For information about transclusion on Wikipedia, see Wikipedia:Transclusion

In computer science, transclusion is the inclusion of part of a document into another document by reference. It is a feature of substitution templates.

Some hypertext systems, including Ted Nelson's Xanadu Project, support transclusion. For example, an article about a country might include a chart or a paragraph describing that country's agricultural exports from a different article about agriculture. Rather than copying the included data and storing it in two places, a transclusion embodies modular design, by allowing it to be stored only once (and perhaps corrected and updated if the link type supported that) and viewed in different contexts. The reference also serves to link both articles.

In Ted Nelson's original proposal for hypertext, where he coined the terms "transclusion", "hypertext" and "hypermedia", outlined in his 1982 book, Literary Machines, micropayments could be automatically exacted from the reader for all the text, no matter how many snippets of content are taken from various places.

Nelson has recently delivered a demonstration of Web transclusion, the Little Transquoter (programmed to Nelson's specification by Andrew Pam). It creates a new format built on portion addresses from Web pages; when dereferenced, each portion on the resulting page remains click-connected to its original context-- always a key aspect of transclusion for Nelson, but missing in most implementations of transclusion.

Context neutrality

Transclusion works better when transcluded sections of text are self-contained, so that the meaning and validity of the text is independent of the context in which it appears. For example, formulations like "as explained in the previous section" are problematic, because the transcluded section may appear in a different context, causing confusion. What constitutes "context neutral" text varies, but often includes things like company information or boilerplate.

Use on the Web

In HTML

At present, transclusion in HTML is somewhat limited by lack of standards support in Internet Explorer. Although all graphical browsers can transclude an image, including a document is a bit more difficult. There are currently two methods of achieving this result:

• The IFrame (inline frame) element, which includes whole documents.
• The Object element. In Firefox and other browsers, this allows true document transclusion, enabling one page to be built from several smaller documents.

In addition, Ajax can achieve a similar result across all modern, JavaScript-enabled browsers.

Future versions of HTML may support deeper transclusion of portions of documents using XML technologies such as entities, XPointer document referencing, and XSLT manipulations.

The practice of 'remote loading'—including data, such as images, directly from other sites—is usually frowned upon because of the use of bandwidth and computing power required from the remote computer system (see Bandwidth theft). This is said to "tax" another server, and is often considered an example of leeching. Increased distribution of proxy servers has the potential to turn this around and take advantage of transclusion to reduce redundant transmissions of the same data.

A major exception to this rule is web advertising, where advertisements supplied by an advertiser are published with other content by a publisher. An advertiser prefers to serve an advertisement and be able to detect when it was viewed, rather than have it served by the publisher and have to trust the publisher. (See also Hit counter, Web bug).

Mashups are a recent phenomena that are similar to transclusion.

Server-side Transclusion

Transclusion can also be accomplished on the server side, provided the server software includes this functionality. This can be done through multiple different technologies, including Server Side Includes and markup entity references resolved by the server software. Client-side transclusion is generally preferable as data transcluded into several pages can be cached by the client rather than sent out again for every page.

Publications

• Di Iorio, A.; Vitali, F. (August 2003). "A Xanalogical Collaborative Editing Environment" (PDF). Proceedings of the Second International Workshop of Web Document Analysis 2003 (WDA2003). 
• Kolbitsch, J.; Maurer, H. (June 2006). "Transclusions in an HTML-Based Environment". Journal of Computing and Information Technology 14 (2): 161-174. 
• Kolbitsch, J. (June 2005). "Fine-Grained Transclusions of Multimedia Documents in HTML". Journal of Universal Computer Science 11 (6). 
• Krottmaier, H. (2002). "Transcluded Documents: Advantages of Reusing Document Fragments" (PDF). Proceedings of the 6th International ICCC/IFIP Conference on Electronic Publishing (ELPUB2002): 359–367, Czech Republic: Karlovy Vary. 
• Krottmaier, H.; Helic, D. (2002). "Issues of Transclusions" (PDF). Proceedings of the World Conference on E-Learning in Corporate, Government, Healthcare, & Higher Education (E-Learn 2002): 1730–1733. 
• Krottmaier, H.; Maurer, H. (July 2001). "Transclusions in the 21st Century". Journal of Universal Computer Science 7 (12): 1125–1136. 
• Moore, A.; et al (2001). "Personally tailored teaching in WHURLE using conditional translucion". Proceedings of the Twelfth ACM Conference on Hypertext and Hypermedia: 163–164. 
• Nelson, T. H. (1981). Literary Machines. Mindful Press. 
• Nelson, T. H. (1995). "The Heart of Connection: Hypermedia Unified by Transclusion". Communications of the ACM (8): 31–33. 
• Nelson, T. H. (1996). Generalized Links, Micropayment and Transcopyright. Almaden Research Center.
• Nelson, T. H. (1998). Transcopyright: Pre-Permission for Virtual Republishing.
• Nelson, T. H. (1999). "Xanalogical Structure, Needed Now More than Ever: Parallel Documents, Deep Links to Content, Deep Versioning and Deep Re-Use". ACM Computing Surveys (4es). 
• Pam, A. (1997). Fine-Grained Transclusion in the Hypertext Markup Language (Text). Internet Draft.
• Wilde, E.; Lowe, D. (2002). "XML Linking Language", XPath, XLink, XPointer, and XML: A Practical Guide to Web Hyperlinking and Transclusion (PDF), Addison-Wesley Professional, 169-198. 

External links

• The Little Transquoter

See also

• Compound document
• Macro
• Single source publishing
• Subroutine
• Cross-reference