XML Reference Guide

Mar 14, 2003

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When you're building an HTML document, it's not uncommon to create links to other documents. These links can be absolute, meaning that the link itself contains all of the information necessary for following it, or they can be relative, meaning that the the link contains only directions on how to get to the new document from here.

Wherever "here" may be.

For example, you might create a document with a link that contains an absolute URL:

<a href="http://www.nicholaschase.com/books/index.html">Nick's Page</a>

The page itself doesn't matter. This is an absolute, fully-qualified URL. No matter what page it's on, clicking that link will always take you to the same page, http://www.nicholaschase.com/books/index.html.

Not all URLs are absolute, however. A relative URL depends greatly on the context in which it's found. Consider this example:

<html>
<head><title>Moving on ...</title></head>
<body>

    Check out <a href="books/index.html">Nick's books</a>.

</body>
</html>

This is a relative URL. Where you go when you click on it is going to depend on the context in which you find the page. If the URL for the actual page were

http://www.nicholaschase.com/

then this relative URL would resolve to

http://www.nicholaschase.com/books/index.html

On the other hand, if the URL for the page were

http://www.vanguardreport.com/nick

then the URL would resolve to

http://www.vanguardreport.com/nick/books/index.html

Web page authors have been able to get around this problem by using the HTML base tag. For example:

<html>
<head><title>Moving on ...</title>
<base href="http://www.nicholaschase.com/" />
</head>
<body>

    Check out <a href="books/index.html">Nick's books</a>.

</body>
</html>

In this case, it wouldn't matter where the overall page was located; the relative URL will always resolve itself relative to the base tag. This gives you the advantage of being able to locate the page anywhere without having to make all of your URLs absolute, which provides the further advantage of being able to change all of their ultimate destinations simply by altering the base tag.

So what does this have to do with XML?

XML has similar functionality in the form of XML Base. XML Base is a recommendation intended to create a way to provide an XML document with the same information an HTM document gets from the base tag.

Unlike other XML specifications and recommendations that designate dozens or hundreds of elements and/or attributes, XML Base consists of exactly one attribute: xml:base.

The xml:base attribute belongs to the default XML namespace, http://www.w3.org/XML/1998/namespace. (You don't have to declare this namespace because it's recognized by all namespace-aware XML applications.) It sets a "reference point" for any relative URLs (or to be more proper, Uniform Resource Identifiers, or URIs) within the document. Consider this (slightly modified) example from the XML Base Recommendation:

<?xml version="1.0"?>
<doc xml:base="http://example.org/today/"
     xmlns:xlink="http://www.w3.org/1999/xlink">
  <head>
    <title>Virtual Library</title>
  </head>
  <body>
    <paragraph>See <link xlink:type="simple" xlink:href="new.xml">what's
      new</link>!</paragraph>
    <paragraph>Check out the hot picks of the day!</paragraph>
    <olist xml:base="hotpicks/">
      <item>
        <link xlink:type="simple" xlink:href="pick1.xml">Hot Pick #1</link>
      </item>
      <item>
        <link xlink:type="simple" xlink:href="pick2.xml">Hot Pick #2</link>
      </item>
      <item>
        <link xlink:type="simple" xlink:href="pick3.xml">Hot Pick #3</link>
      </item>
    </olist>
  </body>
</doc>

The "what's new" link resolves to

http://example.org/today/new.xml

because it's in the scope of the first xml:base attribute.

The "hot pick" links show what happens when you nest these definitions, and are a little trickier. Because the second xml:base attribute is also relative, we first have to resolve it relative to the first one. This makes the second xml:base attribute equivalent to

http://example.org/today/hotpicks

We can then use that to resolve the inner URLs, which resolve to

http://example.org/today/hotpicks/pick1.xml
http://example.org/today/hotpicks/pick2.xml

and

http://example.org/today/hotpicks/pick3.xml

XML Base isn't really intended to be used on its own; instead, it has an effect on other specifications. For example, the XML Infoset Recommendation defines a "base URI" property that uses XML Base as it's method of determination. XLink (seen briefly in the above example) also relies on XML Base. Some recommendations, such as XPath, don't lend themselves to XML Base support. As time goes on, however, it's likely that support will increase.

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