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When CrossRef went live with its collaborative linking service in June 2000, it had enabled reference links in roughly 1,100 journals from a member base of 33 publishers, using a functional prototype system. The DOI-X prototype was described in an article published in D-Lib Magazine in February of 2000 [2]. On the occasion of CrossRef's first birthday as a live service, this article provides a non-technical overview of our progress to date and the major hurdles ahead.

 

 

The electronic medium enriches the research literature arena for all players - researchers, librarians, and publishers - in numerous ways. Information has been made easier to discover, to share, and to sell. To take a simple example, the aggregation of book metadata by electronic booksellers was a huge boon to scholars seeking out obscure backlist titles, or discovering books they would never otherwise have known to exist. It was equally a boon for the publishers of those books, who saw an unprecedented surge in sales of backlist titles with the advent of centralised electronic bookselling.

 

In the serials sphere, even in spite of price increases and the turmoil surrounding site licenses for some prime electronic content, libraries overall are now able to offer more content to more of their patrons. Yet undoubtedly, the key enrichment for academics and others navigating a scholarly corpus is linking, and in particular the linking that takes the reader out of one document and into another in the matter of a click or two. Since references are how authors make explicit the links between their work and precedent scholarship, what could be more fundamental to the reader than making those links immediately actionable? That said, automated linking is only really useful from a research perspective if it works across publications and across publishers. Not only do academics think about their own writings and those of their colleagues in terms of "author, title, rough date" - the name of the journal itself is usually not high on the list of crucial identifying features - but they are oblivious as to the identity of the publishers of all but their very favourite books and journals.

 

Citation linking is thus also a huge benefit to journal publishers, because, as with electronic bookselling, it drives readers to their content in yet another way. In step with what was largely a subscription-based economy for journal sales, an "article economy" appears to be emerging [3] . Journal publishers sell an increasing amount of their content on an article basis, whether through document delivery services, aggregators, or their own pay-per-view systems. At the same time, most research-oriented access to digitised material is still mediated by libraries. Resource discovery services must be able to authenticate subscribed or licensed users somewhere in the process, and ensure that a given user is accessing as a default the version of an article that their library may have already paid for. The well-known "appropriate copy" issue is addressed below.

 

Another benefit to publishers from including outgoing citation links is simply the value they can add to their own journals. Publishers carry out the bulk of the technological prototyping and development that has produced electronic journals and the enhanced functionality readers have come to expect. There is clearly competition among them to provide readers with the latest features.

 

That a number of publishers would agree to collaborate in the establishment of an infrastructure for reference linking was thus by no means predictable. CrossRef was incorporated in January 2000 as a collaborative venture among 12 of the world's top scientific and scholarly publishers, both commercial and not-for-profit, to enable cross-publisher reference linking throughout the digital journal literature. The founding members were Academic Press, a Harcourt Company; the American Association for the Advancement of Science (the publisher of Science); American Institute of Physics (AIP); Association for Computing Machinery (ACM); Blackwell Science; Elsevier Science; The Institute of Electrical and Electronics Engineers, Inc. (IEEE); Kluwer Academic Publishers (a Wolters Kluwer Company); Nature; Oxford University Press; Springer-Verlag; and John Wiley & Sons, Inc. Start-up funds for CrossRef were provided as loans from eight of the original publishers.

 

 

Since its inception, CrossRef has grown to include 70 leading journal publishers, both commercial and non profit, with over three million deposited records from 3,875 journals [4] . We are on track to add between 500,000 and one million new records per year. At present, the DOI Handle System resolves roughly 400,000 CrossRef DOI clicks per month. Hence, CrossRef has already transformed the experience of many scholars using electronic journals, by allowing them to go reliably from a citation in a given article to the full text of the cited material at another publisher's website.

 

CrossRef aims to become nothing less than the complete reference-linking backbone for all scholarly literature available in electronic form. Like the centralised electronic bookseller who provides an invaluable research tool because their aggregated metadata is so comprehensive, CrossRef will be measured by the robustness of its linking coverage; the usefulness of the system is directly proportional to the volume of linkable content. By that measure, we are still in our infancy. There are thousands of electronic journals and issues still to be added to the system. And, of course, journal articles do not exclusively cite other journal articles. Our linking capabilities must be extended to e-prints, conference proceedings, reference works, etc. Hence, we are rapidly recruiting new member publishers, adding other content types to the linking process, and expanding the range and depth of the citation coverage of participating journals to include more backlist issues.

 

CrossRef is not an exclusive club. Any publisher of primary research material can become a contributing member. Members are automatically assigned a DOI prefix, and their DOIs are registered as part of the metadata submission process. Affiliate access to our database of journal metadata and associated DOIs is also open to libraries, secondary publishers, aggregators, and researchers in the digital library arena.

 

 

CrossRef is a process, not a product. Each member publisher creates a DOI incorporating its own DOI prefix for each journal article, tagging it to article metadata and a URL. Records are assembled into batch file submissions to the CrossRef metadata database (MDDB) in a strict XML-based DTD format. CrossRef then registers each article DOI and URL in a central DOI directory. In a separate process, the publisher also submits the reference citations contained in each article to the reference resolver, a front-end component of the MDDB that allows for the retrieval of DOIs. The publisher can insert CrossRef links into any of an article's citations that point to content already registered in the CrossRef system. Figure 1 presents a schematic of this process.

 

 

In September of 2000, CrossRef became the first official DOI Registration Agency, authorised by the International DOI Foundation (IDF) [1] to allocate DOI prefixes, register DOIs, and provide the infrastructure to enable our members to declare and maintain metadata and state data. DOI-URL pairs are registered in the DOI System, which is an implementation of the Handle System^® [5] (a distributed computer system for naming digital objects and storing the names and the information needed to locate and access the objects via the Internet) managed by the Corporation of National Research Initiatives (CNRI) [6] on behalf of the IDF.

 

The CrossRef system uses open standards; the DOI syntax is itself an NISO standard [7]. CrossRef also employs a uniform set of rules to accommodate the publishers' right to control their business policies and maintain branding, while allowing the researcher to navigate through the widely distributed content of multiple publishers. The rules cover what can be named by a DOI, what types of data can be stored in the DOI System, and the way prefix holders can use DOIs on the Web. A researcher clicking on a CrossRef link will be connected to a page on the publisher's website showing a full bibliographical citation of the article, and, in most cases, the abstract as well. The reader can then access the full-text article. While subscribers can generally go straight to the text, non-subscribed users are presented with options for access. See Figure 2 .

 

 

In short, CrossRef provides a database of DOIs and metadata that enables DOI lookup, akin to a telephone book and directory assistance. If you know the DOI for an article, that's all you need to know in order to locate it persistently. If a publisher changes the location of an article, it need only update the URL for the article in one place. Some member publishers and affiliates have availed themselves of local hosting, which provides them with regularly refreshed local copies of the MDDB. By implementing some additional functionality, they can effect dynamic linking, generating links on the fly from an updated bank of deposits.

 

 

The shortcomings of the DOI-X functional prototype system were described in the Atkins et al. article published in February 2000 [2] . CrossRef has made considerable investment to date in refining the system hardware and software. Perhaps more importantly, we have established the legal, technical, and managerial framework to transform the prototype vision into a functioning company. With the groundwork firmly in place, there is now a major system upgrade underway that will greatly improve overall performance. Core requirements are highlighted and briefly discussed here.

 

Data validation. At present, roughly 10 percent of citation queries submitted to the MDDB fail to retrieve the correct DOI link for reasons of poor data quality. This is due mainly to errors in the bibliographic metadata itself, with a small number of failures attributable to inaccuracy of the URLs registered in the DOI Handle System. Stricter validation is thus needed to ensure the integrity of the data. While uploaded XML data is currently parsed and validated according to the upload DTD and data rules, one core specification of the system rewrite is a more sophisticated syntax check.

 

Parameter passing. The DOI is simply a number that redirects the user to a URL at the publisher's site. The publisher can tell very little about where the user is coming from and what terms of access should be granted. Some CrossRef publishers would understandably like to know more about the inbound links to their content. Parameter passing, which refers to when a key or some encoded text is sent along with a DOI link, would enable extra functionality that will benefit both publishers and end users.

 

The "parameter" could be information about the source article (i.e., the article containing the reference the user clicked on). Therefore, the publisher receiving the link would know the exact article and publisher the user came from. Crucially, no information about the individual user would be tracked or stored in a central location; resolution occurs within the DOI system, which is distinct from the CrossRef system. Each publisher could track the links that arrive at its own site, and would thus know which journals were linking to its own journals.