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ARK: Archival Resource Key

ARKs are URLs designed to support long-term access to information objects. They identify objects of any type:

  • digital objects – documents, databases, images, software, websites, etc.
  • physical objects – books, bones, statues, etc.
  • living beings and groups – people, animals, companies, orchestras, etc.
  • intangible objects – places, chemicals, diseases, vocabulary terms, performances, etc.

ARKs are assigned for a variety of reasons:

  • affordability – there are no fees to assign or use ARKs
  • self-sufficiency – you can host ARKs on your own web server
  • flexibility – you can host ARKs on other servers without losing their core identities
  • portability – you can host ARKs at a well-known domain, such as, for global resolvability

Some unique advantages of ARKs:

  • simplicity – access relies only on mainstream web "redirects" and ordinary "get" requests
  • versatility – with "inflections" (different endings), an ARK should access data, metadata, promises, and more
  • transparency – no identifier can guarantee stability, and ARK inflections help users make informed judgements
  • visibility – syntax rules make ARKs easy to extract from texts and to compare for object variant and containment relationships

To date about 100 organizations have registered to assign ARKs.  Some of the largest users are

  • The California Digital Library
  • The Internet Archive
  • National Library of France (Bibliothèque nationale de France)
  • Portico Digital Preservation Service
  • University of California Berkeley
  • University of Chicago

We are very interested in building a community of users and will be announcing an email forum soon.  Here is a brief summary of other resources relevant to ARKs.

ARK Anatomy

An ARK is represented by a sequence of characters that contains the label, "ark:".  When embedded in a URL, it is preceded by the protocol  ("http://") and name of a service that provides support for that ARK. That service name, or the "Name Mapping Authority" (NMA), is mutable and replaceable, as neither the web server itself nor the current web protocols are expected to last longer than the identified objects. The immutable, globally unique identifier follows the "ark:" label. This includes a "Name Assigning Authority Number" (NAAN) identifying the naming organization, followed by the name that it assigns to the object.

Here is a diagrammed example:
\________________/ \__/ \___/ \______/ \____________/
(replaceable) | | | Qualifier
| ARK Label | | (NMA-supported)
| | |
Name Mapping Authority | Name (NAA-assigned)
(NMA) |
Name Assigning Authority
Number (NAAN)

The ARK syntax can be summarized,


The NMA part, which makes the ARK actionable (clickable in a web browser), is in brackets to indicate that it is optional and replaceable. ARKs are intended to work with objects that last longer than the organizations that provide services for them, so when the provider changes it should not affect the object's identity. A different provider hosting the object would simply replace the NMA to reflect the new "home" of the object. For example,

might become

Note that the ark:/NAAN/Name remains the same.

NAAN: Name Assigning Authority Number

The NAAN part, following the "ark:" label, uniquely identifies the organization that assigned the Name part of the ARK. Often the initial access provider (the first NMA) coincides with the original namer (represented by the NAAN), however, access may be provided by one or more different entities instead of or in addition to the original naming authority.

The NAAN used in the ARK anatomy diagram, 13030, represents the California Digital Library.  As of 2012, roughly a hundred organizations have registered for ARK NAANs, including numerous universities, Google, the Internet Archive, WIPO, the British Library, and other national libraries.

Any stable memory organization may obtain a NAAN at no cost and begin assigning ARKs. Please contact the CDL if you are interested in generating and using ARKs for your information objects.

UC3/CDL maintains a complete registry of all currently assigned NAANs, which is mirrored at the (U.S.) National Library of Medicine and the Bibliothèque nationale de France.

Generating ARKs

Because long-term identifiers often look like random strings of letters and digits, organizations typically use software to generate (or mint, in ARK parlance) and track identifiers. To mint ARKs, you may use any software that can produce identifiers conforming to the ARK specification. CDL uses the open-source NOID (nice opaque identifiers, rhymes with "employed") software, which creates minters and accepts commands that operate them. The noid software documentation explains how to use noid not only to mint identifiers but also to serve as an institution's "identifier resolver".

As an alternative to running your own infrastructure for creating, managing, and resolving ARKs and other identifiers, you may wish to use a service such as EZID. Operating on a cost-recovery basis, EZID permits you to create and manage an unlimited number of ARKs and DOIs (support for other kinds of identifiers is planned).

Once minted and publicized as being associated with a specific object, the ARK becomes a stable, unique, and compact reference that can be included in metadata records, databases, redirection tables, etc. It is often useful to generate and assign ARKs well before institutional commitment has been decided because it is easier than changing the original object identifier that may have been in long established use prior to that decision.

ARKs in Action – Inflections

An ARK provides extra services above and beyond that of an ordinary URL. Instead of connecting to one thing, an ARK should connect to three things:

  • the object itself,
  • a brief metadata record if you append a single question mark to the ARK, and
  • a maintenance commitment from the current server when you append two question marks.

This is a achieved through the use of "inflections", or different kinds of endings. With no ending, the ARK (in a URL) gives you what you expect from a web browser. If you add a single '?' to the end, for example,

it returns a brief machine- and eye-readable metadata record, such as

    who:   (:unav) unavailable
    what:  Truckee River, below Truckee Station, looking towards Eastern
            Summit. -- Photographer's number: 222 -- Photographer's series:
            Central Pacific Railroad, California.
    when:  (:unav) unavailable

Adding '??' to the end should return a policy statement. It is a side-benefit of ARKs that an object's metadata doesn't need an identifier different from that for the object, which cuts in half the number of identifiers that need to be generated and managed.

CDL Name Assignment and Support Policy Statements

The CDL assigns identifiers within the ARK domain under the NAAN 13030 and according to the following principles:

  • No ARK shall be re-assigned; that is, once an ARK-to-object association has been made public, that association shall be considered unique into the indefinite future.
  • To help them age and travel well, the Name part of CDL-assigned ARKs shall contain no widely recognizable semantic information (to the extent possible).
  • CDL-assigned ARKs shall be generated with a terminal check character that guarantees them against single character errors and transposition errors.

Institutions that generate ARKs may want to follow similar principles or develop their own assignment policies.

Similarly, but in the role of an NMA and not an NAA, institutions will want to develop service commitment statements for their objects.

In developing such statements, it is useful to recognize first, that managing a digital object may require altering it as appropriate to ensure its stability, and second, that the declared level of commitment may change as the requirements and policies for persistence become better understood over time, and as the institution implements procedures and guidelines for maintaining the objects that it manages.

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