As soon as in summer 1999, Blackhorse's Insight On-Line Library became a registrant of the Digital Object Identifier (DOI®). The Digital Object Identifier (DOI®) is a system for persistent identification and interoperable exchange of intellectual property on digital networks. It provides an extensible framework for managing intellectual content in any form, at any level of granularity and in any digital environment. The International DOI Foundation, a non-profit organization, manages development, policy, and licensing of the DOI system to registration agencies. DOIs may be used to identify any intellectual property entity, including those already identified by systems such as ISBN, and can be used compatibly with ISBN.

Structure of a DOI

The DOI has two components, the prefix and the suffix, which together form the DOI. There is no limitation on the length of a DOI. A DOI may be assigned to any item of intellectual property, which must be precisely defined by means of structured metadata. The DOI itself remains persistent through ownership changes, and unaltered once assigned. A prefix is assigned to an organization that wishes to register DOIs; any organization may choose to have multiple prefixes. Following the prefix (separated by a forward slash) is a suffix (unique to a given prefix) to identify the entity. The combination of a prefix for the Registrant and unique suffix provided by the Registrant avoids any necessity for the centralized allocation of DOI numbers. An existing standard identification system number such as ISBN may be incorporated into a DOI, by using this as the suffix, if the registrant finds it convenient to do so (it is course recommended that precisely the same entity be identified by the two systems). DOI is not alone in being a system that can incorporate existing identifiers: for example, physical bar codes can be used to express ISBNs.

Features of DOI

The value of the DOI system lies in its combination of Resolution, Metadata and Policy. The DOI system uses a Resolution System which ensures persistence by resolving the DOI to a current associated value such as a URL; users of DOIs need not be aware of changes to URLs in order to use the system. The DOI system is a URI and URN implementation. The Resolution System is the Handle System, an open standard scalable architecture. Resolution may be to multiple pieces of data. The DOI system uses a Data Model based on the (interoperability of data in e-commerce systems) activity, consistent with metadata systems such as ONIX and MPEG-21 RDD. The Data Model enables mappings between application areas to be made consistently. Initial DOI applications used the features of the resolution system; later applications build on the data model feature. The Data Model embraces both a data dictionary and a framework for applying it. The data dictionary component is designed to ensure maximum interoperability with existing metadata element sets, such as ONIX (with which IDF works closely) and Dublin core. The framework allows the terms to be grouped in meaningful ways so that certain types of DOIs all behave the same way in an application. DOI policy and governance provide rules and mechanisms for implementation which achieve practical implementation in a similar way to ISBN, EAN/UCC codes, Visa numbers etc., by means of a number of Registration Agencies which operate under the same rules as an operational federation. Added value services may be built using DOI features. These include the use of multiple resolution (associating DOIs with several items of data); associating related pieces of intellectual property (versions, derivations, etc); use with other tools (e.g. OpenURL for contextual local use).

Benefits of DOI

By integrating an identifier into a DOI, the identifier becomes actionable as a standard hyperlink (but unlike URL, persistent), and can function in DOI applications across a variety of platforms. A variety of different identifier systems become readily interoperable when incorporated into DOIs. DOIs may be assigned to ISBN entities (books) to achieve this; DOIs may be used to identify related entities or linked material in any form. Using one system enables one set of tools to be applicable across many platforms, media, standard identification schemes, etc. and promotes interoperable transactions of intellectual property.

Furthermore, Blackhorse joined the unique initiative called CrossRef. CrossRef is a not-for-profit network founded on publisher collaboration, with a mandate to make reference linking throughout online scholarly literature efficient and reliable. As such, it is an infrastructure for linking citations across publishers, and the only full-scale implementation of the Digital Object Identifier (or DOI) System to date. What a DOI is a unique alphanumeric string assigned to a digital object – in this case, an electronic journal article or a book chapter. In the CrossRef system, each DOI is associated with a set of basic metadata and a URL pointer to the full text, so that ituniquely identifies the content item and provides a persistent link to its location on the internet. For more information on the DOI itself, which is a NISO standard syntax, please visit the International DOI Foundation website at www.doi.org. For details on use of the DOI within CrossRef, please see our DOI Guidelines.

What CrossRef is not:

• A product for sale
• An article database
• A direct-to-end-user service
• A search interface
• A broker of full-text content
• Made up of just big commercial publishers

CrossRef's mission

To serve as the complete citation linking backbone for all scholarly literature online, as a means of lowering barriers to content discovery and access for the researcher. We are currently expanding our citation linking services beyond journal articles, to conference proceedings and books.

Who developed CrossRef?

Scholarly publishers developed CrossRef. The initial service was based on a prototype developed by John Wiley & Sons and Academic Press, in cooperation with the International DOI Foundation (IDF). It built on the DOI-X project led by the IDF, Association of American Publishers (AAP) and the Corporation for National Research Initiatives (CNRI).

Key benefits of the CrossRef system:

• No state links: Because a DOI link is a persistent link, unlike a URL , publishers and others who use CrossRef create reliable, persistent links in citations and database records.
• A single agreement with CrossRef serves as a linking agreement with all participating publishers. Avoid having to sign numerous bilateral linking agreements with publishers.
• Add value to your electronic publications: Readers have come to expect online material to contain outbound links to cited sources. At the same time, CrossRef linking will augment the accessibility of your content through inbound links.

In a separate process, the publisher also submits the citations contained in each deposited article to the Reference Resolver, the front-end component of the MDDB that allows for the retrieval of DOIs.This way, the publisher can, as part of its electronic production process, add outbound links to any of an article’s citations that point to content already registered in the CrossRef system. The CrossRef website includes technical specifications for querying and a demo of the DOI look-up process. 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 with CrossRef.