Digital object identifier
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A digital object identifier (or DOI) is a permanent identifier given to a document, which is not related to its current location. A typical use of a DOI is to give a scientific paper or article a unique identifying number that can be used by anyone to locate details of the paper, and possibly an electronic copy. In this way it functions as a permalink. Unlike the URL system used on the Internet for web pages, the DOI does not change over time, even if the article is relocated (provided the DOI resolution system is updated when the change of location is made).
The DOI System uses the CNRI Handle System, a generic system for assigning names to objects, for name resolution. DOIs are handles having the prefix "10.", whereas other namespaces in the handle system have other handles. DOIs can be resolved through the DOI resolver at http://dx.doi.org; but, being handles, they can also be resolved through the global handle resolver at http://hdl.handle.net
Comparison with other standards
A DOI differs from commonly used internet pointers to material such as the Uniform Resource Locator because it identifies an object as a first-class entity, not simply the place where the object is located. A DOI also differs from identifiers such as International Standard Book Numbers (ISBNs), International Standard Recording Codes (ISRCs), etc.) because it can be associated with defined services and is immediately actionable on a network.
A DOI can apply to any form of document expressed in any digital environment, including both physical and digital manifestations, performances and abstract works: DOIs can be used to identify e-texts, images, audio or video items, software, etc. An entity can be identified at any arbitrary level of granularity. This means that, for instance, DOIs can identify a journal, an individual issue of a journal, an individual article in the journal, or a single table in that article.
The DOI consists of a unique alphanumeric character string divided into two parts: a prefix and a suffix.
An example of a complete DOI is:
- 10.1000 is the prefix, or publisher ID, composed by a part identifying the string as a DOI (10) and a part identifying the registrant (1000);
- 182 is the suffix, or item ID, identifying the single object. (Typical suffixes are longer than this example.)
The prefix is assigned by a DOI Registration Agency to a specific registrant. The suffix is assigned by the registrant and must be unique within a prefix. It can integrate existing standard identifiers such as an ISBN or ISSN, or Serial Item and Contribution Identifier (SICI). The DOI is case insensitive and is considered an "opaque string": nothing can be inferred from the number with respect to its use in the DOI System.
The correct way to cite a DOI on a webpage or in a publication is doi:10.1000/182
DOI resolution redirects the user from a DOI to one or more pieces of typed data: URLs representing instances of the object, services such as e-mail, or one or more items of metadata.
"What the DOI identifies" and "what the DOI resolves to" are two different concepts: it is possible that a DOI does not resolve to the identified entity, but just to some related information wished by the publisher.
DOI resolution is provided through the Handle System technology, developed by the Corporation for National Research Initiatives, and is freely available to any user encountering a DOI.
To resolve a DOI, just type in the address bar of any browser the string "http://dx.doi.org/" followed by the DOI. For example, to resolve the DOI 10.1000/182, enter into your browser the address: http://dx.doi.org/10.1000/182. Of course, web pages or other hypertext documents can include hypertext links in this form, as in this sentence which links to the DOI Handbook. Some browsers allow the direct resolution of a DOI (or other handles) with an add-on (e.g. Mozilla)
The DOI organization has applied for a "doi:" URI scheme to allow a DOI to be expressed as a Uniform Resource Identifier (URI) without requiring reference to a specific HTTP server as in the previous paragraph. As of April, 2006, this had not been approved. 
Each DOI is associated with a series of metadata, a set of bibliographical and commercial information concerning the content (title, author, publication date, copyright, price, etc.) and its position within the whole registrant's publishing offer (the belonging of a title to a series, of an article to a serial, the availability of one publication in more formats and/or through different media, etc.). By means of metadata, the DOI configures not simply as an identifying string, but takes the form of a powerful and unambiguous tool for data storage and exchange.
Metadata, as well as the DOI they are associated to, are persistently connected to the object they describe, so they can be easily communicated to other subjects across the productive and distributive chain, enhancing a content producer's ability to trade electronically. Furthermore, metadata represent the key for the development of DOI-based services, such as transnational databases and search engines for different kinds of contents. Asserting that metadata are persistent does not mean they are unmodifiable: registrants may update metadata about their contents any time they wish (whether some publication data change, when the primary URL the DOI resolves to is modified, etc.).
There are three main values granted by DOI adoption:
- Persistent Identification: each DOI unequivocally and permanently identifies the object to which it is associated
- Network Actionability: through Handle System technology, each DOI resolves to one or more web pages assigned by the publisher
- Semantic Interoperability: metadata allow to unambiguously communicate – to any user, from any place, at any point of the productive/distributive chain – all the pieces of information about the related objects and their hierarchical relationships
International DOI Foundation (IDF)
The International DOI Foundation (IDF), a non-profit organisation created in 1998, is the governance body of the DOI System, which safeguards all intellectual property rights relating to the DOI System. IDF supports the development and promotion of the Digital Object Identifier system as a common infrastructure for content management, and works to ensure that any improvements made to the DOI system (including creation, maintenance, registration, resolution and policymaking of DOIs) are available to any DOI registrant, and that no third party licenses might reasonably be required to practice the DOI standard.
IDF is controlled by a Board elected by the members of the Foundation, with an appointed full-time Director who is responsible for co-ordinating and planning its activities. Through the elected Board, the activities of the Foundation are ultimately controlled by its members. Membership is open to all organizations with an interest in electronic publishing and related enabling technologies.
A DOI Registration Agency (RA) is an authority recognized by the IDF, whose primary role is to provide services to DOI registrants: allocating DOI prefixes, registering DOIs and providing the necessary infrastructure to allow registrants to declare and maintain metadata and state data. RAs are also expected actively to promote the widespread adoption of the DOI, to cooperate with the IDF in the development of the DOI System as a whole and to provide services on behalf of their specific user community.
Currently, eight major RAs are active worldwide, as listed at www.doi.org:
- CrossRef (USA) - website
- R.R. Bowker (USA) - website
- Copyright Agency Limited (Australia) - website
- mEDRA (Europe) - website
- Nielsen BookData (UK) - website
- TIB (Germany) - website
- Publications Office (European Union) (EU) - website
- Wanfang Data (China) - website
- Uniform Resource Name
- Persistent Uniform Resource Locator
- Life Science Identifiers
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