Eve Maler (Sun): XLink and XPointer

The simple HTML one-way link mechanism is too limited for the XML world. We need to be able to express link relationships when we might not have write access to either the source or the destination. Sometimes we need one-to-many links. Sometimes we need links stored externally from the files that are linked. Eve Maler (Sun), co-chair of the W3C's XML Linking working group and co-editor of the W3C XLink and XPointer specifications, described XLink, XPointer, and the related XBase.

She distinguished between:

• links - associates set of resources or subresources with optional titles; comes in two types: simple (like HTML) or extended
• participants - any number, local vs. remote, whole vs. subresource; two kinds are locator and resource
• arcs - the critical piece that expresses traversal from start to end, plus traversal behavior

Arcs can be divided into:

• outbound arc - starts locally, ends remotely
• third party arc - starts and ends remotely
• inbound arc - starts remotely and ends locally

There are also associated behaviors:

• show - what happens when a link is encountered; possible values are: new, replace, embed, none, or other.
• actuate - how the link is activated; possible values are: onLoad, onRequest, none, or other.

While remarking that "XPointer is mostly XPath", Eve also discussed XPointer for fragment identifiers and described four flavors:

• bare-name - Example: #foo (based on attribute of type ID)
• child-sequence - Example: #/1/2/5 is the 5th child of the 2nd child of the 1st child of the root
• full XPointer - Example: #xpointer(//prod[@num='22'])
• ranges and string ranges - series of non-contiguous strings

Anthony Duhig, Advanced Technologies, Empolis UK, spoke about Separating Links from Content Using XML, XLink and XPointer. One of his concerns was link management, especially dealing with out-of-line links, such as third-party arcs stored in a special linkbase. This approach is quicker to search and more scalable. Empolis UK (formerly STEP UK) created X2X, the first commercial XLink engine.

See also Eve's concise introduction, XML Linking: An Executive Summary. For an in-depth coverage of XLink, see the three-part XML.com article by Fabio Arciniegas A., What is XLink?

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