XML and Namespaces

This article explains namespaces in XML: when to use namespaces, and how to declare a namespace.

When you see a piece of XML like this:

<foo:bar xmlns:foo="http://foo.example.com/2008/foo">
  <foo:baz mumble="1"/>
</foo:bar>

it is using a namespace.

History

XML 1.0 itself doesn’t define a namespace mechanism, although it does already reserve the colon for future namespace use. It also reserves names starting with the letters X, M and L in any case for official use in future specifications. The minimal conformance requirement for an XML document is that it is well-formed. The other side of the coin is that anything that’s not well-formed is not an XML document, whatever the sender may tell you.

Namespaces in XML became a W3C recommendation in 1999, a year after XML 1.0. It is an optional addition to XML.

Purpose

The purpose of namespaces is to separate different uses of the same names. For example, you may have a list of people with element names like <name> and <address>, and I can have a list of computers with element names like <hostname> and <address> (for IP-addresses). To unambiguously point to my meaning of <address> I can introduce a namespace.

Even if the names don’t overlap exactly, namespaces serve to separate different vocabularies.

Nature

An XML namespace is a URI reference (I’ll use URI for short) and you can make them up any way you want. However, because we wanted to separate my element’s names from yours, it makes sense to use URIs that point to your own organisation. A first stab at my namespace URI could be mailto:computerss@example.com but notice that it contains very little information about what it represents. It is also hard to find out what it means, except perhaps by sending a mail to the address. The same problem exists for URIs with the file: and urn: schemes. As a best practice, use http: URLs and try to maintain some documentation at that address. So, we’ll go for http://namespaces.example.com/2008/computers instead.

Notation

URI references can get pretty long, so if every element and attribute in a namespace would suddenly be preceded by the URI, then the size of your XML documents would explode. That is why the elements themselves use something shorter called a namespace prefix, which is declared using a namespace declaration. The idea is that URI references ensure global uniqueness, while prefixes are enough to ensure local uniqueness within a document, or even a part of a document. Here is an example of a namespace declaration:

Term Example

Term	Example
namespace declaration	`xmlns:foo="http://foo.example.com/2008/foo"`
namespace	`http://foo.example.com/2008/foo`
namespace prefix	`foo`

namespace declaration

xmlns:foo="http://foo.example.com/2008/foo"

namespace

http://foo.example.com/2008/foo

namespace prefix

foo

Notice that it looks like it is just an attribute with the funny name xmlns:foo, but since it starts with the letters xml and it contains a colon, it can’t be a normal attribute. What it does is make foo a shorthand for http://foo.example.com/2008/foo inside the current element and its children. This shorthand can be used on element names, like so <foo:bar/> and on attribute names, like so foo:baz="1". Both the bar element and the baz attribute are said to be in the http://foo.example.com/2008/foo namespace, but you’ll mostly abbreviate that to just the foo namespace.

Next episode

This is part one in a series of articles about XML for programmers. In the next parts, we will look at the available tools for working with XML and namespaces.