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Re: CURIEorURI Value Space Collisions

From: Ivan Herman <ivan@w3.org>
Date: Tue, 12 Apr 2011 06:46:15 +0200
Cc: public-rdfa-wg <public-rdfa-wg@w3.org>
Message-Id: <44FE7141-9BD7-406D-869A-8955A7833D7F@w3.org>
To: Niklas Lindström <lindstream@gmail.com>

(This are my private musing, not an official answer from the WG.)

We cannot deny that there is a danger of mixing here, and the WG had to put different issues into the balance. One of the concerns that clearly came up (witness also the discussion in other groups) that some part of the user base felt uneasy about using CURIEs altogether, especially when a specific property appears only once or twice in a larger context (in RDFa 1.0 even if a property appeared only once in a @rel, one has to define a prefix for it). So the issue is really to choose between not-100%-satisfactory options...

However... see also some comments below

On Apr 12, 2011, at 24:02 , Niklas Lindström wrote:

> Hi all!
> Is it correct that the RDFa WG is currently recommending letting
> CURIEs share the same value space as regular URIs, and so that any
> prefix defined with the same value as a scheme, like "http", "https",
> "news" etc. will change the URI for any absolute URI using those
> schemes?

I am not sure I follow exactly how you formulated this, so just to make it sure: any prefix definition in RDFa 1.1 will take precedence over an absolute URI. 

> I remember worrying about this last year, but I haven't followed the
> decision process in detail since then. It just worries me that letting
> these things collide will blow up for anyone who happens to use at
> least "http" or "https" as prefixes (perhaps rendering prefixes using
> a tool, or getting them from a profile out of their control). Or
> perhaps worse, people believing it safe to use anything but "http(s)"
> as prefixes, which will work until something other than those two
> comes along in the next 10 years or so. It might happen; and if it
> does, it may quite probably be beyond the controls of RDFa specs and
> tools.
> (An example: some vocabulary "Wide Exceptional Graphs" becomes
> popular, using "wxg" as a prefix. Then Google comes along with a new
> wxg scheme ("Web Extended by Google"), and soon lots of resources are
> linked with that instead of old "http". Or for that matter, that some
> other scheme [3] becomes popular again for whatever reason.)

And yes, it is not only http(s) but any existing URI scheme. So it has to be made very clear that using prefixes like urn, ftp, etc, is also bad practice.

However, if I have an RDFa source where I use, today, the 'wxg' prefix for something, and I define it as part of my profile or in a @prefix, and then Google has a new URI scheme wxg:, then my RDFa content remains valid and unchanged. Indeed, the CURIE resolution takes precedence, ie, the old usage remains. Of course, if I wanted to _use_ the new URI scheme in my old code, then I would have to make changes, that is correct. But that is not very frequent.

> I vaguely recall the WG saying something about defining "http" as a
> prefix is bad practise. But this turns up here and there, not least
> since the HTTP Vocabulary Draft [1] (<http://www.w3.org/2006/http#>)
> recommend it as a prefix. And I just ran across "http" as a prefix in
> the Tabulator source as well [2].

The tabulator source is unaffected by this, of course. As for the HTTP vocabulary draft: they will change that, and they will use a different prefix. (To be clear, this is not due to RDFa, they have received many comments on that from elsewhere, hence their decision to change that in future.)



> While I understand that it is confusing to use it as a prefix, I am
> not convinced that it is safe to combine the CURIE and URI value space
> like this. At least not without a limit on the CURIEs allowed in the
> joint CURIEorURI space. For instance, not allowing CURIEs in that
> space to use anything after the prefix+':' other than say an
> isegment-nz-nc from RFC 3987, or something to that effect (like a
> "[A-Za-z0-9_-.]+" regexp).
> If there was such a restriction on the format of CURIEs are allowed in
> the CURIEorURI mix (and that anything not matching it would be
> considered a full URI), I would definitely sleep better. :)
> Am I missing something crucial, or overly worried about the risk of collisions?
> Best regards,
> Niklas
> [1]: http://www.w3.org/TR/HTTP-in-RDF10/
> [2]: http://dig.csail.mit.edu/hg/tabulator/file/9a135feff10f/chrome/content/js/rdf/rdflib.js#l5644
> [3]: http://en.wikipedia.org/wiki/URI_scheme

Ivan Herman, W3C Semantic Web Activity Lead
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Received on Tuesday, 12 April 2011 04:45:50 UTC

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