This is an extended, revised version of . Findability and interoperability of some PIDs and their compliance with the FAIR data principles are explored, where ARKs were added in this version. It is suggested that the wide distribution and findability (e.g. by simple 'googling') on the internet may be as important for the usefulness of PIDs, as the resolvability of PID URIs. This version also includes new reasoning about the failure to use PIDs such as DOIs for citation. The prevalence of phenomena such as link rot implies that the persistence of URIs cannot be trusted. By contrast, the well distributed, but seldom directly resolvable ISBN identifier has proved remarkably resilient, with far-reaching persistence, inherent structural meaning and good validatability, by means of fixed string-length, pattern-recognition, restricted character set and check digit. Examples of regular expressions used for validation of PIDs are supplied or referenced. The suggestion to add context and meaning to PIDs, making them "identify themselves", through namespace prefixes and object types is more elaborate in this version. Meaning can also be conferred by means of structural elements, such as well defined, restricted string patterns, that at the same time make PIDs more "validatable". Concluding this version is a generic, refined model for a PID with these properties, in which namespaces are instrumental as custodians, meaning-givers and validation schema providers. A draft example of a Schematron schema for validation of "new" PIDs in accordance with the proposed model is provided.

Introduction: Identifiers in science

Identifiers in science may refer to digital or physical objects, or concepts. They may also refer to persons, researchers, as ORCiD IDs , or to research organizations, as the recently launched RORs . This paper will focus on persistent identifiers, PIDs, for research outputs, 'things' such as articles, datasets, samples, concepts etc. But, as suggested in section 7, ORCiDs or RORs may be an optional part of a modular, integrated identifier for research outputs. PIDs may be general or domain-specific. Among the more prevalent general PID-types are ARK, DOI, Handle and UUID. There are also old, bibliographic identifiers like ISBN. Created in the 1960's and -70's of the print era, how come they survived into this digital age? Some reasons might be: they are well distributed across the internet, and widely used by stake-holders (libraries, publishers, readers). They have a semantic structure, identifying well-defined objects, and a fairly precise validation mechanism through fixed string-lengths, limited character-set and check digits. Some of these properties of good identifiers are shared by ARKs, DOIs, Handles and UUIDs, or other more domain specific identifiers used for scholarly data, but seldom all of them simultaneously. The focus here is on findability and 'validatability' of PIDs of different types.

Identifiers - why do we need them?

The general purpose of identifiers is to serve as references to the objects that they are supposed to identify. This requires identifiers to indicate, preferably in and by themselves, what type of objects they are meant to identify. Far from all identifiers fulfil this requirement. It is often left to the names of things to describe the objects identified, providing context and meaning. Context may be added by means of location within an hierarchical system, e.g. as in Linnéan taxonomy, where scientific names situate a species within a genus, sometimes also containing the provenance of that name, serving to disambiguate beween names of species belonging to widely different genera, e.g. Asterina gibbosa Gaillard 1897 (a fungus), and Asterina gibbosa(Pennant, 1777) (an echinoderm, a starfish). For the same purpose, it also happens that 'things', objects are renamed later, as with the preceding fungus species now having the accepted scientific name Asterolibertia gibbosa (Gaillard) Hansf. 1949 and / or are assigned an identifier: urn:lsid:catalogueoflife.org:taxon:02af8238-ac8f-11e3-805d-020044200006:col20150401. . However, even if a PID may well serve the need for disambiguation by uniquely identifying an object, it may still be no better - sometimes, as in this case, perhaps even worse at giving access to said object, or at least a landing page with metadata about it. The identifier assigned above is neither directly resolvable nor 'googlable', while the scientific name is at least easily findable via a search engine.

While scientific names are often useful for describing objects, they have other drawbacks compared to PIDs, some of which were identified by . For example, homonymy and disambiguation should be no problem for globally unique identifiers. And while concatenations or abbreviations may be problematic in the use of names for identification, string-length and pattern restrictions are useful for validation of identifiers. Missing or added characters, all types of misspellings are easier to detect and validate in standardized identifiers of fixed string-length or well-defined character patterns. Inconsistent encoding should also not be a problem in good PIDs, with limited character sets. However, these assets of some identifiers may conflict with the interest in having also transparent, meaningful PIDs that at least in part "speak for themselves". The result of a compromise between these two interests may be seen in the Handle system (below).

FAIR principles

The FAIR guiding principles aim to make (meta)data Findable, Accessible, Interoperable, and Re-usable. As such they concern also PIDs, as is seen from some of the principles:

The FAIR data principles.

The FAIR data principles.

The FAIR principles clearly need interpretation to become fully operational, and such work is also well in progress. Further explications of some of the principles are also available in . Figuring prominently in the explications of all these principles, particularly for interoperability, is the requirement that metadata should be machine actionable, a conditio sine qua non for FAIRness.] This is also used by fairmetrics.org as a measure of Findability.

However, the FAIR principles do not say anything explicitly about validation. Particularly for the principles of Interoperability and Re-usability, it is crucial that metadata can be properly validated against a schema, as adhering to an accepted metadata standard. It has been remarked that this is already implied by the FAIR principle R1.3 above, but if so, only indirectly and still open to interpretation. There are several cases where general data repositories, professing to be FAIR and adhere to accepted metadata standards both for their default output and export formats, nevertheless fail to validate against schemas of these same standards. Fairmetrics.org explicates R1.3, as measuring a "Certification from a recognized body, of the resource meeting community standards", by means of a valid electronic signature, such as a verisign signature. However, one might ask whether general data repositories such as Harvar'ds Dataverse,Figshare or Zenodo, qualify as "recognized bodies" in this respect, all being part of the test reported in "Evaluation_Of_Metrics/Supplementary Information_ FM Evaluation Results.pdf" , but none of which could be evaluated on R1.3. This comes as no surprise, since there is already a comment on R1.3 saying that "Such certification services may not exist, but this principle serves to encourage the community to create both the standard(s) and the verification services for those standards." True, in the rationale for FM_R1.3 there is mention of validation: "... As such, data should be (individually) certified as being compliant, likely through some automated process (e.g. submitting the data to the community's online validation service)". But it remains unclear what the "community" here refers to a general metadata standard, or a repository, using it's own standard and validation service? Some output metadata files from repositories even lack a schemaLocation reference, making it difficult to validate them, or, the schemaLocation given might be erroneous, as observed in one case. We cannot just wait for repositories themselves to provide verification of compliance with standards. We must use available validation tools, testing to what extent they are keeping their promises of compliance compliance. This concerns metadata in general, but naturally includes also identifiers. We must be sure that they are of the type or format they claim to be, even if they cannot be resolved to a dedicated landing-page. Failed validation, e.g. due to simple typos or wrong namespace, may even be one way of checking why an identifier or URI does not resolve as expected. Validation is also important for the possibility to export metadata to another format, thereby promoting the re-use of data, without exporting also potential errors. Resistance to transcription errors, e.g. by means of a restricted character set, using base32 for encoding, and fixed string-length (suffix has 2 times 4 characters, separated by a hyphen), has been promoted as an advantage of so-called "cool DOIs". These are precisely the kind of properties that make PIDs eminently "validatable" and thereby machine-actionable, a crucial requirement for FAIRness. Although transformation or harvesting of metadata might be possible even without validation, trust in the results and quality as well as the eventual findability of the data (and so again the re-usability) might be seriously affected. For enhanced findability, it is also important that standard, widely distributed identifiers are used.

Validation of an identifier means ensuring that it is true to its proclaimed type, for example, making sure that what is flagged as an ISBN is not in fact an ISSN (real use case), or that the string-length and check-sum is compliant with its type. An advantage of promptly validatable identifiers is that validation can often be performed also off-line, by means of a pattern expressed by a regular expression, a piece of script (JavaScript, Python, etc.), an HTML form, or a schema (e.g. XSD or Schematron) and a piece of software such as an XML-editor.

Resolvable or findable?

In the present FAIR principles the focus is very much on resolvability of identifiers, despite the general awareness of phenomena like 'link rot' and 'reference rot'. It has even been suggested to put up digital tombstones over disappeared resources, with metadata from their last known whereabouts serving as epitaphs. A 2013 study in BMC Bioinformatics analyzed nearly 15,000 links in abstracts from Thomson Reuters’ Web of Science citation index and found that the median lifespan of web pages was 9.3 years, and just 62% were archived. This happens although there is an understanding that [u]nique identifiers, and metadata describing the data, and its disposition, should persist -- even beyond the lifespan of the data they describe. A recent study of some 40 research data repositories found that only one of these (3%) was compliant with the FAIR principle of Accessibility requiring a clear policy statement (or various examples of data this has actually happened to) indicating that metadata is still available even if the data is removed. The argument here is not that resolvable, persistent URIs should be avoided as identifiers; most often they serve their purpose of more persistently giving access to a metadata source than "ordinary", plain URLs. But, as has been eloquently remarked, "persistent URIs must be used to be persistent". Resolvable URIs as PIDs work by decoupling the location and the identification functions of URIs.

The custodian of a web resource maintains the correspondence between the identifying URI and the locating URI in the resolver’s look-up table as the resource’s location changes over time. ... The solution comes at a price because it requires operating a resolver infrastructure and maintaining the look-up table that powers it.

This is true of ARKs, DOIs, as well as Handles, PURLs and URNs. There are in fact numerous cases when the lookup-table is not maintained and updated as required. That is why it may be wise not to rely on a single 'custodian' for the resolution of identifiers and access to associated metadata. Note that we are not talking here about simply having more than one proxy server acting as resolvers of the same PIDs. We already have that; provided the lookup-table is managed properly, the three different DOI-URIs from three different proxy-servers all resolve to the same landing-page location: https://doi.org/10.1007/978-3-319-53637-8_11, https://hdl.handle.net/10.1007/978-3-319-53637-8_11 and https://identifiers.org/doi:10.1007/978-3-319-53637-8_11. ARKs (Archival Resource Keys) are resolved by identifiers.org and n2t.net, as well as by their "mother institutions", e.g. n2t.net/ark:/67531/metapth346793/, identifiers.org/ark:/67531/metapth346793/ and digital.library.unt.edu/ark:/67531/metapth346793/ resolve the same content. It is rather the distribution and use of identifiers - whether resolvable or not - that is important here. It seems not even the authors of are true to their own principles, since three of their references that actually have DOIs are cited without them: . So, despite having DOIs or other PIDs assigned, documents are often not cited by those PIDs. One possible reason might be that the PID is not clearly displayed in the landing page with metadata, or in the document itself. In the case of above it takes an extra click on a link 'Cite as' to actually have the DOI displayed. But that should hardly be the reason why it was not used for citation in , since the citation there is actually much more verbose and complex, than it would have been to just copy-paste from the 'Cite as' page above. Another, slightly ironic case concerns , the founding paper of the FAIR principles, where you either have to download the citation with the DOI from the landing page, where it is not displayed, or find it at the bottom of each page in the actual paper, but not prominently marked. This may partly explain why a recent paper on software sustainability and reproducibility, while arguing that one of the ways to make software more reproducible is to "use a persistent identifier such as a Digital Object Identifier (DOI) to help find and cite code" , failed itself to use the DOI when citing . Another reason, gathered from one of the authors by personal communication, but which I believe could be generalized, is that inclusion of the DOI (or other PID) was not part of the citation style of the publisher. In fact, it sometimes happens that publishers impose their own citation formats or standards, excluding the use of PIDs. Again, PIDs must be used and cited to persist. Citations help in achieving wide distribution, depending on good 'validatability', in order not to multiply errors and 'non-resolution' as a result.

Again, going back to the question of resolvability, the relationship between identifiers such as DOIs and URIs/IRIs is not always straightforward, and sometimes involves a chain of redirects ('303s'), before reaching a destination holding also the appropriate metadata. . Another reason resolvability may not be sufficient, even if the metadata is somehow in place, is that the file on the destination page resolved to is behind a paywall. In a case from December 2016, public domain content more than 110 years old was hidden behind a DOI-resolver charging 50$ for release of the content.

When someone in an ensuing Twitter conversation complained about this, an answering tweet seemed to mean, that was the price we have to pay for something as useful as DOIs. There was also a remark to the effect that every object gets one and only one DOI. But this is not necessarily true. It is possible to mint several DOIs for the same resource by different agents, such as Dataverse, Figshare, Zenodo etc. If not, that would give commercial publishers the opportunity to pro-actively seize whatever public domain content there is out there on the internet, quickly mint and assign their DOI to it and then lock it up behind paywalls. In any event, it would certainly not promote the use of PIDs instead of simple, ephemeral URLs for citation.

The DOI in question then was: 10.1080/00222930908692639. The remedy proposed to "jump over" the paywall was by means of what was then oadoi.org , which still worked quite well. In several steps it eventually led, via an API, to an XML-file with a link to the freely accessible fulltext at http://www.biodiversitylibrary.org/part/60220. However, things have changed. When tried again in November 2018, the replacement unpaywall.org and oadoi-API for 10.1080/00222930908692639 was not working anymore for this DOI; the response received was: best_oa_location: null. But the resource sought, free from paywall, although no longer detected by unpaywall.org, could still be found e.g. at biodiversitylibrary.org. However, often you will not find a free replacement copy of paywalled resources. An earlier "error message" (December, 2016) from oadoi.org for , said this holds for around 80% of scholarly articles. Maybe the situation has changed for the better since then. In January 2018, the unpaywall.org FAQ-page stated: We find fulltext for 50-85% of articles, depending on their topic and year of publication. But, this message is no longer found there (in November 2018). And as we have seen in the example above, apparently in some cases unpaywall.org no longer finds open access fulltext alternatives, that was previously found by the oadoi.org application used then.

If unpaywall.org or an oadoi-API (e.g. https://api.oadoi.org/10.3897/biss.2.25805?email=p@x - the 'email' argument as given here may well be "fake", as long as it holds the @-character) fails to find a freely accessible publication (as with https://api.oadoi.org/10.1007%2Fs10654-018-0449-x?email=n@y), an alternative might be to try the identifiers.org SPARQL endpoint . But, it does not necessarily give us an open access URI in return. And it only works if the potential corresponding URIs have been assigned the property owl:sameAs just as the submitted subject URI. Unfortunately, in neither of our cases above these conditions are met.

Assuming we have finally found a single seemingly reliable custodian of our PIDs and URIs, promising 24/7 resolution and top quality metadata, should we rest content with that? Most serious lawyers and journalists probably would agree: it is wise not to judge by the testimony of a single witness, a single source alone. The evidence of at least two, mutually independent witnesses is generally preferred. Multiple resolution of any PID by several different proxy servers, as we already know, still means single parenthood, single custodianship of that lookup-table that has to be managed and updated in order for the PID to resolve as expected. Clark describes it as representing a stage in the evolution of PIDs, that will eventually be surpassed by a more mature age when we supply also data types to come with the PIDs, in order to make them more machine actionable . But we want more than that. We want backup custodians, relatives or friends as caretakers when the poor single parent fails duty. Providing multiple access to, or identification of resources through PIDs, that are capable of serving as trustworthy, competent, valid independent witnesses from different moments in time, at different sites, in different places is a good idea. Thus, we accept that an object may have multiple PIDs. Ideally these multiple PIDs should get to "know about" each other as a way towards interoperability. This can be achieved already, e.g. by means of linked open data (LOD), sameAs-relationships and tools provided by n2t.net, unpaywall.org and the identifiers.org SPARQL endpoint referred to above. Multiple identifiers from different namespaces for the same object may even be desirable in order to ensure interoperability in different environments. . It is also in line with the principle of the semantic web known as the NUNA, Non-Unique Naming Assumption, implying that things described in RDF data can have more than one name and any object may be identified by more than one URI, serving in RDF as 'names' of things.

However, this does not imply that any identifier, any PID is as good as the other. In fact, there are significant differences in quality between identifiers, particularly in terms of 'validatability' and 'meaningfulness'. We are getting there a bit later.

But first, having referred to linked data and sameAs-relationships as a possible solution to achieving interoperability, what about long-term sustainability? Are LOD, relying heavily on URIs, fit for survival? Archival records for long-term preservation need to be self-sustained, carrying meaning within themselves, while the references may no longer be resolvable. In e-archives compliant with the OAIS-model and Trustworthy Digital Repositories standards for self-sustenance, this means that URIs lacking an inherently meaningful structure will often serve only as another set of dumb identifiers. Unless they can import some meaning from outside, through resolution or sameAs links, such opaque, non-resolvable URIs should henceforth rather be described as "non-semantic".

Which identifiers are FAIR enough?

We must ask about PIDs, Persistent Identifiers, just how persistent they are really? Even if not always resolvable, are they in general still 'findable', well distributed over the internet in time and space? Are they 'validatable' (e.g. through fixed string-length, pattern-recognition, restricted character set, built-in checksum, built-in type?) Are they FAIR?

Findability: Beginning with the F for findability, for comparison we go back in time to 'old-fashioned' ISBNs, Internaional Standard Book Numbers. Publicly declaring what type of objects they are meant to identify, ISBNs are rarely directly resolvable. But they are widely distributed, they have good findability in terms of precision hits, as seen by simple 'googling', with good survival rate, longer than the median age of web-pages 9.3 years. For example, look at ISBN 0-14-029161-X: The Diversity of Life / Edward O. Wilson (2001). Simple googling of 014029161X, unprefixed and without hyphens results in 57/57 precision hits (date: 2017-01-30). ISBNs could also be searched in library catalogs, the most comprehensive of which is probably the Karlsruhe Virtual Catalog – KVK worldwide. Result of the query '014029161X', with the same unprefixed ISBN without hyphens yields 123/123 precision hits, recall being difficult to compute since in 55 of 72 catalogs the search could not be successfully processed or no records were found. To counteract the possibly unfair bias with a modern classic like this, we try instead an even older, and presumably less well-known example: ISBN: 2130381030. L'Identité : séminaire interdisciplinaire dirigé par Claude Lévi-Strauss, 1974-1975 (Paris: PUF, 1983). Googling without prefix (2130381030) the precision is between 14/39 and 22/50; with prefix (ISBN2130381030) it reaches as high as 17/18 (date: 2017-01-30).

Accessibility: Data and (digital) objects are accessible only in so far as identifiers are findable or resolvable preferably to open access landing pages with either direct availability of resources, or sufficient metadata to direct the user to such an access point. In this respect DOIs are often, but not always, as good as or sometimes better than ISBNs (for obvious reasons regarding print only material), while gni-UUIDs as described above are all but useless.

Interoperability and Re-usability are both intimately associated with 'validatability', as argued above. We will look more into detail at the performance of different PIDs regarding this below.

Archival Resource Key (ARK) Identifiers: ARKs have a well defined syntax : [http://NMA/]ark:/NAAN/Name[Qualifier] , where NMA is a (changeable) Name Mapping Authority, a "host" or proxy resolving agent. This is not part of the ARK proper, as marked out by the encompassing brackets. The NAAN is the Name Assigning Authority Number, corresponding to the prefix starting with '10.dddd' in a DOI, and serving as a namespace for the following /Name. The NMA-supported [Qualifier] is not further defined in , but an example is given by the suffix s3/f8.05v.tiff, including also a file extension as we can see. As examples below, none of them having a qualifier, we find ARKs giving direct access to digital fulltext of Buffon’s Histoire naturelle at BnF and a 20th Century Guide for mixing fancy drinks in Internet Archive, here with identifiers.org as resolving NMA. In the third case, we see a resource with a prominent feature of ARKs, their possible inflections, here represented by the two '??' at the end of the URI, giving both metadata for the 'document' itself, a photo of the Dallas Police Department from 1963, and the name and location of the collection holding it. This inflection property of ARKs might serve as a response to the requirement ensuing from the FAIR principle of accessibility (A2), that "metadata are accessible, even when the data are no longer available" , i.e. resources that have for some reason been removed from the net should at least leave a gravestone with metadata of last known whereabouts behind to the afterworld. Another interesting, perhaps unintentional feature of this particular case is that when the resolving agent, the NMA is changed from texashistory.unt.edu to either identifiers.org or n2t.net, the very same ARK does not resolve to the same landing page, the same location.

What about the "validatability" of ARKs, then? As obvious already from the examples above, apart from the specific structure, there is no specific pattern or definite string-length of an ARK. The only restrictions on the Name and Qualifier parts "as strings of visible ASCII characters" is that they "should be less than 128 bytes in length" using "letters, digits, or any of these six [Sic!] characters: = # * + @ _ $", allowing also four more characters with reserved meaning: % - . / These general restrictions are hardly sufficient for efficient validation by means of a regular expression. It is possible to impose restrictions defining a specific pattern and string-length within a namespace, for a particular NAAN, using for example a NOID template , but this is not something that is required by the ARK specification.

More heavily incumbent on an ARK seems to be the demand that, "[i]f longevity is the goal, it is important to keep the prefixes free of recognizable semantics" . This, I believe, is a misconception. It is not the absence of semantic content that guarantees longevity; it is rather the continued use of the identifier that enhances its persistence, as observed again by above, and this use may well be promoted by at least some semantic content in the identifier, allowing a user to recognize it as an identifier of precisely that particular object that it is supposed to identify. The Findability by simple 'googling' and current Accessibility of the example ARKs above presently (Nov. 2018) still seems quite good. At least the first of these three examples seems to be well distributed, producing an impressive precision score of 25/25 by simple googling of '12148/bpt6k97497t' (each page in the hitlist actually containing a reference to the same document by Buffon in the Gallica collection). The second example apparently has a narrower distribution, but the few items found still display good precision, 4/4. The third example, without inflection, has been used extensively as a paradigmatic case, so should perhaps be considered outside competition here, but anyway also shows good precision. And, given the limited validatability, the degree to which the resources they identify will also be Interoperable and Re-usable will very much depend on the difficult to predict long-term sustainability of these two first FAIR properties of these ARKS.

DOI: DOIs can look just like anything. Here are some real cases, all at the time of writing resolvable and with multiple Findability also by simple googling, some of them pretty 'old', although they got their DOIs assigned fairly recently. One is even from 1977 (doi: 10.1177/030631277700700112), but it still produces an impressive precision score of 68/68 (date: 2018-11-07), mostly due to it quite high citation rate, yielding hits for all the citing sources.

Now, following are two old DOIs from Wiley Online Library 1996 and Springer 2001 that still do not seem to resolve properly (neither on 2017-01-31, nor on 2018-11-11):

However, the two following old DOIs, again from Wiley Online 1996 and 1998, that were similarly unresolvable at the same date (2017-01-31), are proof that some PIDs might regain there resolvability later.

Obviously, all these DOIs, whether resolvable or not, vary substantially in string-length, from just 17 to over 60 characters, some involving abbreviations of journals or organisations, one an ISBN, and some containing characters in need of special XML-encoding, different from URI. Note that although the two last items in the first group are from the same journal, Scientometrics, they are quite different in structure. Anyway, all the above DOI examples are valid in accordance with the best we can offer as a regular expression restriction, with only partial pattern recognition: ^10[.][0-9]{4,}\/\S+$

meaning that any valid DOI must start by '10.' followed by a minimum of 4 digits, before the slash '/' and then a suffix of any length or characters, but no spaces in between.

But then, according to the same partial restriction, defined by the regex above, this entirely fake DOI is equally valid:

To be sure, there are other regular expression restrictions suggested for DOIs, those that are even more permissive (as DataCite 4.1, with the pattern value for doiType set to "10\..+/.+" , apart from not being PHP or JavaScript compliant, allowing also inline spaces, or the pattern registered for DOIs at identifiers.org as "^(doi\:)?\d{2}\.\d{4}.*$" MIR:00000019, both of which also allow for the fake DOI above as valid, when tested in regex101.com). There are other patterns that are more restrictive, but then obviously not catching all the now prevalent and permitted DOIs by one singular regular expression. , Thus, unlike ISBNs, DOIs are difficult to validate properly. Or rather, it is hard to find sufficiently discriminatory criteria to distinguish proper DOIs from fake ones. They have no fixed string-length, and very little of character set restrictions. All we can have is a partial pattern recognition; the more restrictive the validation rule or regular expression it is based on, the more actually existing DOIs it will leave out.

Handle: The Handle identifier system, of which DOIs are only a special case, seems fairly easy and handy at first glance. It comes in two different flavors. One is the semantically opaque, which has the structure: Prefix/noid (10079/sqv9sf1), where the NOID-part (for Nice Opaque Identifier ) is a short alphanumeric string from the restricted character set "0123456789bcdfghjkmnpqrstvwxz", with random minting order. The other flavor is the semantically transparent, which could be of three different types: the URL handle: Prefix/local-PID (10079/bibid/123456) , the user handle: Prefix/netid/netid (10079/netid/guoxinji), which as demonstrated here seems to be less persistent, as people tend to move, and the simpler group handle: Prefix/group (10079/ISPS). While those of the second flavor might be more instantly "meaningful", providing context, how are Handles fairing regading Findability and Accessibility? The findability by googling will again, as for other PIDs, largely depend on the use and citation rate of items, while the accessibility again rests largely on the maintenance of the lookup-table by the custodian. Even so, as just demonstrated, Handles may not always resolve to the page expected, especially when used as context dependent identifiers of individuals. In these cases, ORCiD ids should be preferred. What about the Interoperability and Re-usability of Handles then? Those of the NOID type, with a restricted character set, will in principle at least be effectively "validatable", to the extent that the "namespace" or minting agent restricts the string-length, as e.g. 2077/36687 – Gothenburg university: 4/5 characters, and 10079/31zcrtn – Yale university: 5/7 characters. Those of the second, "semantic" flavor will apparently prove less "validatable" in the sense that there is no longer any fixed string-length or restricted character-set.

UUID: UUIDs v5 are used within the field of biodiversity taxonomy, as a complement to scientific names. They were introduced to the field in 2015 by the Global Names Architecture - GNA . The arguments for using them instead of name strings for certain functions are that they save space as index keys in databases, they have a fixed string length (36 characters, including the dashes) while scientific names are of different length. UUIDs do not suffer, as names sometimes do, from encoding problems that are difficult to detect and they are more easily distinguishable one from the other than name strings for closely related species variants. Specifically, it is argued that UUIDs v5 ... can be generated independently by anybody and still be the same to the same name string... Same ID can be generated in any popular language following well-defined algorithm. Note, however, that it is actually the specific name string that is identified here, not the object, the organism, the 'thing itself'. Thus, the resulting UUID is completely dependent upon the particular name string (with its encoding), it cannot be used as a bridge between different name forms for the same organism, telling us that they are naming the same object. This is due to the fact that it is generated by hashing a namespace identifier and name. As a result, UUIDs generated in this way by the gna name resolver, e.g. "707f84e1-e5b8-5063-8256-369ba9d72e13" for Antiaris toxicaria are next to useless as instruments of Findability, often yielding 0 hits by simple googling, all the while a search on the scientific name alone will give plenty of precision hits for the sought after organism, providing rich metadata for the 'thing' itself. Likewise, the same UUID is seldom or never Accessible, by being resolvable on its own. As an example, consider one of the most well studied organisms of all, the fruitfly Drosophila melanogaster. Using the Global Names Resolver to get a UUID v. 5 for Drosophila melanogaster, <gni-uuid>1bc2f359-47e4-5da6-a748-74676b7c8c5d</gni-uuid>, googling it either unprefixed or prefixed gives a zero result (0 recall, 0 precision, date: 2017-01-30). Trying instead the same UUID in a general search of all databases of NCBI, the US National Center for Biotechnology Information), we get 0 hits (2018-11-15): 1bc2f359-47e4-5da6-a748-74676b7c8c5d. Most notably, we get 0 hits in the NCBI Taxonomy database, that on the face of it would seem to be the most relevant to our search.

By contrast, the UUID v5 is eminently "validatable", with a character set restricted to digits and lower case [a-f], and a fixed string length, 36 characters including hyphens, in a recognizable, precise pattern: "8-4-4-4-12", allowing for validation by a regular expression such as ^[a-f\d]{8}-[a-f\d]{4}-[a-f\d]{4}-[a-f\d]{4}-[a-f\d]{12}?$, or by means of an online validator. On the other hand, given the fact that these generated UUIDs are seldom or ever used for citation, and are not "fed back" to the source databases, where the corresponding scientfic names were found, it is doubtful whether this "validatability" is also sufficient to make them qualify for Interoperability and Re-usability. To improve its findability and re-usability the UUIDs v5 issued by the Global Name Resolver could "ping-back" and assign themselves to the records in the biodiversity database sources they were drawn from and further use schema.org markup to get incoming links and a better ranking by search engines.

Why context?

Generally speaking, although it is preferable that identifiers be findable and identifiable also in their unprefixed, pure form, typed identifiers give context by means of namespace prefixes of a metadata standard, a vocabulary or ontology. A typed identifier "intoduces itself", telling us what kind of identifier it is, and what type of objects it is used for. Most importantly the namespace tells us what schema(s), which rules should be used for its validation.

Page claimed that e.g. "dc:title" is adding "unnecessary complexity (why do we need to know that it's a "dc" title?)" in the JSON expression:

 { "@context": { "dc:title": "http://purl.org/dc/terms/title" },
                "dc:title": "Darwin Core: An Evolving Community-Developed Biodiversity Data
                Standard" }

A simple answer is that namespaces are important to retain meaning from context, serving as a key to interpretation for the future. Self-sustained long-term preservation should ideally mean in a case like this that the dc specification and schemas valid at the time be archived together with the records, or at least that there is provenance metadata including timestamps and namespace of terms used. Metadatafiles in XML usually have a xsi:schemaLocation indicating which schema to validate against. This information, together with timestamped metadata elements such as 'dateIssued' should be sufficient to provide context. For JSON metadata there are name/value pairs such as { "protocol": "doi", ... "createTime": "2017-01-12T10:49:03Z", ...} that could fill the same function. And then, context is just as important for validation of records also in the present.

A "new" contextual, integrated, validatable PID?

As seen in the case of Handle above, validatability sometimes comes at a cost: transparency lost. Are we forced to make a choice between the two? Can we create identifiers that are both fully validatable and at the same time more meaningful, providing context? So, here we suggest a model for a "new" PID, with a limited character set, at least for the object id part, defined by namespace specifications and schemas.

Model: [namespacePrefix].[objectType].[objectId: 10 positions].[issuedDate: YYYY-MM-DD].[registrant: ORCID or ROR-ID]
Example (expression of this paper): fabio.PositionPaper.pp1255qv43.2018-11-12.0000-0001-5699-994X

It is a model of a structured, contextual, modular, validatable identifier. To make it easier to implement, and more generalizable, there is no character set or string-length restrictions for the first two modules, except that they should not contain the dot (.), which is reserved as a module separator. Nevertheless, this means already existing namespaces and object types could already be used to create a PID in accordance with this model.

The third module, the objectId (local ID) has a limited character set, selected to avoid ambiguous interpretations and, to avoid making local uniqueness case-dependent , restricted to lower case letter characters and digits. The full stop or dot (.) was chosen as module separator, since it works well in both xml- and http-environments, without encoding, and is not subject to confusion as sometimes hyphens and dashes (en-dash and em-dash) can be. It also works for tokenization of strings. The object type identified in the second module should belong to the initial namespace prefix. Every namespace can have as many object types as needed. Namespace schemas could also define valid data types for their different object types, thus supplying PIDs with data types, in order to make them even more machine actionable.

The scalability of this model will mainly depend on the 10 character objectId and the size of the permitted character set. An objectId limited to the proposed character set [a-kmnp-z0-9] will have 3410 permutations within each namespace (and possibly objectType), still better than e.g. a 7 character Handle with NOID.

The objectId module, thus, could be validated separately by a regular expression restricted to ^[a-kmnp-z0-9]{10}$. It may also be part of a more comprehensive validation schema, involving a random or pattern based minting algorithm, preferably including a checksum, with different rules invoked for different namespace contexts, checking also for example the correspondence between namespace (module 1) and objectType (module 2) as in this still crude Schematron schema:

<schema xmlns="http://purl.oclc.org/dsdl/schematron"  queryBinding="xslt2">  
  <ns prefix="rdf" uri="http://www.w3.org/1999/02/22-rdf-syntax-ns#"/>
  <ns prefix="fabio" uri="https://w3id.org/spar/fabio"/>
  <ns prefix="local" uri="local"/>
  <rule id="newPid-rule" context="local:newPid">  
   <let name="objectType" value="for $i in (.) return tokenize($i,'\.')[2]"/>
   <let name="objectId" value="for $i in (.) return tokenize($i,'\.')[3]"/>
   <let name="x" value="'https://w3id.org/spar/fabio'"/>
   <let name="objectTypeList" value="(for $i in  (doc($x)//rdf:type[@rdf:resource='http://www.w3.org/2002/07/owl#Class']/parent::rdf:Description/@rdf:about) 
   return substring-after($i,'fabio/'))"/>
   <let name="objectTypeString" value="string-join($objectTypeList,',')"/>
   <assert test="matches($objectId,'^[a-kmnp-z0-9]{10}$')">
   An identifier of type 'newPid' must have as its third module a namespace unique objectId of 10 characters from the set [a-kmnp-z0-9].</assert>
   <assert test="matches($objectTypeString,$objectType)">The objectType, the second module of the newPid must belong to the namespace of the first module.

One might consider generalizing such a validation schema to the extent possible, so that the namespace URI in $objectTypeList, from which the $objectType should be drawn, was automatically construed based on the namespace prefix (module 1) of the newPid instance to be validated. This could be achieved by having the namespacePrefix (module 1) expressed as a link with namespace URI, e.g. as fabio in our case above. But that would also make the validation schema a bit more complicated, notably the tokenization and separation of modules.

It is also conceivable, to allow for integration of already existing identifier schemes, that a namespace sets its own character set and string-length restrictions, to be declared by the validation rules of that namespace. For "narrow" namespaces, lacking defined different object types, possibly since they comprise basically only one type of object (as for e.g. ISBNs and ISSNs) we suggest as default second module value 'NOT' = No Object Type. So we could have an IGSN, International Geo Sample Number , with string-length of objectID set to 9, expressed in this model:

Example: IGSN.NOT.IECUR0002.2005-03-31.gswa-library 

The identifier should be fully validatable, as a whole or in part (modules), in the corresponding namespace(s). The last two modules are optional, but they are meant to offer built in data provenance. For organisation identifiers, we hope that the recently launched ROR-IDs will become a global standard, like the ORCID for persons. Then we could replace the last module in the IGSN-PID above with "05h2dda38".

The resulting PIDs should be minted within the corresponding namespaces, who would also be the 'custodians' and resolving authorities of their PIDs, responsible for their uniqueness within their namespace. Another task would be to monitor and assign sameAs-properties to PIDs that refer to the same 'thing' in other namespaces.

It has been suggested that in order to build more connected, cross-linked and digitially accessible Internet content it is necessary to assign recognizable, persistent, globally unique, stable identifiers to ... data objects. . The model proposed here aims to make "new" PIDs fully recognizable, universally unique, stable, but always in a well-known context, meaningful, and with a good potential for backup.


The purpose of this paper was to analyse some of the more prevalent general PIDs used in scholarly communication, identify some of their shortcomings and find out how PIDs could be made more FAIR. Real examples of PIDs were analysed to find out what additional requirements there might be to make them fully Findable, Accessible, Interoperable and Re-usable - FAIR. The "novelty" of the paper, if any, is then the "widening" of the FAIR principles to include also Findability as rate of distribution or dissemination (e.g. as measured by means of 'googling') and Interoperability or Re-usability to include also 'validatability'. Further, as against earlier insistence on the opaqueness of PIDs as a warrant for persistence, we argued for the importance of adding meaning to PIDs, through namespace prefixes and object types, as possibly enhancing their future use, distribution, findability and interpretability and safeguarding against failed resolvability. The custodianship and minting of PIDs, we suggested to be the responsibility of namespaces, as these are already assuming the administration of specifications, validation schemas, vocabularies or ontologies, and should be well qualified for the task. The minting algorithm, the patterns for PID-recognition, restriction in character set, string-length (with possible checksum) of objectId module should all be part of the validation schemas. These namespaces should then be able to register their schemes with n2t.net or identifiers.org, as already happens. And there might be several services such as the SPARQL endpoint of identifiers.org for registering sameAs-links. To create, maintain and make our PIDs truly persistent, widely used and FAIR should be a cooperative effort of the whole scholarly community.


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