Almost two years ago I wrote a blog entry entitled Useful vs. Useless Abstractions which stated that the invention of URIs by the IETF/W3C crowd to replace the combination of URLs and URNs was a step backwards. I wrote
URIs are a merger of the syntax of URLs and URNs which seem to have been repurposed from their original task of identifying and locating network retrievable documents to being more readable versions UUIDs which can be used to identify any person, place or thing regardless of whether it is a file on the Internet or a feeling in your heart.
This addition to the URN/URL abstraction seemed to address some of the bits which may have been considered to be leaky (if I enter http://www.yahoo.com in my browser and it loads it from its cache then the URL isn't acting as a location but as an identifier). Others also saw URIs as a way for people who needed user friendly UUIDs for use on the Web. I've so far come into contact with URIs in two aspects of my professional experience and they have both left a bad taste in my mouth.
URIs and the Semantic Web: Ambiguity2
One problem with URIs is that they don't uniquely identify a single thing. Consider the following hyperlinked statements
Dare is a Georgia Tech alumni.
Dare's website is valid XHTML.
In the above statements I use the URI "http://www.25hoursaday.com" to identify both myself and my web page. This is a bad thing for the Semantic Web. If you read Aaron Swartz's excellent primer on the Semantic Web you will notice where he talks about RDF and its dependence on URIs
...
Now consider...
<http://aaronsw.com/> <http://love.example.org/terms/reallyLikes> <http://www.25hoursaday.com/> .
Can you tell whether Aaron really like my website or me personally from the above RDF statement? Neither can I. This inherrent ambiguity is yet another issue with the vision of the Semantic Web and the current crop of Semantic Web technologies that are overly dependent on URIs.
Over the past few years I've been on the W3C Technical Architecture Group mailing list I've seen this inherent ambiguity of URIs result in many lengthy, seemingly never-ending discussions about how to workaround this problem or whether it is even a problem in itself. The discussion thread entitled Information resources? which morphed into referendum on httpRange-14 is the latest incarnation of this permathread on the WWW-TAG mailing list.
I was much heartened to see that Tim Berners-Lee is beginning to see some of the problems caused by the inherent ambiguity of URIs. In his most recent response to the "referendum on httpRange-14 " thread he writes
> It is a best practice that there be some degree of consistency
> in the representations provided via a given URI.
Absolutely.
> That applies *both* when a URI identifies a picture of
> a dog *and* when a URI identifies the dog itself.
>
> *All* URIs which offer consistent, predictable representations will be
> *equally* beneficial to users, no matter what they identify.
Now here seems to be the crunch.
The web architecture relies, we agree I think, on this consistency
or predictability of representations of a given URI.
The use of the URI in the web is precisely that it is associated
with that class of representations which could be returned for it.
Because the "class of representations which could be returned"
is a rather clumsy notion, we define a conceptual thing
which is related to any valid representation associated with the URI,
and as the essential property of the class is a similarity in
information content, we call the thing an Information Resource.
So a URI is a string whose sole use in the web architecture
is to denote that information resource.
Now if you say in the semantic web architecture that the same will
identify a dog, you have a conflict.
>
>> The current web relies on people getting the same information from
>> reuse of the same URI.
>
> I agree. And there is a best practice to reinforce and promote this.
>
> And nothing pertaining to the practice that I and others employ, by
> using http: URIs to identify non-information resources, in any way
> conflicts with that.
Well, it does if the semantic web can talk about the web, as the
semantic web can't be ambiguous about what an identifier identifies in the way that
one can in english.
I want my agent to be able to access a web page, and then use the URI
to refer to the information resource without having to go and find some
RDF somewhere to tell it whether in fact it would be mistaken.
I want to be able to model lots and lots of uses of URIs in existing
technology in RDF. This means importing them wholesale,
it needs the ability to use a URI as a URI for the web page without
asking anyone else.
The saga continues. The ambiguity of URIs have also been a problem in XML namespaces since users of XML often wonder assume a namespace URI should lead to a network retrievable document when accessed. Since they are URIs, this isn't necessarily true. If they were URLs it would be and if they were URNs they would not be.