ARK namespaces and sub-namespaces

ARK namespace overview

ARK namespaces work much the same way that all namespaces work. Given a prefix associated with a namespace, this prefix can be “extended” (adding characters to the end of it) to create a new sub-namespace (directly under it) associated with the extended prefix. If the extended prefixes don’t conflict, nor will the names in the associated namespaces. There can be a namespace associated with any prefix you can think of, each with a potentially infinite number of names (ARKs) that start with it.

ARK namespaces begin with the Name Assigning Authority Number (NAAN), and can be further extended by a shoulder.

Set of all ARKs startingAssociated namespaceExample ARK in that namespace
ark:/All ARKsark:/99999/fk4gt2m
ark:/12345/ARKs under the NAAN 12345ark:/12345/p987654
ark:/12345/x5 ARKs under the 12345/x5 shoulderark:/12345/x5wf6789
ark:/12345/x5wf6789/ARKs under the 12345/x5wf6789 objectark:/12345/x5wf6789/c2/s4.pdf

The above table shows examples of four common namespace/sub-namespace levels. The first is for all ARKs and the second is for all ARKs under ark:12345. The third is the shoulder concept, described further below, which is the next subdivision under the NAAN; note that it has no “/” after it.

The fourth, a complete ARK-as-prefix example, shows that an object ARK is itself also a namespace, with an infinite number of “sub-ARKs” that could descend from it to name object parts and variants. Creating new namespaces to avoid naming conflicts is an ancient practice. For example, a family may refer to someone as Sam, the community as Sam Smith, the government as Sam Smith, 4321 Main Street, Springfield, and history as Sam Smith, 4321 Main Street, Springfield, 1888-1997.

ARK shoulder as namespace

A shoulder is a sub-namespace under a NAAN. It is the set all ARKs starting with a short, fixed extension to the NAAN. For example, in


the shoulder, /x5, extends the NAAN, 12345. The short designation, /x5, isn’t unique in many contexts, so the fully qualified, globally unique designation should be used (for example, ark:/12345/x5). In the classic namespace tradition, the shoulder is the set of all possible ARKs starting with the shoulder name. Our use of this term is borrowed from locksmithing, which understands sets of keys to be defined by fixed, unvarying “shoulders” that precede the varying “blades” (shapes that differ among keys sharing the same shoulder) that follow it.

Shoulders help organize a NAAN namespace for the long term. Just because a namespace contains an infinite number of possible ARKs does not mean that finding an unassigned ARK is easy, especially when over time there are – or were, or may be – different independent ARK assignment operations under it. Just as the ARK community sets aside organizations’ NAAN namespaces, each organization is encouraged to set aside shoulder sub-namespaces. If you don’t use shoulders from the beginning, even for one simple stream of assignments, you risk creating mild but permanent chaos in your NAAN namespace, and you may end up requesting an additional NAAN (which is discouraged) for future assignment streams.

A shoulder is analogous to a guest room in your house. Imagine a colleague, Sally, who takes in a long term lodger, Larry. Although her home is extremely spacious (in fact it is infinite), Sally complains that Larry leaves things permanently lying around in random spots all over the house: his coat on the kitchen chair, glasses on the dining table, book on Sally’s desk, slippers next to the sofa, coffee cup on the bathroom sink, etc. By the terms of his lodging agreement, Larry’s things, once placed, cannot be moved. But Sally, who also needs places for her things and might later take on new lodgers, is stuck forever noticing and trying not to disturb Larry’s stuff in parts of the house that she uses often.

Understanding Sally’s troubles, you might vow to require any guest of yours to agree to place things only in their room (their shoulder). Under such an agreement, not only would Sally’s home have been minimally disturbed by Larry’s stuff, but also she would be able to take on any number of new lodgers (new assigning operations) under similar agreements.

So shoulders allow ARK assignment under a NAAN to be delegated to autonomous projects or divisions, just as NAANs do under the overall ARK namespace. Even if an organization initially only needs to create ARKs for one project, plans may change. If other needs for ARKs arise later, setting aside a new shoulder for each new project or division makes it easy to ensure that independent assignment streams – present, past, or future – won’t conflict with each other, thanks to non-overlapping namespaces. (Shoulders can also ease the namespace splitting problem.) If you would like to learn more about shoulders, please see the brief ARK Shoulders FAQ.

Shared NAANs

There are four shared NAANs with special semantics that you might want to take advantage of. Normally, long term ARKs and their NAANs should be opaque, revealing little about what they’re assigned to, but the semantics in the table below are considered so immutable as to not risk their longevity. Each shared NAAN has particular connotations that software and people with enough training can recognize and benefit from, and this offers some relief from the challenge of using opaque identifiers. 

Shared NAANs are not owned by any one organization. In order to create ARKs without conflict under a shared NAAN requires, as you might imagine, reserving a shoulder, and that requires filling out an online form to request a shoulder under a shared NAAN (please don’t use this for shoulders under your own, non-shared NAAN).

Shared NAAN meaning Purpose, meaning, or connotation of ARKs with this NAAN.
(It’s ok for these NAANs to be non-opaque since their meanings are immutable.)
Expect to resolve? OK for long term reference?
12345 examples Example ARKs appearing in documentation. They might resolve, but no link checker need be concerned if they don’t. They should not be considered viable for long term reference.maybeno
99152 termsARKs for controlled vocabulary and ontology terms, such as metadata element names and pick-list values. They should resolve to term definitions and are suitable for long term reference.yesyes
99166 agentsARKs for people, groups, and institutions as “agents” (actors, such as creators, contributors, publishers, performers, etc). They should resolve to agent definitions and are suitable for long term reference.yesyes
99999 test idsARKs for test, development, or experimental purposes, often at scale. They might resolve, but no link checker need be concerned if they don’t. They should not be considered viable for long term reference.maybeno

The 99999 and 12345 ARKs (“non-real”) are especially useful if you are responsible for reviewing broken link reports. Unless you know otherwise, errors for ARKs with these NAANs can be ignored. This can save lots of wasted effort since, despite providers’ best efforts, such non-real ARKs frequently “escape into the wild” for all to see. Recipients (eg, people and link checkers) that would normally be concerned with broken links have only to recognize these two special NAANs in order to avoid being distracted by them. (Note that the non-real semantics remain even if the things don’t exist.)

The NAAN’s role in N2T ARK resolution

NAANs also play a key role in resolution. For example, if the resolver cannot find an incoming ARK in its database, it looks at the incoming NAAN and redirects the ARK to the local resolver registered with the NAAN. Any local resolver could be configured to return the favor for incoming ARKs containing NAANs that it doesn’t know about, simply by redirecting them to N2T.

How to make changes to a NAAN

Because the information in the NAAN registry can play a significant role in resolving your ARKs, you want to keep it up to date any time there is a change to your local resolver. You can request a change to the registry entry for a NAAN related to your organization by filling out the same online form used for requesting a new NAAN. For security purposes requests are processed manually. Example reasons for a change may include

  • notifying N2T of a change in your organization’s contact person or resolver URL,
  • updating your organization’s name assignment policy (sample policy),
  • requesting an additional NAAN, eg, to support a significant new body of ARKs or new organizational division, and
  • transitioning your NAAN to another organization that will carry on your work and future use of your NAAN.

NAANs are portable. If your organization transitions into or out of a vendor relationship, there is no impediment to taking your NAAN with you.