

<p>Contains a cooking recipee written down by R. Dolores for a COAP 2180 CSS exercise. Everything below the line is XML styled with a CSS stylesheet</ p>

<hr/>

<re:recipe>

<re:recipe_head>

<re:recipe_name>Cold Salmon in Creamy Spiced Sauce</re:recipe_name>

	<re:recipe_author>Hilaire Walden</re:recipe_author>
	<re:meal_type>Fish and Shellfish</re:meal_type>

</re:recipe_head>

.....

</re:recipe>

</body>

</html>

XML with some XHTML included

Works with Firefox 1x/2x and IE7 (probably also with IE6)
An HTML namespace is declared in the root element and we use it twice (for the img and a tags).
Btw this is a trick to get around non-implementation of the xlink standard in IE. If need to create a valid DTD, see " " [12]

XML plus XHTML (file xml_plus_xhtml.xml):

<?xml version="1.0" ?>
<?xml-stylesheet href="xml_plus_xhtml.css" type="text/css"?>
<page xmlns:html="http://www.w3.org/1999/xhtml" updated="jan 2007">
<title>Hello friend</title>
<list>
<!-- we use an HTML tag below to include a picture -->
<html:img src="photo.jpg"/>
<item price="10"> White plate  </item>
<item price="20"> Gold plate  </item>
<item price="15"> Silver plate  </item>
</list>
<comment> Written by <html:a href="http://tecfa.unige.ch/tecfa-people/ schneider.html">DKS/Tecfa</html:a> , feb 2007 </comment>
</page>

XML namespace

Contents

Definition

Purpose and history of XML namespace

Standards

Namespaces

An example demonstrating the need for namespaces

Declaring namespaces

Scoping

Default namespaces

Validation of composite documents

Client-side XML languages and namespaces overview

Compound documents

Combining your XML with XHTML

Navigation menu

XML namespace

Definition

Purpose and history of XML namespace

Standards

Namespaces

An example demonstrating the need for namespaces

Declaring namespaces

Scoping

Default namespaces

Validation of composite documents

Client-side XML languages and namespaces overview

Compound documents

Combining your XML with XHTML

Navigation menu

Slow Search