Handle System

The Handle System is the Corporation for National Research Initiatives's proprietary registry assigning persistent identifiers, or handles, to information resources, and for resolving "those handles into the information necessary to locate, access, and otherwise make use of the resources".[1]

As with handles used elsewhere in computing, Handle System handles are opaque, and encode no information about the underlying resource, being bound only to metadata regarding the resource. Consequently, the handles are not rendered invalid by changes to the metadata.

The system was developed by Bob Kahn at the Corporation for National Research Initiatives (CNRI). The original work was funded by the Defense Advanced Research Projects Agency (DARPA) between 1992 and 1996, as part of a wider framework for distributed digital object services,[2] and was thus contemporaneous with the early deployment of the World Wide Web, with similar goals.

The Handle System was first implemented in autumn 1994, and was administered and operated by CNRI until December 2015, when a new "multi-primary administrator" (MPA) mode of operation was introduced. The DONA Foundation[3] now administers the system's Global Handle Registry and accredits MPAs, including CNRI and the International DOI Foundation.[4] The system currently provides the underlying infrastructure for such handle-based systems as Digital Object Identifiers and DSpace, which are mainly used to provide access to scholarly, professional and government documents and other information resources.

CNRI provides specifications and the source code for reference implementations for the servers and protocols used in the system under a royalty-free "Public License", similar to an open source license.[5]

Thousands of handle services are currently running. Over 1000 of these are at universities and libraries, but they are also in operation at national laboratories, research groups, government agencies, and commercial enterprises, receiving over 200 million resolution requests per month.

Specifications

The Handle System is defined in informational RFCs 3650,[1] 3651[6] and 3652[7] of the Internet Engineering Task Force (IETF); it includes an open set of protocols, a namespace, and a reference implementation of the protocols. Documentation, software, and related information is provided by CNRI on a dedicated website[8]

Handles consist of a prefix which identifies a "naming authority" and a suffix which gives the "local name" of a resource. Similar to domain names, prefixes are issued to naming authorities by one of the "multi-primary administrators" of the system upon payment of a fee, which must be renewed annually. A naming authority may create any number of handles, with unique "local names", within their assigned prefixes. An example of a handle is:

  • 20.1000/100
  • 2381/12345

In the first example, which is the handle for the HANDLE.NET software license, 20.1000 is the prefix assigned to the naming authority (in this case, Handle.net itself) and 100 is the local name within that namespace. The local name may consist of any characters from the Unicode UCS-2 character set. The prefix also consists of any UCS-2 characters, other than "/". The prefixes consist of one or more naming authority segments, separated by periods, representing a hierarchy of naming authorities. Thus, in the example 20 is the naming authority prefix for CNRI, while 1000 designates a subordinate naming authority within the 20 prefix. Other examples of top-level prefixes for the federated naming authorities of the DONA Foundation are 10 for DOI handles; 11 for handles assigned by the ITU; 21 for handles issued by the German Gesellschaft für wissenschaftliche Datenverarbeitung mbH Göttingen (GWDG), the scientific computing center of the University of Göttingen; and 86 for the Coalition of Handle Services – China. Older "legacy" prefixes issued by CNRI before the "multi-primary administrator" (MPA) structure was instituted are typically four of five digits, as in the second example above, a handle administered by the University of Leicester. All prefixes must be registered in the Global Handle Registry through an DONA Foundation approved registrar, normally for a fee.

As with other uses of handles in computing, the handle is opaque; that is, it encodes no information about the underlying resource and provides only the means to retrieve metadata about the resource.

This may be contrasted with a Uniform Resource Locator (URL), which may encode within the identifier such attributes of the resource as the protocol to be used to access the server holding the resource, the server host name and port number, and perhaps even location specifics such as the name of a file in the server file system containing the resource. In the Handle System, these specifics are not encoded in the handle, but are found in the metadata to which the handle is bound.

The metadata may include many attributes of the information resource, such as its locations, the forms in which it is available, the types of access (e.g. "free" versus "paid") offered, and to whom. The processing of the metadata to determine how and where the resource should be accessed, and the provision of the resource to the user, are performed in a separate step, called "resolution", using a Resolver, a server which may be different than the ones involved in exchanging the handle for the metadata. Unlike URLs, which may become invalid if the metadata embedded within them becomes invalid, handles do not become invalid and do not need to change when locations or other metadata attributes change. This helps to prevent link rot, as changes in the information resource (such as location) need only be reflected in changes to the metadata, rather than in changes in every reference to the resource.

Each handle may have its own administrator(s) and administration of the handles can be done in a distributed environment, similar to DNS domain names. The name-to-value bindings may also be secured, both via signatures to verify the data and via challenge response to verify the transmission of the data, allowing handles to be used in trust management applications.

It is possible for the same underlying information resource to be associated with multiple handles, as when two university libraries generate handles (and therefore possibly different sets of metadata) for the same book.

The Handle System is compatible with the Domain Name System (DNS), but does not require it, unlike persistent identifiers such as PURLs or ARKs, which are similar to handles, but which utilise domain names. However, unlike these domain-name based approaches, handles do require a separate prefix registration process and handle servers separate from the domain name servers.

Handles can be used natively. or expressed as Uniform Resource Identifiers (URIs) through a namespace within the info URI scheme;[9][10] for example, 20.1000/100 may be written as the URI, info:hdl/20.1000/100. Some Handle System namespaces, such as Digital Object Identifiers, are "info:" URI namespaces in their own right; for example, info:doi/10.1000/182 is another way of writing the handle for the current revision of the DOI Handbook[11] as a URI.

Some Handle System namespaces define special presentation rules. For example, Digital Object Identifiers, which represent a high percentage of the extant handles, are usually presented with a "doi:" prefix: doi:10.1000/182.

Handles may also be expressed as Uniform Resource Locators (URLs) through the use of a HTTP proxy server,[12] such as:

Implementation

Implementation of the Handle System consists of Local Handle Services, each of which is made up of one or more sites that provide the servers that store specific handles. The Global Handle Registry is a unique Local Handle Service which stores information on the prefixes (also known as naming authorities) within the Handle System and can be queried to find out where specific handles are stored on other Local Handle Services within this distributed system.

The Handle System website provides a series of implementation tools, notably the HANDLE.NET Software[13] and HANDLE.NET Client Libraries.[14] Handle clients can be embedded in end user software (e.g., a web browser) or in server software (e.g., a web server) and extensions are already available for Adobe Acrobat[15] and Firefox.[16]

Handle client software libraries are available in both C and Java. Some applications have developed specific add-on tools, e.g., for the DOI System.[17]

The interoperable network of distributed handle resolver servers (also known as the Proxy Server System) are linked through a Global Resolver (which is one logical entity though physically decentralised and mirrored). Users of Handle System technology obtain a handle prefix created in the Global Handle Registry. The Global Handle Registry maintains and resolves the prefixes of locally maintained handle services. Any local handle service can, therefore, resolve any handle through the Global Resolver.

Handles (identifiers) are passed by a client, as a query of the naming authority/prefix, to the Handle System's Global Handle Registry (GHR). The GHR responds by sending the client the location information for the relevant Local Handle Service (which may consist of multiple servers in multiple sites); a query is then sent to the relevant server within the Local Handle Service. The Local Handle Service returns the information needed to acquire the resource, e.g., a URL which can then be turned into an HTTP re-direct. (Note: if the client already has information on the appropriate LHS to query, the initial query to GHR is omitted)

Though the original model from which the Handle System derives dealt with management of digital objects, the Handle System does not mandate any particular model of relationships between the identified entities, nor is it limited to identifying only digital objects: non-digital entities may be represented as a corresponding digital object for the purposes of digital object management. Some care is needed in the definition of such objects and how they relate to non-digital entities; there are established models that can aid in such definitions e.g., Functional Requirements for Bibliographic Records (FRBR), CIDOC CRM, and indecs content model. Some applications have found it helpful to marry such a framework to the handle application: for example, the Advanced Distributed Learning (ADL) Initiative[18] brings together Handle System application with existing standards for distributed learning content, using a Shareable Content Object Reference Model (SCORM),[19] and the Digital Object Identifier (DOI) system implementation of the Handle System has adopted it together with the indecs framework to deal with semantic interoperability.

The Handle System also makes explicit the importance of organizational commitment to a persistent identifier scheme, but does not mandate one model for ensuring such commitment. Individual applications may choose to establish their own sets of rules and social infrastructure to ensure persistence (e.g., when used in the DSpace application, and the DOI application).[20]

Design principles

The Handle system is designed to meet the following requirements to contribute to persistence[21]

The identifier string:

  • is not based on any changeable attributes of the entity (location, ownership, or any other attribute that may change without changing the referent's identity);
  • is opaque (preferably a ‘dumb number’: a well known pattern invites assumptions that may be misleading, and meaningful semantics may not translate across languages and may cause trademark conflicts);
  • is unique within the system (to avoid collisions and referential uncertainty);
  • has optional, but nice to have, features that should be supported (human-readable,cut-and-paste-able, embeddable; fits common systems, e.g., URI specification).

The identifier resolution mechanism:

  • is reliable (using redundancy, no single points of failure, and fast enough to not appear broken);
  • is scalable (higher loads simply managed with more computers);
  • is flexible (can adapt to changing computing environments; useful to new applications):
  • is trusted (both resolution and administration have technical trust methods; an operating organization is committed to the long term);
  • builds on open architecture (encouraging the leverage efforts of a community in building applications on the infrastructure);
  • is transparent (users need not know the infrastructure details).

Applications

Among the objects that are currently identified by handles are journal articles, technical reports, books, theses and dissertations, government documents, metadata, distributed learning content, and data sets. Handles are being used in digital watermarking applications, GRID applications, repositories, and more. Although individual users may download and use the HANDLE.NET software independently, many users have found it beneficial to collaborate in developing applications in a federation, using common policy or additional technology to provide shared services. As one of the first persistent identifier schemes, the Handle System has been widely adopted by public and private institutions and proven over several years. (See Paradigm, Persistent identifiers.)[22]

Handle System applications may use handles as simple persistent identifiers (as most commonly used, to resolve to the current URL of an object), or may choose to take advantage of other features. Its support for the simultaneous return as output of multiple pieces of current information related to the object, in defined data structures, enables priorities to be established for the order in which the multiple resolutions will be used. Handles can, therefore, resolve to different digital versions of the same content, to mirror sites, or to different business models (pay vs. free, secure vs. open, public vs. private). They can also resolve to different digital versions of differing content, such as a mix of objects required for a distance-learning course.

There are thousands of handle services running today, located in 71 countries, on 6 continents; over 1000 of them run at universities and libraries. Handle services are being run by user federations, national laboratories, universities, computing centers, libraries (national and local), government agencies, contractors, corporations, and research groups. Major publishers use the Handle System for persistent identification of commercially traded and Open Access content through its implementation with the Digital Object Identifier (DOI) system.

The number of prefixes, which allow users to assign handles, is growing and stands at over 12,000 as of early 2014. There are six top-level Global Handle Registry servers that receive (on average) 68 million resolution requests per month. Proxy servers known to CNRI, passing requests to the system on the Web, receive (on average) 200 million resolution requests per month. (Statistics from Handle Quick Facts.)

In 2010, CNRI and ITU (International Telecommunication Union) entered into an agreement to collaborate on use of the Handle System (and the Digital Object Architecture more generally) and are working on the specific details of that collaboration; in April 2009 ITU listed the Handle System as an "emerging trend".[23]

Licences and use policy

Handle System, HANDLE.NET and Global Handle Registry are trademarks of the Corporation for National Research Initiatives (CNRI), a non-profit research and development corporation in the USA. The Handle System is the subject of patents by CNRI, which licenses its Handle System technology through a public license,[24] similar to an open source license, in order to enable broader use of the technology. Handle System infrastructure is supported by prefix registration and service fees, with the majority coming from single prefix holders. The largest current single contributor is the International DOI Foundation. The Public License allows commercial and non-commercial use at low cost of both its patented technology and the reference implementation of the software, and allows the software to be freely embedded in other systems and products. A Service Agreement[5] is also available for users who intend to provide identifier and/or resolution services using the Handle System technology under the Handle System public license.

Related technologies

The Handle System represents several components of a long-term digital object architecture. In January 2010 CNRI released its general-purpose Digital Object Repository software,[25] another major component of this architecture. More information[26] about the release, including protocol specification, source code and ready-to-use system, clients and utilities, is available.[27][28]

See also

References

  1. ^ a b "RFC 3650: Handle System Overview".
  2. ^ "Kahn/Wilensky Architecture". CNRI. 1995-05-13. Retrieved 2013-03-13.
  3. ^ "Welcome to The DONA Foundation".
  4. ^ "The DOI System".
  5. ^ a b "Redirect to Current Handle.Net web site content". www.handle.net. Retrieved 15 March 2018.
  6. ^ "RFC 3651: Handle System Namespace and Service Definition".
  7. ^ "RFC 3652: Handle System Protocol (ver 2.1) Specification".
  8. ^ "handle.net". handle.net. Retrieved 2013-03-13.
  9. ^ "About "info" URIs – Frequently Asked Questions". Info-uri.info. Retrieved 2013-03-13.
  10. ^ "RFC 4452: The "info" URI Scheme for Information Assats with Identifiers in Public Namespaces".
  11. ^ "DOI Handbook". doi:10.1000/182.
  12. ^ "HDL.NET Services: Proxy Server System". Handle.net. Retrieved 2013-03-13.
  13. ^ "HS Software Download". Handle.net. Retrieved 2013-03-13.
  14. ^ "Software Client Libraries". Handle.net. Retrieved 2013-03-13.
  15. ^ "HDL Plug-in for Adobe Acrobat and Acrobat Reader". Handle.net. Retrieved 2013-03-13.
  16. ^ [1] Archived September 5, 2015, at the Wayback Machine
  17. ^ "DOI System Tools". Doi.org. 2012-07-12. Retrieved 2013-03-13.
  18. ^ "adlnet.gov". adlnet.gov. Retrieved 2013-03-13.
  19. ^ "SCORM". adlnet.gov. Archived from the original on 2008-06-06.
  20. ^ "doi.org". doi.org. 2013-01-08. Retrieved 2013-03-13.
  21. ^ "Identifier Systems in Network Architecture, Laurence Lannom, CNRI. Video of presentation (or presentation PDF only) from the Digital Motion Picture Metadata Symposium, Science & Technology Council, Academy of Motion Picture Arts & Sciences, 11 June 2009". Oscars.org. 2012-08-24. Archived from the original on 2013-03-30. Retrieved 2013-03-13.
  22. ^ "workbook on digital private papers | administrative and preservation metadata | persistent identifiers". paradigm. 2008-01-02. Retrieved 2013-03-13.
  23. ^ "Handle System". Itu.int. 2010-04-16. Retrieved 2013-03-13.
  24. ^ http://www.handle.net/HSj/hdlnet-2-LICENSE.pdf
  25. ^ "dorepository.org". dorepository.org. 2013-01-08. Retrieved 2013-03-13.
  26. ^ "Digital Object Repository Server: A Component of the Digital Object Architecture". Dlib.org. 2010-02-04. Retrieved 2013-03-13.
  27. ^ "DO Repository". DO Repository. doi:10.1045/january2010-reilly. Retrieved 2013-03-13.
  28. ^ "Cordra".

External links

Corporation for National Research Initiatives

The Corporation for National Research Initiatives (CNRI), based in Reston, Virginia, is a non-profit organization founded in 1986 by Robert E. Kahn as an "activities center around strategic development of network-based information technologies", including the National Information Infrastructure (NII) in the United States.CNRI develops the Handle System for managing and locating digital information. CNRI obtained DARPA funding for the development of JPython (Jython), a Python implementation in and for Java, initially created by Jim Hugunin.

Digital identity

A digital identity is information on an entity used by computer systems to represent an external agent. That agent may be a person, organization, application, or device. ISO/IEC 24760-1 defines identity as "set of attributes related to an entity".The information contained in a digital identity allows for assessment and authentication of a user interacting with a business system on the web, without the involvement of human operators. Digital identities allow our access to computers and the services they provide to be automated, and make it possible for computers to mediate relationships.

The term "digital identity" has also come to denote aspects of civil and personal identity that have resulted from the widespread use of identity information to represent people in computer systems.

Digital identity is now often used in ways that require data about persons stored in computer systems to be linked to their civil, or national, identities. Furthermore, the use of digital identities are now so widespread that many discussions refer to "digital identity" as the entire collection of information generated by a person’s online activity. This includes usernames and passwords, online search activities, birth date, social security, and purchasing history. Especially where that information is publicly available and not anonymized, and can be used by others to discover that person's civil identity. In this wider sense, a digital identity is a version, or facet, of a person's social identity. This may also be referred to as an online identity.The legal and social effects of digital identity are complex and challenging. However, they are simply a consequence of the increasing use of computers, and the need to provide computers with information that can be used to identify external agents.

Digital object identifier

In computing, a Digital Object Identifier or DOI is a persistent identifier or handle used to identify objects uniquely, standardized by the International Organization for Standardization (ISO). An implementation of the Handle System, DOIs are in wide use mainly to identify academic, professional, and government information, such as journal articles, research reports and data sets, and official publications though they also have been used to identify other types of information resources, such as commercial videos.

A DOI aims to be "resolvable", usually to some form of access to the information object to which the DOI refers. This is achieved by binding the DOI to metadata about the object, such as a URL, indicating where the object can be found. Thus, by being actionable and interoperable, a DOI differs from identifiers such as ISBNs and ISRCs which aim only to identify their referents uniquely. The DOI system uses the indecs Content Model for representing metadata.

The DOI for a document remains fixed over the lifetime of the document, whereas its location and other metadata may change. Referring to an online document by its DOI is supposed to provide a more stable link than simply using its URL. But every time a URL changes, the publisher has to update the metadata for the DOI to link to the new URL. It is the publisher's responsibility to update the DOI database. If they fail to do so, the DOI resolves to a dead link leaving the DOI useless.

The developer and administrator of the DOI system is the International DOI Foundation (IDF), which introduced it in 2000. Organizations that meet the contractual obligations of the DOI system and are willing to pay to become a member of the system can assign DOIs. The DOI system is implemented through a federation of registration agencies coordinated by the IDF. By late April 2011 more than 50 million DOI names had been assigned by some 4,000 organizations, and by April 2013 this number had grown to 85 million DOI names assigned through 9,500 organizations.

E-Theses Online Service

E-Theses Online Service (EThOS) is a bibliographic database and union catalogue of electronic theses provided by the British Library, the National Library of the United Kingdom. As of March 2018 EThOS provides access to approximately 480,000 doctoral theses awarded by over 140 UK higher education institutions, with around 3000 new thesis records added every month.

EIDR

EIDR, or the Entertainment Identifier Registry, is a global unique identifier system for a broad array of audio visual objects, including motion pictures, television, and radio programs. The identification system resolves an identifier to a metadata record that is associated with top-level titles, edits, DVDs, encodings, clips, and mash-ups. EIDR also provides identifiers for Video Service providers, such as broadcast and cable networks.

As of February, 2018, EIDR contains over 1.8 million records, including 298K movies, and 794K episodes of over 25K TV series.

EIDR is an implementation of a Digital Object Identifier (DOI).

F5 Networks

F5 Networks, Inc. is a global company that specializes in application services and application delivery networking (ADN). F5 technologies focus on the delivery, security, performance, and availability of web applications, as well as the availability of servers, cloud resources, data storage devices, and other networking components. F5 is headquartered in Seattle, Washington, with additional development, manufacturing, and sales/marketing offices worldwide.

Known originally for its load balancing product, today F5's product and services line has expanded into all things related to the delivery of applications, including local load balancing and acceleration, global (DNS based) load balancing and acceleration, security through web application firewall and application authentication and access products, DDoS defense. F5 technologies are available in the data center and the cloud, including private, public, and multi-cloud environments based on platforms such as AWS, Microsoft Azure, Google Cloud, and OpenStack.

HDL

HDL may refer to one of the following:

Hardware description language

Handle System identifier (Hdl.handle.net)

High-density lipoprotein, so-called "good cholesterol"

Les Hurlements d'Léo, an alternative rock band from France

GE HDL diesel engine

Huey, Dewey, and Louie, Disney-characters, nephews of Donald Duck

HDL System, HDL Universal Tactical role-playing game system produced by Tremorworks, LLC

Huntington's disease-like syndromes, a family of genetic neurological diseases

Hybrid library

Hybrid library is a term used by librarians to describe libraries containing a mix of traditional print library resources and the growing number of electronic resources.

Identifier

An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique class of objects, where the "object" or class may be an idea, physical [countable] object (or class thereof), or physical [noncountable] substance (or class thereof). The abbreviation ID often refers to identity, identification (the process of identifying), or an identifier (that is, an instance of identification). An identifier may be a word, number, letter, symbol, or any combination of those.

The words, numbers, letters, or symbols may follow an encoding system (wherein letters, digits, words, or symbols stand for (represent) ideas or longer names) or they may simply be arbitrary. When an identifier follows an encoding system, it is often referred to as a code or ID code. For instance the ISO/IEC 11179 metadata registry standard defines a code as system of valid symbols that substitute for longer values in contrast to identifiers without symbolic meaning. Identifiers that do not follow any encoding scheme are often said to be arbitrary IDs; they are arbitrarily assigned and have no greater meaning. (Sometimes identifiers are called "codes" even when they are actually arbitrary, whether because the speaker believes that they have deeper meaning or simply because they are speaking casually and imprecisely.)

The unique identifier (UID) is an identifier that refers to only one instance—only one particular object in the universe. A part number is an identifier, but it is not a unique identifier—for that, a serial number is needed, to identify each instance of the part design. Thus the identifier "Model T" identifies the class (model) of automobiles that Ford's Model T comprises; whereas the unique identifier "Model T Serial Number 159,862" identifies one specific member of that class—that is, one particular Model T car, owned by one specific person.

The concepts of name and identifier are denotatively equal, and the terms are thus denotatively synonymous; but they are not always connotatively synonymous, because code names and ID numbers are often connotatively distinguished from names in the sense of traditional natural language naming. For example, both "Jamie Zawinski" and "Netscape employee number 20" are identifiers for the same specific human being; but normal English-language connotation may consider "Jamie Zawinski" a "name" and not an "identifier", whereas it considers "Netscape employee number 20" an "identifier" but not a "name". This is an emic indistinction rather than an etic one.

Info URI scheme

In computing, info is a Uniform Resource Identifier (URI) scheme which enables identifiers from public namespaces to be represented as URIs, when they would otherwise have no canonical URL form, such as Library of Congress identifiers, Handle System handles, and Digital object identifiers.

InterStream Transit Protocol

Protocol Helper: istp://

Organization: InterStream Industry AssociationThe InterStream Transit Protocol (ISTP) is a secure protocol that allows Internet Service Providers (ISPs) to offer high bandwidth multimedia services over their networks. The ISTP application is installed as a new protocol handler into browsers. It includes components which enable Internet Service Providers to obtain incremental revenue from subscribers, advertisers, other ISPs, and streaming video providers in exchange for using premium bandwidth services on their networks.

ISTP uses the handle system to index and reference rich media objects on the existing Web. Content is indexed into the handle system by users with ISTP clients who access existing HTTP, RTP, or MMS content. Once the content is indexed, it can be natively referenced by the handle system and streamed over the InterStream Media Grid infrastructure. Indexed rich media objects from the InterStream Media Grid are associated with individual portals. Portals are designated by a "handle prefix", istp://prefix_name. The prefix_name designates a portal for members of the InterStream association. By default, istp:// designates the "default" InterStream portal.

By linking ISTP content, Web Site owners and hosts may embed video from other providers. An overt goal of the protocol has been to fix the pervasive copyright issues surrounding the distribution of video on the Internet. ISTP as a new MIME subtype and protocol handler enables web site owners to link to the video in the form in which the content owner wishes it to be presented.

International Geo Sample Number

The International Geo Sample Number or IGSN is a sample identification code of typically nine characters. As an active persistent identifier it can be resolved through the Handle System. The system is used in production by the System for Earth Sample Registration (SESAR), Geoscience Australia, Commonwealth Scientific and Industrial Research Organisation Mineral Resources, Australian Research Data Commons (ARDC), University of Bremen MARUM, and German Research Centre for Geosciences (GFZ). Other organisations are preparing the introduction of the IGSN.

The IGSN preserves the identity of a sample even as it is moved from lab to lab and as data appear in different publications, thus eliminating ambiguity that stems from similar names for samples from the earth. The IGSN unique identifier allows researchers to track the analytical history of a sample and build on previously collected data as new techniques are developed. Additionally, the IGSN provides a link between disparate data generated by different investigators and published in different scientific articles.

Link rot

Link rot (or linkrot) is the process by which hyperlinks on individual websites or the Internet in general tend to point to web pages, servers or other resources that have become permanently unavailable. There is no reliable data on how long web pages and other resources survive: the estimates vary dramatically between different studies, as well as between different sets of links on which these studies are based (see the #Prevalence section).

NIC handle

A NIC handle (Network Information Centre handle) is a unique alphanumeric character sequence that represents an entry in the databases maintained by Network Information Centres. When a new domain name is registered with a domain name registrar, a NIC handle is assigned by the registrar to the particular set of information associated with that domain name (such as who registered it and a contact e-mail address). Once a domain name has been registered, its NIC handle can be used to search for that record in the database.The NIC handle was developed in 1982 by Ken Harrenstien and Vic White working at the early Network Information Center at SRI International.

Persistent identifier

A persistent identifier (PI or PID) is a long-lasting reference to a document, file, web page, or other object.

The term "persistent identifier" is usually used in the context of digital objects that are accessible over the Internet. Typically, such an identifier is not only persistent but actionable: you can plug it into a web browser and be taken to the identified source.

Of course, the issue of persistent identification predates the Internet. Over centuries, writers and scholars developed standards for citation of paper-based documents so that readers could reliably and efficiently find a source that a writer mentioned in a footnote or bibliography. After the Internet started to become an important source of information in the 1990s, the issue of citation standards became important in the online world as well. Studies have shown that within a few years of being cited, a significant percentage of web addresses go "dead," a process often called link rot. Using a persistent identifier can slow or stop this process.

An important aspect of persistent identifiers is that "persistence is purely a matter of service." That means that persistent identifiers are only persistent to the degree that someone commits to resolving them for users. No identifier can be inherently persistent.

Persistent identifiers are often created within institutionally administered systems. These include:

Archival Resource Keys (ARKs)

Electronic Identifier Serial Publications (EISPs)

Digital Object Identifiers (DOIs), the Handle System

International eBook Identifier Numbers (IEINs)

Persistent Uniform Resource Locators (PURLs)

Uniform Resource Names (URNs)

Extensible Resource Identifiers (XRIs)

Magnet link (decentralized, with BitTorrent)However, some regular URLs (i.e. web addresses), maintained by the website owner, are intended to be long-lasting; these are often called permalinks.

Web archiving services such as perma.cc, archive.is, and WebCite offer anyone the ability to archive a web page and create their own persistent identifier for it.

Programmable logic array

A programmable logic array (PLA) is a kind of programmable logic device used to implement combinational logic circuits. The PLA has a set of programmable AND gate planes, which link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output. It has 2N AND Gates for N input variables, and for M outputs from PLA, there should be M OR Gates, each with programmable inputs from all of the AND gates. This layout allows for a large number of logic functions to be synthesized in the sum of products canonical forms.

PLAs differ from Programmable Array Logic devices (PALs and GALs) in that both the AND and OR gate planes are programmable.

Snips

Snips, also known as shears, are hand tools used to cut sheet metal and other tough webs. There are two broad categories: tinner's snips, which are similar to common scissors, and compound-action snips, which use a compound leverage handle system to increase the mechanical advantage.

Uniform Resource Name

A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) that uses the urn scheme.

Use error

The term use error has recently been introduced to replace the commonly used terms human error and user error. The new term, which has already been adopted by international standards organizations for medical devices (see #Use errors in health care below for references), suggests that accidents should be attributed to the circumstances, rather than to the human beings who happened to be there.

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