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June 24, 2009

The lifecycle of a URI

Via a post to the W3C Linked Open Data mailing list recently, I came across a short document by David Booth on The URI Lifecycle in Semantic Web Architecture.

It's particularly interesting, I think, because it highlights that different agents have varying relationships to, or play various roles with respect to, a URI, and those different roles bring with them varying responsibilities for maintaining the stability of the URI as an identifier. And it is the collective action of these different parties that serve to preserve that stability (or not).

The foundations of the principles articulated here are, of course, those presented in the W3C's Architecture of the World Wide Web. But David's document amplifies these base guidelines by emphasising both the social and the temporal dimensions of URI "minting", use (both by authors/writers of data and consumers/readers of data), and, potentially, obsoleting.

As the introduction emphasises, the lifecycle of a URI is quite distinct from that of the resource identified by that URI:

a URI that denotes the Greek philosopher Plato may be minted long after Plato has died. Similarly, one could mint a URI to denote one's first great-great-grandson even though such a child has not been conceived yet.

For David, a key part of the "minting" stage is the publication of what he calls a URI declaration to be accessible via the "follow-your-nose" mechanisms of the Web and the Cool URIs conventions. It is this which forms the basis of a "social expectation that the URI will be used in a way that is consistent with its declaration"; it "anchors the URI's meaning". (More precisely, the documents refer to the "core assertions" of such a declaration.)

An author using/citing that URI in their data is then responsible for using that URI in ways which are consistent with the owner's URI declaration. And a consumer of that data should make an interpretation of the URI consistent with the declaration. However, there is a temporal aspect to these actions: a URI declaration may be changed in the period between the point an author cites a URI and the point at which a consumer reads/processes that data, and in that case the author's implied commitment is to the declaration at the time their statement was written, and the consumer may also choose to select that specific version of the declaration over the most recent one.

In the discussion of the document on the public-lod mailing list, there's some analysis of what happens when actors do not meet such expectations or discharge these responsibilities, and indeed to what extent the existence of these expectations and responsibilities leaves room for flexibility. Dan Brickley describes the case of the "semantic drift" of a FOAF property called foaf:schoolHomepage, where the URI owners' initial intent was that this referred to the home pages of the institutions UK residents know as "schools" (i.e. the institutions you typically attend up to the age of 16 or 18), but which authors from outside the UK interpreted as having a broader scope (one in which the notion of "schools" included, e.g., universities) and deployed in that way in their data. As a consequence, the "URI declaration" was updated to reflect the community's interpretation.

There is a tension here between "nailing down" meaning and allowing for the sort of "evolution" that takes place in human languages, and the scope for accommodating this sort of variability was, I think, perhaps the main point of contention in the discussion. In conclusion, David emphasises:

The point of a URI declaration is not to forbid semantic variability, it is to permit the bounds of that variability to be easily prescribed and determined.

All in all, it's a clear, thoughtful document which addresses several complexities in our management of URI stability over time in a social Web.

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