Towards XML Transclusions Angelo Di Iorio, Silvio Peroni, Fabio Vitali John Lumley, Tony Wiley Dept. of Computer Science HPLabs Bristol University of Bologna Filton Road, Stoke Gifford Mura Anteo Zamboni, 7 Bristol BS34 8QZ 40127, Bologna, Italy United Kingdom {diiorio | speroni | fabio}@cs.unibo.it {john.lumley | anthony.wiley}@hp.com ABSTRACT Transclusions were the core idea of Xanadu. Xanadu doc- The idea of transclusion has been at the same time the uments were built on-the-fly from fine-grained references so strength and weakness of Xanadu: some people considered that users could access, modify and reuse any fragment from it as an extremely powerful mechanism to get any version any document in a safe and controlled way. The implemen- of any fragment of any document in a global shared docu- tation of transclusions relies on external referencing to lo- ment space, others as a very complex solution too difficult to cations in a text data stream, through complex addressing be actually implemented and delivered. We believe transclu- mechanisms. Mark-up information, links and metadata are sions are still worth implementing and would allow designers expressly distinct from the flow of text in order to guarantee to build very sophisticated hypermedia applications. On the flexibility and expressiveness. other hand, we are aware that the original design of Xanadu cannot be implemented without uprooting current systems, The world of XML (and SGML) relies on a completely differ- protocols and technologies - in primis the World Wide Web ent strategy: mark-up is embedded, documents are strictly and XML. In fact, there is a great distance between the hierarchical and assertions about text fragments are made original data model of transclusions - strongly based on ex- by wrapping them with elements, enriched by attributes. ternal referencing mechanisms - and the XML data model Ted Nelson himself pointed out disadvantages of such an - strongly based on hierarchical structures and embedded approach in relation with transclusions[12]. The three objec- markup. tions he raised can be summarized as: (1) SGML approach This paper investigates to what extent the concept of tran- interposes a ‘forced’ structure between users and actual con- sclusion can be shaped for the world of XML, and stud- tent while editing, (2) SGML approach only supports well- ies simplified models for building functionalities inspired by formed inclusions and does not allow users to change in- Xanadu. Particular attention is given to the support for cluded content, (3) SGML approach does not support over- tracing fragments provenance in multi-source documents and lap and non-hierarchical relationships. for synchronizing distributed content through transclusions. The paper also traces a roadmap to actually implement tran- Thus, these two positions seem to be irreconcilable. The sclusions for XML - identifying three incremental steps - and goal of this paper is to investigate whether and how tran- briefly describes some experimental prototypes. sclusions can be implemented for XML documents and trace a possible course towards that goal. Categories and Subject Descriptors H.5.4 [ INFORMATION INTERFACES AND PRE- The preliminary step is to understand what we mean by SENTATION (I.7)]: Hypertext/Hypermedia ‘XML transclusions’. Our goal is not to re-implement a re- vised version of Xanadu based on XML technologies, rather to support users in creating and editing composite XML 1. INTRODUCTION documents that make some Xanalogical functionalities pos- sible. In particular, we are interested in: tracing fragment The concept of transclusion is rooted in the early days of provenance and remote manipulation. First of all, compos- hypertext[13]. A transclusion is a very advanced inclusion, ite XML documents would benefit from rich information whose content is not actually copied but stored as a virtual about the origin of each fragment. That makes it possible to reference to the original source. There is only one copy of identify single contributions in collaboratively edited docu- each fragment in the whole document space and transcluded ments, to display multiple changes in a single document, to data is permanently connected to the original. go back to original resources and navigate documents in a free and powerful way. The permanent connection between transcluded content and original sources would also make possible sophisticated forms of editing. Changes to remote documents could be propagated through transclusions or - the other way round - local modifications could actually up- date remote resources. Yet, such scenarios also require other tricky issues to be addressed such as content merging, syn- chronization, access permissions, reliability and so on. 23 The core of this paper discusses three approaches for imple- The design and implementation of CB-OHS follows the ‘phi- menting XML transclusions, in section 4. These solutions losophy of the primacy of structure over data’[16]. This is use different syntaxes and are progressively expressive and a key feature of the Open Hypermedia approach, strongly powerful. Before that, we briefly review the recent litera- related with the idea of transclusions: the separation be- ture about transclusions and XML. The paper also mentions tween the relationships and the information they relate to. some prototypes we are developing. It makes possible to handle data separately, to create links pointing to read-only reasorces, to create multiple and over- 2. TRANSCLUSIONS AND XML lapped links on the same content and so on. The two most recent research efforts for implementing tran- sclusions are outside the XML universe. Nelson and his 3. A CASE-STUDY: COLLABORATIVE RE- team proposed Transliterature[11] a revision of the original VIEWS Xanadu project built on newer technologies. Transliteral The idea of transcluding fragments from and to XML doc- documents are dynamically built on top of transcluded frag- uments is still under-developed. Hereinafter we discuss a ments, so that rich and fine-grained connections between road-map to make that development possible, by using a documents are permanently available. Two prototypes are case study throughout the paper. worth mentioning: Transquoter, that allows users to hide, highlight and surf multi-source quotations and XanaduS- Let us suppose we are building a system for supporting mul- pace, that provides users a 3D view of the overall document tiple users to write collaborative reviews about movies. It space and shows some of the advanced surfing and display- would be useful to let reviewers quote fragments from other ing functionalities envisioned for such inter-connected doc- reviews, keep trace of their source and maintain a ‘live’ chan- uments. nel between connected fragments. In fact, a review process can be improved by quoting opposite criticism, by letting Kolbitsch at al.[8] investigated transclusions for HTML, pre- reviewers access related reviews, by automatically updating senting a prototype that allows users to select content from distributed reviews with new material and by fostering dis- web pages and transclude them into new documents. Tran- cussion among reviewers. It is worth noting that such an sclusions directives are stored as in-line elements and very idea is both rooted in the early days of hypermedia (the complex URLs. Interesting issues are still open about con- essence itself of hypermedia is the inter-connection between tent addressing and merging, especially considering that web documents) and central in the recent trend of the World pages may change often and may have complex (and badly- Wide Web (one of the milestones of the so-called Web 2.0 is formed) internal structures. just collaboration). The closest solution to transclusions in the XML universe Consider now that reviewers are writing their comments is XInclude[10]. XInclude is a W3C standard for merging about ‘Australia’, starring Nicole Kidman and Hugh Jack- XML documents, by writing inclusion directives and retriev- man1. The film is one of the most controversial of the early ing other (parts of) documents. The focus of XInclude is 2009. David wrote a quite positive review: “Baz Luhrmann’s on well-formed XML fragments. Although even text frag- Australia is good, but not a masterpiece”.2 Brad has an op- ments can be included (parse="text"), XInclude does not posite opinion: “A major miscalculation if there ever was allow users to include bad-formed fragments or ranges. That one”3. Assume that Brad replied to David quoting his orig- makes it impossible to implement fully-fledged transclusions. inal note. In fact, a third reviewer - say Mike - might be Simplified forms of transclusions are possible through XIn- interested in quoting both these opinions, even by citing a clude anyway. XIPr[17] is an implementation of XInclude different comment by a fourth reviewer (for instance, Re- in XSLT 2.0, very efficient and simple to be integrated in becca saying “I actually regret having seen the film through other XML applications. XSLT technologies could be also to the end.”4). Figure 1 shows a possible view of such a used straightforwardly for simplified transclusions: a general composite review, highlighting multiple contributions5. approach is discussed in [5]. The multi-contribution view is only one of the applications XLink[3] could also be cited as a way to transclude pieces of such an advanced quoting. Users might also be given the of content. The @show and @actuate attributes allow users possibility to surf to the original review, in order to col- to define hot-links, that are similar to transclusions apart lect more information about the movie. Moreover, perma- from implementation details. On the other hand, such a nent connections between fragments could be enriched with solution is only partial and XLink does not seem to succeed metadata (stating, for instance, that the remote review is as expected. ‘positive’ or ’negative’), so that advanced search could be performed over the network of documents. Note also that XLink is a relative recent effort, rooted in a very long ex- 1http://www.australiamovie.com/ perience in the hypermedia research.