XML namespace
This is a draft for a tutorial article, but it needs to be fixed in several places.
Note on its production: It's a Framemaker to Word to Mediawiki translation from parts of teaching slides - 21:50, 20 March 2007 (MET)
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 involved
The Namespace standard:
Namespaces are identified with URIs
A URI is either a URL or a URN
Compound W3C documents
See also: The standards overview article. Most XML-based standards in education (e.g. IMS Content Packaging make heavy use of namespaces ! In addition most IMS and SCORM standards use XML Schema which are associated to XML contents via namespace declarations.
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
- Therefore if some namespace URLs below are broken there is absolutly nothing to worry about. 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 variants to declare namespaces
(1) declaring a namespace that will require insertion of prefixes
<prefix:element xmlns:prefix="URI">
e.g.
<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">
e.g.
<html xmlns=’http://www.w3.org/1999/xhtml’> .....
Scoping
"Scoping" 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"?> <!-- initially, the default namespace is "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> <notes> <!-- make HTML the default namespace for some commentary --> <p xmlns=’http://www.w3.org/1999/xhtml’> 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> <head> <title> <xsl:value-of select="title"/> </title></head> <body bgcolor=" ffffff"> <xsl:apply-templates/> </body> </html> </xsl:template> <xsl:template match="title"> <h1 align="center"> <xsl:apply-templates/> </h1> </xsl:template>
<xsl:template match="content"> <p align="center"> <xsl:apply-templates/> </p> </xsl:template>
<xsl:template match="comment"> <hr /> <xsl:apply-templates/> </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://tecfa.unige.ch/">TECFA</el> <el xlink:href="http://tecfa.unige.ch/" xlink:type="simple">TECFA</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
- RDF namespace
RDF-based languages are very prolific and add to the complexity of semantic web initiatives
See 100 most common RDF namespaces
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 + other vocabularies
- 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> <Author>DKS -</Author> <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 /> <svg xmlns="http://www.w3.org/2000/svg" version="1.1" width="400" height="300"> <!-- 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 XHTML with your XML
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://tecfa.unige.ch/ns/cooking"> <head> <title>A Compound XHTML-XML Document</title> <!-- this should work with IE but doesn’t, maybe IE6 ? --> <link href="cooking.css" type="text/css" rel="stylesheet"/> </head> <body> <h1>A Compound XHTML-XML Document</h1> <p>Contains a cooking recipee written down by R. D. as an 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>H.W.</re:recipe_author> <re:meal_type>Fish and Shellfish</re:meal_type> </re:recipe_head> ..... </re:recipe> </body> </html>
Combining your XML with XHTML
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.
- 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="....">DKS/Tecfa</html:a> , feb 2007 </comment> </page>