||High Energy Physics Libraries Webzine|
The impact of XML on library procedures and services
For example, in the High Energy Physics world, the omnipresence of the TeX text processing system and the availability of the Internet have achieved that all preprints are now electronically distributed and retrieved via the Internet. This development took both publishers and libraries by surprise. Now that the distribution of information is being taken care of by the HEP community, scientific publishers are forcing themselves out of this market by continuing to increase their journal prices. The organization of peer review is the only role left for the publisher and even this role could soon be taken away from them. Paul Ginsparg already claimed in 1994 that "certain physics journals currently play NO role whatsoever for physicists. Their primary role seems to be to provide a revenue stream to publishers, a revenue stream invisibly siphoned from overhead on research contracts through library systems..." If there are no more printed journals to store on shelves, the role of the library in the preprint publication process also becomes less clear. Since preprints are stored in full-text form, servers offer very good search capabilities, as good as those of a conventional library. With the Los Alamos preprint server available on every physicist's desktop, I would argue that the flow of scientific information via preprints would not be seriously impacted without libraries.
More traditional, and non HEP-specific library activities have also changed. Acquisition, cataloging, searching and borrowing are done online using automatic library systems with Web interfaces these days. Of course these systems can and are also used to store information on preprints but, in view of the above, we feel that this is redundant. At CERN, the library has gone a step further and has made its system available for the archival of physics experiment's internal documents. One drawback of this database is that the documents are only available in PostScript or PDF and not in full-text form. It is not impossible, but difficult and costly to recover meta data of the information contained in these documents, which is essential if a specific query from a search engine is to be successful. On the other hand, the current level of granularity (i.e. entire documents in PostScript or PDF form) seems to satisfy most users.
XML (the eXtensible Markup Language, see the XML web site) is a recent technology that has been hailed as the revolutionizer of the Web. Will XML have an impact on the library, and more generally on the flow of information in the HEP community?
The "X" familyXML (see its specification) is the language proposed by the W3 Consortium for describing documents that are to be delivered via the World Wide Web. It has the status of a W3 Consortium Recommendation (since February 1998). Although this is not the same as a full International Standard issued by the ISO, XML appears to be widely accepted today. A large amount of XML software is available (for free) from mainstream vendors such as Microsoft, IBM and others. The intention is that XML will add richer functionality to the Web where HTML is too limited to do so.
XML (see some literature) is a simplified subset of SGML (the "Standard Generalized Markup Language"), and as such, its use largely surpasses documents to be sent over the Internet. XML data is vendor and application independent. XML fits well into any software architecture that aims at re-using its components over a long period of time in a robust manner.
XML originated in 1996 from an initiative organized by some SGML software vendors. At that time, SGML was growing but struggling to become mainstream. The Web, on the other hand, had by that time become fully mainstream. HTML, the language that Web documents are written in, conforms to SGML, but the SGML vendors were almost unanimous in rejecting HTML because of its simplicity, its lack of flexibility and its too practical way of approaching hypertext links. In my opinion it was thanks to these "disadvantages" that HTML became such a succes! Anyhow, the SGML software industry had an impressive amount of products and services ready for the mainstream market: and so XML was born, a simplified subset of SGML designed to overcome the limitations of HTML.
Following XML, a number of other standards were developed, all starting with an "X".
In the past, the SGML community had also produced the HyTime and DSSSL standards, for describing hypertext linking and document formatting respectively. SGML, HyTime and DSSSL, although rigorously written and well thought out, all suffered from the fact that they were far too complex to understand or to implement. The titles of the standards themselves are already meaningless (actually, the "X" family doesn't score much better on that account). Now the people who made XML tried to learn from the difficulties of SGML, and XML clearly is much simpler to understand and implement than SGML. The HyTime and DSSSL (Document Style Specification and Semantics Language) standards were also re-implemented for XML and became XLINK (XML Link Language for robust hypertext links) /XPOINTER (XML Pointer Language) and XSL (XML Stylesheet Language, the standard for describing the visual representation of XML documents). Documents in XML are, as is the case for SGML, independent of a particular application. This is good, if you want to maximize exchange possibilities. If, however, you need to standardize on a particular application, you need a way of describing the semantics, which can be done using the XML Schema Language. Other standards exist, and more will undoubtedly follow.
The "X" activity has shown an eXplosive growth. Can this enthusiasm be indicative of a real change with a positive impact for users, not only consultants and software vendors?
Current users and activitiesA measure of its success can be seen from the number of industry sectors and projects where XML is being employed. Some examples are:
Three global areas for XML applicationsAnalysing the places where XML is being used, there seem to be three areas of potential use for XML:
Advantages of XML for librariesDespite its great potential, there are only a few places on the Web where the impact of XML on libraries is discussed. There are even less concrete examples of XML projects. XML is currently being used for literary textual documents with a fairly simple structure; it is being used instead of HTML because people can define their own tags. Some points in favour of XML (for libraries) are:
I should point out that ever since the beginning of the Web, there has been a misunderstanding of what HTML is about. The SGML community never understood that the use of HTML was a pure accident. HTML is a display language; any ASCII system could have replaced it, e.g. the 20 most common TeX commands, nroff/troff or even GML would have been adequate. A document's structure is irrelevant from the point of view of displaying it in an Internet browser. However, the fact that it is so simple, and so small, meant that it was easy to implement fast browsers. I don't think that the Web people consciously choose HTML because it was an SGML application (the majority of us didn't understand what an SGML application was in 1989). The HTML DTD was written a posteriori, under pressure from the SGML vendors. And so there was a lobby in favour of displaying SGML - an SGML browser is a lot more complicated to make, and it is usually sold for big $$$'s. But HTML is just fine for online display in most cases.
What's stopping us today from using XML instead of PDF and/or PostScript?As I have observed in the above, the HEP world likes to use PDF and/or PostScript as the format for distributing physics documents. The touchstone for a successful introduction of XML would be if we could replace PDF and/or PostScript by XML for storing and displaying HEP documents. The following major obstacles prevent us from doing this:
Provided that it will be sufficiently easy to create XML, both the library and its users will gain a significant advantage by using it, because XML documents will be fully searchable, and its meta data will be automatically extractable. Thus search engines will be able to profit from this by better responding to user's queries.
Even the XML Linking Language requires the endpoint of a link to be persistent, although it has some facilities for generating collections of links. Use of XML might make the results of a search engine more reliable, but this would not help for keeping it up to date. Use of the XML Linking Language would help to make hyperlinks inside static documents more robust. In any case broken links inside a document tend to be less of a problem as authors are fairly conscientious and Web authoring tools allow quick identification of broken links on the level of an entire Web site. However, the XML Linking Language will not solve the (more annoying) problem with search engines pointing to documents that no longer exist - because the search engine's database is not in sync with the status of the real data.
A tool such as FrameMaker, which already has full support for SGML, will undoubtedly soon have good support for XML, i.e. it will be as easy to extract XML from Frame as it is now to make PDF (or HTML from Word). As there is growing support using FrameMaker for complex technical documents at CERN (excluding mathematics), one can imagine that a lot of technical design reports for the LHC experiments will be available to the library in XML. As these documents are a priori not sent to a preprint server, the library could have an important role to play in this development. But for documents containing mathematical formulas TeX, PostScript and PDF will remain the norm.
The interchange medium need not necessarily be the Internet. By standardizing on XML, the number of conversion programs (symbolized by the blob containing "parser"), is linear in the number of different formats used by each respective company, instead of squared. By using XML as the exchange language, the exchanging partners become independent of each other, provided they can agree on a standard "Document Type Definition" (DTD, = a specific collection of XML tags). No wonder that the most commonly found applications for XML are in the domain of e-commerce and business-to-business applications.
But it's easy to get carried away by the hype. Surprisingly enough, XML is sometimes used as a layer inbetween a database and an HTML Web interface. Data is extracted from the database into XML, then converted into HTML. If one of the parties is a client on the Internet, and the data input is to be done from a Web browser, it's much simpler to use HTML forms. In this case, it seems to me that introducing XML is an extra, unnecessary complication.
In fact, one of the most often quoted applications of XSL is to convert documents from XML into HTML. Unless this is used as a temporary measure, awaiting the day when full XML support will be available in the common Web browsers, this use of XML seems to introduce an extra layer of complication.XML is to non textual, but numerical data (N.B. not binary data!). HEP is full of this, and it is no wonder that there are now many examples of physicists at CERN who are turning to XML as a general purpose data description language.
Some examples are described in the following.
WIREDWIRED is a system for displaying events (i.e. particle tracks, hits) from a collision inside a detector. The data on the display can be manipulated in many ways: one can zoom in and out, look at specialized projections, hide parts from the display and so on. In the figure below you can see hits on wires in a tracking chamber with a few reconstructed tracks.
Tracks generated by a Monte Carlo simulation program can be distinguished from reconstructed tracks by changing their colour or representation. WIRED is written in Java. To get the event and detector data (i.e. the coordinates of points, lines and polygons) into WIRED, WIRED has to be integrated into an analysis program (which then needs to be written in Java), or the data should be supplied in an application independent form such as XML. An XML loader has to be written that knows how to represent a <polygon> on the screen.
Traditionally, experiments have built their own event displays. The big advantage of this way of working is that the WIRED developers can concentrate and apply their expertise to the very specialized domain of event visualization, while the problem of getting the data into the system is handled once, by XML. The WIRED client, an experiment, only has to write its data out in XML form (possibly via a Web server). In principle any experiment can do this.
Detector Description Persistency in GaudiWithin the Gaudi framework, the LHCb experiment is using XML as a persistent way to store its detector description. The detector material and 3D geometry C++ objects (following the Geant 4 structure) are converted to XML for saving in a transient data store, to facilitate independent editing and creation by graphic XML tools. The following mapping between C++ objects and XML was used:
The advantage of using XML is that the detector geometry can be easily modified (e.g. adding a new subdetector) using an XML editor; and it can be easily mapped in both directions onto programmatic datastructures. A drawback is that there is no equivalence of neither methods nor inheritance in XML. This means that if a new C++ subclass is added, the XML DTD needs to be modified, and it is not easy to automate this.
The impact of these developments on library procedures will be indirect, as it will enhance people's awareness about XML, and reduce the learning curve for using XML for textual documents.