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Resources, and Versions, and Identifiers! Oh, my!

The only constant is change. —Heraclitus

Data publication, management, and citation would all be so much easier if data never changed, or at least, if it never changed after publication. But as the Greeks observed so long ago, change is here to stay. We must accept that data will change, and given that fact, we are probably better off embracing change rather than avoiding it. Because the very essence of data citation is identifying what was referenced at the time it was referenced, we need to be able to put a name on that referenced quantity, which leads to the requirement of assigning named versions to data. With versions we are providing the x that enables somebody to say, “I used version x of dataset y.”

Since versions are ultimately names, the problem of defining versions is inextricably bound up with the general problem of identification. Key questions that must be asked when addressing data versioning and identification include:

What is being identified by a version? This can be a surprisingly subtle question. Is a particular set of bits being identified? A conceptual quantity (to use FRBR terms, an expression or manifestation)? A location? A conceptual quantity at a location? For a resource that changes rapidly or predictably, such as a data stream that accumulates over time, it will probably be necessary to address the structure of the stream separately from the content of the stream, and to support versions and/or citation mechanisms that allow the state of the stream to be characterized at the time of reference. In any case, the answer to the question of what is being identified will greatly impact both what constitutes change (and therefore what constitutes a version) and the appropriateness of different identifier technologies to identifying those versions.
When does a change constitute a new version? Always? Even when only a typographical error is being corrected? Or, in a hypertext document, when updating a broken hyperlink? (This is a particularly difficult case, since updating a hyperlink requires updating the document, of course, but a URL is really a property of the identifiee, not the identifier.) In the case of a science dataset, does changing the format of the data constitute a new version? Reorganizing the data within a format (e.g., changing from row-major to column-major order)? Re-computing the data on different floating-point hardware? Versions are often divided into “major” versions and “minor” versions to help characterize the magnitude and backward-compatibility of changes.
Is each version an independent resource? Or is there one resource that contains multiple versions? This may seem a purely semantic distinction, but the question has implications on how the resource is managed in practice. The W3C struggled with this question in identifying the HTML specification. It could have created one HTML resource with many versions (3.1, 4.2, 5, …), but for manageability it settled on calling HTML3 one resource (with versions 3.1, 3.2, etc.), HTML4 a separate resource (with analogous versions 4.1, 4.2, etc.), and continuing on to HTML5 as yet another resource.

So far we have only raised questions, and that’s the nature of dealing with versions: the answers tend to be very situation-specific. Fortunately, some broad guidelines have emerged:

Assign an identifier to each version to support identification and citation.
Assign an identifier to the resource as a whole, that is, to the resource without considering any particular version of the resource. There are many situations where it is desirable to be able to make a version-agnostic reference. Consider that, in the text above, we were able to refer to something called “HTML4” without having to name any particular version of that resource. What if that were not possible?
Provide linkages between the versions, and between the versions and the resource as a whole.

These guidelines still leave the question of how to actually assign identifiers to versions unanswered. One approach is to assign a different, unrelated identifier to each version. For example, doi:10.1234/FOO might refer to version 1 of a resource and doi:10.5678/BAR to version 2. Linkages, stored in the resource versions themselves or externally in a database, can record the relationships between these identifiers. This approach may be appropriate in many cases, but it should be recognized that it places a burden on both the resource maintainer (every link that must be maintained represents a breakage point) and user (there is no easily visible or otherwise obvious relationship between the identifiers). Another approach is to syntactically encode version information in the identifiers. With this approach, we might start with doi:10.1234/FOO as a base identifier for the resource, and then append version information in a visually apparent way. For example, doi:10.1234/FOO/v1 might refer to version 1, doi:10.1234/FOO/v2 to version 2, and so forth. And in a logical extension we could then treat the version-less identifier doi:10.1234/FOO as identifying the resource as a whole. This is exactly the approach used by the arXiv preprint service.

Resources, versions, identifiers, citations: the issues they present tend to get bound up in a Gordian knot. Oh, my!