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Re: HTTP URIs and authority

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 23 Oct 2007 18:42:27 -0500
Message-Id: <p06230924c344363508e1@[]>
To: wangxiao@musc.edu
Cc: noah_mendelsohn@us.ibm.com, Alan Ruttenberg <alanruttenberg@gmail.com>, Mikael Nilsson <mikael@nilsson.name>, Richard Cyganiak <richard@cyganiak.de>, W3C-TAG Group WG <www-tag@w3.org>

>Pat Hayes wrote:
>>>noah_mendelsohn@us.ibm.com wrote:
>>>>Xiaoshu Wang writes:
>>>>>- The URI identifies the city.
>>>>>- The *representation* that people gets back by dereferencing 
>>>>>the URI with HTTP protocol is your *impression*.
>>>>I don't think the word impression is really appropriate here. 
>>>>Let's say that I assign resource 
>>>>http://example.org/nonRhymingPoem to the poem that is popular 
>>>>with American school children:
>>>>         Roses are red,
>>>>         Violets are blue,
>>>>         Some poems rhyme,
>>>>         Some don't.
>>>>If you do an HTTP GET to that URI I send you back an HTML page. 
>>>>The text of the poem is more or less centered.  It's set out in 
>>>>some font of my choosing, in 25 point italic.  The background is 
>>>>purple.  I don't think the most appropriate way to describe that 
>>>>in English is to say that it's my impression of the poem. It's 
>>>>the way I choose to render the poem for your perusal.  In fact, 
>>>>it's quite appropriate to say that the HTML page is the way that 
>>>>I choose to represent the program.
>>>>Furthermore, I don't think we need to insist that this particular 
>>>>URI is only for the poem rendered in those fonts, unless that's 
>>>>what I say the URI is for.  If I say that it's for the poem, and 
>>>>in a year or so someone comes up with a font I like better, I see 
>>>>no problem with my changing the page to use that.  The URI still 
>>>>identifies the poem, since I say it does (presuming I've 
>>>>registered example.com).  The HTML pages are still 
>>>>representations of the poem, they are not my impressions of it.
>>>Yes, I agree with all that.  I am perfectly clear about that.
>>>The problem is that at least someone does not think that 
>>>http://www.ihmc.us/users/phayes/PatHayes can be used to refer Pat 
>>>Hayes? They will say httpRange-14 dictates that 
>>>"http://example.org/nonRhymingPoem" can emit 200 but 
>>>http://www.ihmc.us/users/phayes/PatHayes must emit 303?
>>Yes. That page violates httpRange-14 (deliberately, I might add :-)
>>>I cannot see there is anything, at least anything objective, to 
>>>judge that position. The reason people think that Pat's URI is 
>>>wrong because when they dereference the URI with HTTP they gets 
>>>back an HTML page.  They say, nah, Pat cannot be an HTML page, so 
>>>that URI is wrong. But the truth is: it is not that Pat becomes a 
>>>HTML page, it is a representation of Pat becomes page. What I want 
>>>to say in the past few days is let's clarify the difference 
>>>between *a resource* identified by a URI and *a representation* of 
>>>that resource once dereferenced via the HTTP protocol.  You can 
>>>know something about the resource from its representation but how 
>>>much depends on the content of the representation and your ability 
>>>to handle the representation.
>>>If we understand URI dereference from this point of view, there is 
>>>no ground for httpRange-14 anymore. And there is no point to 
>>>distinguish what is information resource and what is not.
>>There is a real issue, however. You or I can read what that page 
>>says and understand it. But programs can't. The Semantic Web needs 
>>a way for a program to reliably infer whether the URI denotes me or 
>>some HTML. If there were some universally agreed way to do this 
>>(say, a world-standard, universally accepted, OWL person ontology) 
>>then the page could use this. But there is no such way for it to 
>>say this in a machine-readable fashion, and so we are left with an 
>If the RDF representation of "http://www.ihmc.us/users/phayes/PatHayes" says:
><http://www.ihmc.us/users/phayes/PatHayes> a foaf:Person.  Why a 
>program would confuse that?  How?
>>One view on this, which I have espoused elsewhere, is to simply 
>>accept that URI denotations are ambiguous and learn to live with 
>>it, by using the context to disambiguate the meaning.
>If we cleanly separate the meaning of URI from its *dereference* 
>protocol, how can the identity of URI ambiguous?

Because people often DO use the URI to denote the page/'information 
resource' that it dereferences. And that's not *illegal*; and in fact 
it is very common. So now, here I am, a dumb-as-dirt SemWeb program, 
and I have this URI. Does it denote what it dereferences? Or 
something else? How can I tell?

>HTTP is just a communication protocol, established for agents to 
>communicate.   It does not establish a context for the meaning.  If 
>we do, we put this as a poker game, where we try to infer someone's 
>hand from its behavior.  Is this what the web wants?

No, which is why I think reading httpRange-14 as a way of 
'signalling' denotation is wrong. It just acknowledges the common, 
widely used, natural convention that a URI denotes whatever it 
dereferences to, but it also allows a minimal way to side-step this 
in some cases, if you want it (the URI) to denote something else. Can 
you see any other way to do this?



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Received on Tuesday, 23 October 2007 23:42:40 UTC

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