What are ARKs?
ARKs (Archival Resource Keys) are high-functioning identifiers that lead you to things and to descriptions of those things. For example, this ARK,
https://n2t.net/ark:/67531/metadc107835/
gets you to a dissertation, and adding a '?' on the end of the ARK should get you to its description:
https://n2t.net/ark:/67531/metadc107835/?
What's an identifier?
On the internet, an identifier is a URL, or part of a URL. For example, this basic ARK identifier,
appears inside two different URLs (Uniform Resource Locators, also known as web links or web addresses):
https://gallica.bnf.fr/ark:/12148/btv1b8449691v/f29
https://n2t.net/ark:/12148/btv1b8449691v/f29
ARKs are especially good at being persistent identifiers.
What's a persistent identifier?
The average lifetime of a URL was once said to be 44 days. At the end of its life, a URL link breaks, meaning it gives you the dreaded "404 Not Found" error that most of us have seen. Irritating as that may be, it's a disaster for libraries, archives, museums, and other memory organizations.
A persistent identifier (sometimes abbreviated PID) is a link that in principle keeps working far into the future, even as things move between websites. Normally when things move, everyone who ever recorded the old links would need to be told what the new links are, which is next to impossible. That's where identifier resolvers come in.
What's a resolver?
A resolver is a website that specializes in forwarding incoming identifiers (the ones originally advertised to users) to whichever websites are currently best able to deal with them. Overall, forwarding is called resolution; one step in a resolution process is called redirection.
For a resolver to work, its hostname must be carefully chosen so won't ever need to be changed. Memory organizations, some of them centuries old, tend to have hostnames well-suited to be resolvers. Some well-known, younger resolvers are n2t.net (the ARK resolver), identifiers.org, doi.org, handle.net, and purl.org.
What are ARKs used for?
For anything and everything. ARKs are mostly used to identify digital content, such as genealogical records (FamilySearch), publisher content (Portico), and digitized manuscripts (Gallica), texts (Internet Archive), and museum holdings (Smithsonian). They are also used for datasets, journals, and vocabulary terms (yamz.net, perio.do). One group uses ARKs to identify historical figures (SNAC), and another to identify live customers.
How does ARK compare to DOI, Handle, PURL, and URN?
These are all major persistent identifier schemes (or identifier types). They have much in common, starting with structure:
https://n2t.net/ark:/99999/12345
https://doi.org/10.99999/12345
https://handle.net/10.99999/12345
https://purl.org/12345
https://<various>/urn:99999:12345
As seen in these examples, they all begin with the protocol (https://) plus a hostname, followed by the Name Assigning Authority (99999, 10.99999, or purl.org), which is the organization that created a particular identifier. Finally there's the name, or local identifier, that it assigned (12345). Some more things these schemes have in common:
- They all fail to stop the major causes of broken links: loss of funding, natural disaster, war, deliberate removal, human error, and provider neglect.
- They all burden the end provider with the responsibility to update forwarding tables as URLs change.
- They all give access to any kind of thing, whether digital, physical, abstract, person, group, etc.
- They all identify content that is subject to change on future visits.
- They all break regularly and in large numbers (thousands and more).
- They all use ordinary redirection built in to web servers since 1994 and provided for free by hundreds of URL shortening services.
- A non-trivial fraction of each scheme's identifiers will fail permanently, requiring forwarding to "tombstone" pages.
Given how little each scheme gives you, it is wise to consider cost, risk, and philosophy (eg, openness) when choosing one.
How do ARKs differ from identifiers like DOIs, Handles, PURLs, and URNs?
The short answer is that ARKs are the only mainstream, non-siloed, non-paywalled identifiers that you can register to use in about 24 hours. Over 500 registered organizations have created an estimated 3.2 billion ARKs in the world, and no one has ever paid for the right to create them.
That's not to say that making ARKs persistent is cost-free. Making any identifier persistent burdens you, the provider, with the costs of content management, hosting, monitoring, and forwarding. You can do those things yourself or with help from a vendor. With ARKs, just as with URLs, you will not be charged separately for your identifiers and you will not be locked in to a special-purpose resolution silo that also locks out other identifiers.
How else do they differ?
Here are some more differences between DOIs, Handles, PURLs, and URNs.
- When, not if (because all things pass), the
https://protocol and the hostname cease to exist, only ARKs and URNs will indicate the type of identifier that remains. - For DOIs, Handles, and PURLs, you are required to use their respective resolvers. ARKs and URNs, permit you to use your own resolver.
- To create DOIs and Handles, you are required to pay a membership fee and, for DOIs, per-DOI charges. There are no fees for ARKs, PURLs, and URNs.
- Although you can use your own or a vendor resolver for your ARKs and URNs, all ARKs can be resolved via n2t.net, making it the closest thing to a "global ARK resolver".
- The envisioned URN resolver was never built, so URNs are currently resolved as URLs, and there is no designated global URN-as-URL resolver. In order to register to create URNs, you must apply for a URN namespace.
- Unlike DOIs and Handles, ARKs don't have any metadata requirements and ARKs that haven't been released into the world can be deleted.
When should I use ARKs compared to DOIs, Handles, PURLs, and URNs?
There are no simple answers. Identifiers (not things, but their names) are tricky to talk about, so if you hear simple answers elsewhere, beware of common fallacies.
Nothing inherent in ARKs, DOIs, Handles, PURLs, or URNs makes them more or less fit for any particular field, domain, or sector. With an identifier resolver and administrative management service, they all provide the core service of resolution (and so do properly managed URLs).
The concrete differences that we experience, such as metadata (object descriptions), landing pages, and tool integration (eg, publishing tools), are not properties of identifier schemes per se, but properties of resolution, management, and citation services that various providers extend to or withhold from different identifier types. Those services are shaped in turn by communities of practice and by markets. Basic services are founded on a reliable database storing each identifier along with metadata elements (creator, title, date, redirection URL, etc) that describe the identified object. Extra services include link checking, duplicate detection, report generation, and searching.
Typically, scheme-based services are designed as silos ("walled gardens") to serve a particular identifier type (eg, Handle, DOI, or PURL). Each silo does the same basic things – mapping names (identifiers strings) to things (objects or metadata). Excluding all but one type of identifier string may help to capture markets, but it's wasteful and non-inclusive. It requires building the same set of services over and over for each type and violates basic principles of openness, so the N2T (Name-to-Thing) resolver and EZID (identifiers made easy) management interface were designed to work with all identifiers. Work put into any new feature can be efficiently leveraged across all types, which sometimes creates surprising flexibility; for example, ARKs are often stored in EZID with "DOI metadata", and every DOI stored in N2T can benefit from "ARK resolution features" such as inflections and suffix passthrough, which are not available via the main DOI resolver (doi.org).
Generalizations about identifier types sometimes apply when resolution and management for that type is locked into one particular vendor or provider. For example, many PURL and Handle features and restrictions are well-defined by their respective administration silos. DOIs, which are built on top of Handles, have the same resolution features and restrictions as Handles, but metadata practices are diverse and evolving across registration agencies. DOIs used to be known primarily as identifiers for scientific and scholarly publications, with a mature community and service offering around "Crossref DOIs", but newer kinds of DOIs, such as those from DataCite and EIDR, are changing the nature of the DOI.
Don't identifier types differ in metadata flexibility, content negotiation, inflections, and suffix passthrough?
Only one resolver, n2t.net, supports all of these features, and it does so for any identifier stored with appropriate metadata. Contrary to popular belief, identifiers don't do anything – it's their resolvers that do or don't support these features. For example, suffix passthrough is a feature supported by n2t.net (and purl.org, where it's called "partial redirect"), but not by doi.org or handle.net.
By metadata flexibility is meant the ability to store any metadata you want, including repeated elements, such as multiple authors and forwarding URLs, or no metadata at all. N2T has full metadata flexibility, while Crossref and DataCite have specific requirements (eg, the DataCite schema) to create their DOIs.
Content negotiation is a way for software to request descriptions of things that are not already in formats that might represent descriptions. Fortunately, to request descriptions without restriction, both humans and software can use inflections, exemplified by the '?' in the first answer. Backed by the right metadata, N2T is one of the few that does both.
Why doesn't the global ARK resolver (n2t.net) have the word "ARK" in it?
N2T (Name-to-Thing) is a resolver originally built for ARKs. Principles of openness prevented N2T from becoming just another DOI/Handle/PURL-type silo, which all perform the same basic functions. Thus the "global ARK resolver" also resolves DOIs, Handles, PURLs, URNs, and 600 other types of identifier.
This counter-silo principle can also be found in micro-service tools such as noid, which was built for ARKs and is widely used by organizations that mint ARKs and those that mint Handles.
I've heard of ORCIDs and UUIDs – where do they fit in?
Those are special kinds of persistent identifiers. ORCIDs only identify researchers, and they link to research works using ARKs, DOIs, etc. ORCIDs look like
https://orcid.org/0000-0001-7604-8041
UUIDs are globally unique, 37-character strings that are easy for software to generate but only become usable as web addresses when made part of a URL, for example, in this ARK:
https://n2t.net/ark:/65665/3c2e39526-e0c3-41ae-be4f-07558a9458eb
