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To anything digital, physical, or abstract. That can include things that don't yet exist but to which you need to refer from objects that you're in the process of creating or planning, such as a link from a draft article to a dataset under preparation, or a link from an archived digital letter to a planned finding aid.
One caution is that you should generally assign ARKs to things that you own, control, or manage. Assigning ARKs to things you don't control is discouraged because such identifiers tend to be fragile.
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You are free to create ARK strings as you wish, provided you use only digits, letters upper- and lower-case letters (ASCII, no diacritics), and the following characters:
= ~ * + @ _ $ . /
The last two characters are reserved in the event you wish to disclose ARK relationships. A unique feature of ARKs is that hyphens ('-') may appear but are identity inert, meaning that strings that differ only by hyphens are considered identical; for example, these strings
ark:/12345/141e86dc-d396-4e59-bbc2-4c3bf5326152
ark:/12345/141e86dcd3964e59bbc24c3bf5326152
identify the same thing. The reason for this feature is that text formatting processes out in the world routinely introduce extra hyphens into identifiers, breaking links to any server that treats hyphens as significant.
Regarding assignment strategy, it is common to leverage legacy identifiers. For example, a museum moth specimen number cd456_f987 might be advertised under the ark:/12345/cd456_f987. Some legacy identifiers may need to be altered in view of ARK character restrictions.
The second common strategy is to make up entirely new strings for your ARKs. In this case it is important to consider whether to make them opaque or non-opaque (or a bit of both).
What are opaque identifiers?
Persistent identifier strings are typically opaque, deliberately revealing little about what they're assigned to, because non-opaque identifiers do not age or travel well. Organization names are notoriously transient, which is why NAANs are opaque numbers. As titles and dates are corrected, word meanings evolve (eg, innocent older acronyms may become offensive or infringing), strings meant to be persistent can become confusing or politically challenging. The generation and assignment of completely opaque strings comes with risk too, for example, numbers assigned sequentially reveal timing information and strings containing letters can unintentionally spell words.
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ARKs are not required to be opaque, but it is recommended that the base object name be made opaque, since it tends to name the main focus of persistence. If any qualifier strings follow that name, it may be less important that they be opaque. To help decide what your approach to opacity, you may wish to consider compatibility with legacy identifiers and ease of string generation and transcription (eg, brevity, check digits). New strings can be created (minted) with date/time, UUID, and number generators, as well as Noid (Nice Opaque Identifiers) minters.
Opaque strings are "mute" and therefore challenging to manage, which is why ARKs were designed to be "talking" identifiers. This means that if there's metadata, an ARK that comes in to your server with the '?' inflection should be able to talk about itself.
Anchor servingARKs servingARKs
How do I make server content addressable with ARKs?
| servingARKs | |
| servingARKs |
First, decide what the user experience of accessing your ARKs will be, for example, a spreadsheet file, a PDF, an image, a landing page filled with formatted metadata and a range of choices, etc. Whichever you choose, plan for your server to be able to respond with metadata if your ARK should arrive with a '?' inflection after it.
Otherwise, serving ARKs is like serving URLs. Normally incoming URL strings address (get mapped to) content that your web server returns. If your server is ARK-aware, incoming ARKs (expressed as URLs) must be mapped to the same content. A common approach is to map the ARK to the URL using a software table that you update whenever the URL changes. In this case your server is acting as a local resolver. If you don't want to implement this yourself, there are ARK software tools and services that can help.
Another approach is to run your web server without change, but instead of updating local tables, you would update ARK-to-URL mapping tables residing at a non-local resolver. Examples of this can be found among vendors and in any organization that updates tables via EZID.cdlib.org (which, due to a special relationship, updates resolver tables at n2t.net).
How do I advertise my ARKs?
ARKs meant for external use are generally advertised (released, published, disseminated) as actionable identifiers, in other words, as ARKs inside URLs. An important decision is whether your URL-based ARKs will use the hostname of your local resolver or the N2T.net resolver.
If local control or branding is important enough, you would advertise ARKs based at your local resolver. If you're concerned about the stability of your local hostname, you would advertise your ARKs based at n2t.net (see examples of both).
Resolving your ARKs through N2T is always possible for users, regardless of how you advertise them.
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Yes, ARKs can be assigned at any level of granularity, such as to a manuscript, to chapters inside it, to chapter sections, subsections, etc. An ARK can also be assigned to a thing that encloses other things. In ARKs the character '/' is reserved to help the recipient understand about containment, for example, the first ARK below contains the second ARK:
ark:/12148/btv1b8449691v
ark:/12148/btv1b8449691v/f29
That's the containment qualifier. There's only one other ARK qualifier, and it indicates variant forms of a thing by using the reserved character '.' in front of a suffix. For example, if these ARKs identify documents,
ark:/12148/btv1b8449691v/f29.pdf
ark:/12148/btv1b8449691v/f29.html
because they differ only by the suffix .pdf or .html, it can be inferred that they identify two different forms of the same document.
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ARKs are unusual in being decentralized. While one can get resolution services from a global ARK resolver called n2t.net, over 90% of the ARKs in the world are published without reference to it. More than 500 registered organizations across the world have created an estimated 3.2 billion ARKs, and, as with URLs, no one has ever paid an identifier fee to create them. Of course maintaining them isn't free. It is never without cost to keep content access persistent in the long term, regardless of identifier type.
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More differences between ARKs, DOIs, Handles, PURLs, and URNs
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- Landing pages: Crossref and DataCite DOIs link to publisher landing pages constructed around but not directly to objects you care about, but ARKs can freely link directly to objects you care about, which is machine- and human-friendly since it does not require an extra human navigation step for common tasks such as
- opening an article's PDF file for reading,
- referencing an image file meant to be incorporated automatically inline into a document, and
- citing a spreadsheet to be used for direct data analysis software.
- DOIs, Handles, etc. do not support ARK-style inflections that permit access to metadata regardless of whether an identifier points to an object or its landing page.
- Unlike DOIs and Handles, ARKs don't have metadata requirements. ARKs that haven't been released into the world are easy to delete.
- All things eventually pass, including hostnames and the web itself and the "
https://"protocol. When that first part of the identifier ceases to have meaning, only ARKs and URNs will include the label (eg, "ark:") indicating 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.
- To create Handles, you are required to install and maintain a local Handle server, which gives you another system to monitor, patch, and troubleshoot.
- Although you can use a local or vendor resolver for your ARKs and URNs, ARKs can be resolved via the global n2t.net resolver.
- The envisioned URN resolution infrastructure 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.
- ARKs have some unique features that support early object development: ARKs can be deleted, can be born with no metadata, and can exist with any metadata you care to store.
But if ARKs can be deleted, how can they be trusted?
Being able to delete identifiers It actually makes ARKs more trustworthy. The ability to delete is a vital part of healthy collection management that is denied to those non-ARK identifier types prohibiting deletion under the presumption that people, once they are asked to commit, won't make mistakes. People armed with identifier management software regularly turn simple human errors into big tangles of systematic large-scale mistakes, even at the threshold of commitment. By making it difficult to clean them up, we force systems to drag those messes forward in perpetuity.
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The concrete differences that we experience, such as metadata, 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.
What are usage trends for ARKs, DOIs, Handles, PURLs, and URNs?
As of 2019, purely on an incomplete and anecdotal level, here are a few trends that have been observed.
- ARKs have seen broad adoption in cultural memory institutions – museums, archives, and libraries. There is strong adoption in France and francophone regions.
- DOIs until recently have mostly been known as reliable identifiers for scientific and scholarly literature, when in fact this applies to a subset of DOIs assigned via Crossref. What it means to be a DOI is becoming harder to pin down because DOIs are being assigned to datasets, data management plans, field stations, etc. via DataCite, as well as to movies (eg, "Kung Fu Panda") via EIDR. Having said that, Crossref and DataCite DOIs have been successful in creating tools and services for scholarly publishers.
- PURLs have seen lots of use in identifying metadata vocabulary and ontology terms.
I've heard of ORCIDs, RORs, and UUIDs – where do they fit in?
Those are special kinds of persistent identifiers. ORCIDs (Open Researcher and Contributor Identifiers) only identify researchers, and they link to research works using ARKs, DOIs, etc. ORCIDs look like
https://orcid.org/0000-0001-7604-8041
ROR (Research Organization Registry) identifiers designate organizations. For example, here's the California Digital Library:
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://somehost.example.com/3c2e39526-e0c3-41ae-be4f-07558a9458eb
While embedding a UUID in an ordinary URL makes it actionable ("clickable"), you could expect more if it were embedded in an ARK such as
https://n2t.net/ark:/65665/3c2e39526-e0c3-41ae-be4f-07558a9458eb
As an ARK, for example, that UUID should return metadata (if available) and be insensitive to the hyphens, making this form equally viable:
https://n2t.net/ark:/65665/3c2e39526e0c341aebe4f07558a9458eb
From cradle to grave
When in my workflow should I create ARKs?
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