|No. issued||8.2 billion|
|No. of digits||variable|
|Check digit||NCDA, 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).
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.
A complete NAAN registry 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.
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).