URIs and Unique IDs

I'm resending this message to the semantic web discussion group for the
record.

On Wed, Oct 29, 2008 at 3:53 PM, Michael F Uschold <uschold@gmail.com>wrote:

> Currently there is no accepted practice on how/whether to migrate to new
> URIs when a new version of an ontology is published. This is largely due to
> the fact that there is no good technology for managing versioning, and the
> W3C consciously (and probably sensibly) decided not to address the issue.
> Versioning information is meant to be placed on a version annotation.
>
> However the current situation is like the wild West, and everyone will be
> doing different things, resulting in a mess.
>
> Wordnet published a new version and minted all new URIs even though many or
> most of the entries were semantically identical.
> The SKOS working group is currently considering the pros and cons of
> various options. One is to adopt all new URIs in a new namespace, just like
> Wordnet. Another is to keep the exact same name space, and change the
> semantics of a small number of terms while keeping the same URI. A third is
> to keep the same URI for the unchanged terms, and mint new URIs for the
> terms with different semantics.
>
> This is a problem because they have no guidelines, they are basically
> stumbling along in the dark.
>
> I believe that this is an urgent matter that needs attention to prevent a
> nightmare from unfolding.
>
> In the current state of semantic web use, it may not matter to much what
> choice the SKOS team chooses. This is mainly relatively few applications
> will be impacted, which may be due to the fact that the applications are not
> driven by the ontologies.
>
> However, when usage of ontologies and ontology-driven applications becomes
> more mainstream, the differences could be profound. Given that this issue is
> intimately tied up with versioning, and that we have no good solutions yet,
> do we continue to throw our hands up and punt? Absolutely not, it is
> essential that a good precedent is set ASAP that is based on sound
> principles.
>
> Here is how.
>
> We should imagine a future where ontology versioning is handled properly
> and do things that are going to make things easy to migrate to that future.
> We don't know how the versioning black box will work, but we should be able
> to make some clear and definitive statements about WHAT it does.
>
> For example, in the future, ontology-driven applications will be fairly
> mainstream. URIs are used as unique identifiers. When applications are
> driven from ontologies, then they will break if you change the semantics in
> mid-stream.  Imagine an application that relied on the semantics of broader
> as it was originally specified with transitivity.  They loaded data that was
> created using that semantics. Then the SKOS spec changes and broader is no
> longer transitive. New datasets are created according to this new meaning.
> The application loads more data. It needs to know which data is subject to
> transitive closure and which is not. This is impossible, if the same SKOS
> URI is used for versions with different semantics.  They are different
> beasts, and thus MUST have different URIs.
>
> Similarly, if SKOS mints a whole new namespace and changes all the URIs,
> the application also has a problem. It has datasets with the old URI and
> datasets with the new URIs. This means that the datasets will not be linked
> like they should, they will treat the two different URIs for the same thing
> as being different.  If one wanted to go into OWL-Full, one can use
> owl:sameAs, but this is not very practical.  The only reasonable solution is
> to have the same URI for things with the same semantics.
>
> Thus, any ontology versioning systemof the future will rely on these two
> principles:
> 1. If the semantics of a term changes, then it needs to have a new unique
> ID.
> 2. If the semantics of a term does NOT change, then it should maintain the
> same ID in any future versions.
>
> If either of these two guidelines are broken, then so will the
> ontology-driven applications of the future.
>
> These maxims hold without exception for any standards that are formally
> released as standards.
> A question arises if we need to hold to the same standards for standards
> like SKOS which was never formally blessed.
>
> The practical difficulties will be the same whether the standard is blessed
> or not. It only really depends on whether the standard is a de facto
> standard,or whether it is getting significant use. If users build things and
> ontology producers break things through carelessness, this will hinder
> semantic web technology adoption.
>
> Another question is what to do if the original standard is belived to be
> incorrect, and the new one is the fixed one. Can one then keep the same URI?
> Again, the answer should be informed by the impact on applications. The
> same problems will occur if you change the semantics and keep the same URI
> even if you are fixing a mistake.  The URI with the wrong semantics must
> keep its original unique ID.
>
> Michael Uschold
>