An Archival Resource Key (ARK) is a Uniform Resource Locator (URL) that is a multi-purpose persistent identifier for information objects of any type. An ARK contains 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 '') gain 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.
A complete NAAN registry is maintained by the California Digital Library and replicated at the Bibliothèque Nationale de France and the US National Library of Medicine. In 2015 it contained over 395 entries, some of which appear below.
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).
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