Indirect Addressing for XML

This paper describes and explains the XIndirect facility, a W3C Note. The XIndirect Note defines a simple mechanism for representing indirect addresses that can be used with other XML-based linking and addressing facilities, such as XLink and XInclude. XIndirect is motivated primarily by the requirements of XML authoring in which the management of pointers among systems of documents under constant revision cannot be easily satisfied by the direct pointers provided by XLink and XInclude. Indirect addressing is inherently expensive to implement because of both the processing demands of multi-step pointers and the increased system complexity required to do the processing. XLink and XPointer (and by extension, XInclude) explicitly and appropriately avoid indirection in order to provide the simplest possible solution for the delivery of hyperlinked documents, especially in the context of essentially unbounded systems, such as the World Wide Web. XIndirect enables indirect addressing when needed without adding complexity to the existing XML linking and addressing facilities — by defining indirection as a separate, independent facility, processors that only need to support delivery of documents are not required to support indirection simply in order to support XLink or XInclude. Rather, when indirection management is required, developers of XML information management systems can limit the support for indirection to closed systems of controlled scope where indirection is practical to implement. This paper illustrates some of the key use cases that motivate the need for the XIndirect facility, describes the facility itself, and discusses a reference implementation of the XIndirect facility.