1. The Purpose of XML Namespaces

2. Namespace Caveats
XML namespaces are the source of criticism and confusion
XML namespaces are the source of many myths
.NET namespaces and XML namespaces are not the same thing
In fact, they have nothing to do with each other

3. The Problem of Name Ambiguity
Names can be ambiguous
When we say “Las Vegas” do we mean “Las Vegas, Nevada” or “Las Vegas, New Mexico”
How many cities have the name “Springfield”?
When we say address, do we mean street address or Internet address
Name ambiguity occurs when two XML documents have elements of the same name but with different meanings

4. The Problem of Name Ambiguity (Example)
<?xml version="1.0" encoding="utf-8" ?>
<office>
<address>123 Oak street</address>
<address> </address>
</office>
The meaning of <address> differs between the two contexts

5. The Purpose of XML Namespaces (1)
They eliminate name ambiguity in element names between documents
Namespaces answer the question “Are we talking about the same thing?”
XML namespaces were released about 1 year after XML itself was released
They don’t quite work right with DTDs
Namespaces are the reason why we don’t use the colon (:) in element names

6. The Implementation of XML Namespaces (1)
XML namespaces use URIs (IRIs) to qualify element names
This allows us to use the same element name with a different URI thereby qualifying the element name
The URI is just a globally unique name
Formally, XML namespaces define a vocabulary or universe of names

7. Namespaces (Quotable Quotes)
What Does a Namespace URL Locate?
There is nothing at all at the end of a namespace URI, except perhaps a 404 Not Found error

8. IT’S JUST A NAME Namespaces (Myths) IT’S NOT NECESSARILY A WEB SITE
IT’S NOT A REFERENCE TO A PROGRAM ON THE WEB
IT’S NOT A POINTER TO A RESOURCE
IT’S JUST A NAME

9. Applications using XML namespaces
XML namespaces are widely used in
The Schema Definition Language (XSDL)
The Extensible Stylesheet Language (XSL)
To describe the schema of an Office 2007 document
A .NET DataSet

10. Namespaces (Illustration)
Namespace (vocabulary) XSD
Parser
Namespace (vocabulary) XSLT
Namespace (vocabulary)
Other
Your document

11. Characteristics of Namespace URIs
You should control the URI that you use or you can’t control the name
It can be any URI whether or not the URI exists on the Web
Thus, the URI has no relevance
URIs are case sensitive
URIs are identical only if the strings are identical
No %-escaping is done

12. Characteristics of Namespace URIs (Example)
The following URIs ARE different:
And so are the following:

13. The XML Namespace Now it should be clear what xml:lang is
The prefix xml is always bound to the namespace name
OK – this one points to a URI but it does not have to!
You can declare this namespace but it is not necessary
If declared, you cannot bind xml: to another namespace URI

14. The Implementation of Namespaces (1)
Namespace names are defined through ‘special’ attributes
Any attribute starting with the prefix xmlns: is considered a prefix defining attribute
The prefix following the attribute is the local abbreviation for the namespace
The prefix name has no relevance although there are common naming conventions
The attribute value is a unique URI

15. The Implementation of Namespaces (Example 1)
The following shows three namespace declarations
The prefix off is associated with the namespace
<off:office xmlns:off="
xmlns:phys="
xmlns:net="
</off:office>
Qualified name <off/office>

16. Applying Namespaces to Attributes
Attributes can apply to a namespace
The default namespace does NOT apply to attribute names