XML in a Nutshell, 3rd Edition

Base URIs

Relative URL references such as sark.jpg, ../pi1/sark.jpg, and turing/pi1/sark.jpg must be resolved relative to an absolute base URI before being retrieved. When relative URLs are found in XLinks, xml-stylesheet processing instructions, system identifiers, and other locations in XML documents, they are normally resolved relative to the absolute base URL of the document or entity that contains them. For instance, if you find the element <image xlink:type="simple" xlink:href="pi1/sark.jpg" /> in a document at the URL http://www.turing.org.uk/turing/index.html, you would expect to find the file sark.jpg at the URL http://www.turing.org.uk/turing/p1/sark.jpg. This isn’t a surprise. It’s pretty much how links have worked in HTML for over a decade.

However, XML does add a couple of new wrinkles to this procedure. First, an XML document may be composed of multiple entities loaded from multiple different URLs, even on different servers. If this is the case, then a relative URL is resolved relative to the base URL of the specific entity in which it appears, not the base URL of the entire document.

Secondly, the base URL may be reset or changed from within the document by using xml:base attributes. Such an attribute may appear on the XLink element itself or on any ancestor element in the same entity. For example, this XLink points to ftp://ftp.knowtion.net/pub/mirrors/gutenberg/etext93/wizoz10.txt:

<novel xmlns:xlink = "http://www.w3.org/1999/xlink" xml:base="ftp://ftp.knowtion.net/pub/mirrors/gutenberg/etext93/" ...