Archival Resource Key
ARK logo.png
OrganisationARK Alliance
Introduced2001 (2001)
No. issued8.2 billion
No. of digitsvariable
Check digitNCDA, optional

An Archival Resource Key (ARK) is a multi-purpose URL suited to being a persistent identifier for information objects of any type. It is widely used by libraries, data centers, archives, museums, publishers, and government agencies to provide reliable references to scholarly, scientific, and cultural objects. In 2019 it was registered as a Uniform Resource Identifier (URI).[1]

A URL that is an ARK is distinguished by the label ark: after the URL's hostname, which sets the expectation that, when submitted to a web browser, the URL terminated by '?' returns a brief metadata record, and the URL terminated by '??' returns metadata that includes a commitment statement from the current service provider. The ARK and its inflections ('?' and '??') provide access to three facets of a provider's ability to provide persistence.

Implicit in the design of the ARK scheme is that persistence is purely a matter of service and not a property of a naming syntax. Moreover, that a "persistent identifier" cannot be born persistent, but an identifier from any scheme may only be proved persistent over time. The inflections provide information with which to judge an identifier's likelihood of persistence.

ARKs can be maintained and resolved locally using open source software such as Noid (Nice Opaque Identifiers) or via services such as EZID. Most implementations are decentralized and no fees are charged for the right to assign ARKs. Some implementations choose to publish ARKs via the centralized N2T (Name-to-Thing) resolver.


  • NAAN: Name Assigning Authority Number - mandatory unique identifier of the organization that originally named the object
  • NMA: Name Mapping Authority - optional and replaceable hostname of an organization that currently provides service for the object
  • Qualifier: optional string that extends the base ARK to support access to individual hierarchical subcomponents of an object,[2] and to variants (versions, languages, formats) of components.[3]

A complete NAAN registry[4] is maintained by the ARK Alliance and replicated at the Bibliothèque Nationale de France and the US National Library of Medicine. It contained 530 entries in June 2018, 633 in July 2020, and 754 in April 2021.


ARKs may be assigned to anything digital, physical, or abstract. Below are examples, as reported (2020) to the ARK Alliance by the linked organizations.

Generic Services

Three generic ARK services have been defined. They are described below in protocol-independent terms. Delivering these services may be implemented through many possible methods given available technology (today's or future).

Access Service (access, location)

  • Returns (a copy of) the object or a redirect to the same, although a sensible object proxy may be substituted (for instance a table of contents instead of a large document).
  • May also return a discriminated list of alternate object locators.
  • If access is denied, returns an explanation of the object's current (perhaps permanent) inaccessibility.

Policy Service (permanence, naming, etc.)

  • Returns declarations of policy and support commitments for given ARKs.
  • Declarations are returned in either a structured metadata format or a human readable text format; sometimes one format may serve both purposes.
  • Policy subareas may be addressed in separate requests, but the following areas should be covered:
    • object permanence,
    • object naming,
    • object fragment addressing, and
    • operational service support.

Description Service

  • Returns a description of the object. Descriptions are returned in either a structured metadata format or a human readable text format; sometimes one format may serve both purposes.
  • A description must at a minimum answer the who, what, when, and where questions concerning an expression of the object.
  • Standalone descriptions should be accompanied by the modification date and source of the description itself.
  • May also return discriminated lists of ARKs that are related to the given ARK.

See also

Notes and references

  1. ^ "Uniform Resource Identifier (URI) Schemes".
  2. ^ Hierarchy qualifiers begin with a slash character.
  3. ^ Variant qualifiers begin with a dot character.
  4. ^ Name Assigning Authority Number registry

External links