RELAX NG

A Ten-Minute Guide to XML Namespaces

Let’s examine the motivations behind XML namespaces. The first motivation is to have namespaces replace the formal public identifier (FPI), an inheritance from SGML. These identifiers provide a way to identify which vocabulary , or set of names, is being used in a document. The XML/SGML way of identifying the vocabulary used in our library would be to add a public identifier to the document type declaration such as:

<?xml version="1.0"?>
<!DOCTYPE library PUBLIC "-//ERICVANDERVLIST//DTD for library//EN" "library.dtd"/>
<library>
...
</library>

This DOCTYPE declaration contains an FPI (-//ERICVANDERVLIST//DTD for library//EN) and the location of the DTD describing the vocabulary (library.dtd). XML requires that the DOCTYPE declaration always provide a SYSTEM identifier—a location—when a PUBLIC identifier is used, though PUBLIC identifiers aren’t required when SYSTEM identifiers are used. The creators of XML 1.0 didn’t want to require parsers to include the tools (typically XML catalog processing) for resolving formal public identifiers to addresses, but they kept the option open. Because the DOCTYPE declaration provides the parser with identification of the DTD rather than the identification of the abstract set of names, this approach is generally sensible.

The first goal of XML namespaces is to provide identifiers for the abstract notions of vocabularies and namespaces without linking these identifiers directly to the technical implementations ...