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Re: Question on the boundaries of content negotiation in the context of the Web of Data

From: Michael Hausenblas <michael.hausenblas@deri.org>
Date: Wed, 18 Feb 2009 19:00:25 +0000
To: Jonathan Rees <jar@creativecommons.org>
CC: <www-tag@w3.org>
Message-ID: <C5C20B49.20E0%michael.hausenblas@deri.org>

Jonathan,

> ... Or were you really asking the broader question about the use of CN that
people like me were eager to answer?

Yes, indeed. Broader or narrower is not up to me to decide, though ;)

Of course the problem is not only with RDF vs. PNG or between two still
image formats such as PNG and GIF. We've experienced the same in linked data
where we *assume* it is ok to conneg between RDF and HTML (as long as the
two representations convey the same information). Again, here it is: 'convey
the same information' - I think that we need a clean explanation that (i)
either tells people, look, so far you may go or these are the criteria you
need to check against to be sure you are within the 'sameness' boundary OR
(ii) tells people, don't worry, this is highly subjective, you as the owner
can do whatever you want and only afterwards the community and the usage out
there decides if you are credible or not (short version of DanC's POV, if I
got it right).

Anyway, thanks Jonathan for taking this further and I remain in the hope
that the TAG as a whole will discuss it ;)

Once again, if there is help needed I'm happy to support this on the
editorial level, based on [1], maybe?

Btw, is there anywhere a list of all TAG Task Forces or is it just my
ignorance that I overlooked this :)?

Cheers,
      Michael

[1] http://esw.w3.org/topic/AwwswHome

-- 
Dr. Michael Hausenblas
DERI - Digital Enterprise Research Institute
National University of Ireland, Lower Dangan,
Galway, Ireland, Europe
Tel. +353 91 495730
http://sw-app.org/about.html
http://webofdata.wordpress.com/


> From: Jonathan Rees <jar@creativecommons.org>
> Date: Wed, 18 Feb 2009 09:32:25 -0500
> To: Michael Hausenblas <michael.hausenblas@deri.org>
> Cc: <www-tag@w3.org>
> Subject: Re: Question on the boundaries of content negotiation in the context
> of the Web of Data
> 
> I started to turn this into a request for TAG telecon agendum, and got
> stuck on the word "equivalent".
> 
> Just to make sure I understand you - by "equivalent" are you referring
> to HTTP 2616 section 13.3.3:
> 
>    Entity tags are normally "strong validators," but the protocol
>    provides a mechanism to tag an entity tag as "weak." One can think of
>    a strong validator as one that changes whenever the bits of an entity
>    changes, while a weak value changes whenever the meaning of an entity
>    changes. Alternatively, one can think of a strong validator as part
>    of an identifier for a specific entity, while a weak validator is
>    part of an identifier for a set of semantically equivalent entities.
> 
> and are you specifically asking about the use of entity tags?  Or were
> you really asking the broader question about the use of CN that people
> like me were eager to answer? Because I think these are two different
> questions.
> 
> If you're asking for advice on "good practice" around the use of
> entity tags, the only example given in RFC 2616 is that of hit
> counters, which seems quite a long way from "semantic equivalence" of
> an image and some RDF. I'd be surprised if anyone would argue in favor
> of allowing a cached PNG to be returned when RDF was available and
> preferred. On the other hand, the question of under which
> circumstances (if any) you are advised to use CN to choose between PNG
> and RDF has a very different character. Perhaps some server software
> has chosen to assume co-representations are equivalent for caching
> purposes, but if this is allowed by RFC 2616 I'd be very interested to
> hear the argument.
> 
> I would think that CN is used (and intended to be used) not just for
> choosing between semantically equivalent entities, but also for
> semantic subsetting, such as abbreviated representations for mobile
> devices, low-resolution displays, audio vs. written, etc. Subsetting
> is certainly *not* equivalence.
> 
> Obviously there is appeal to a slippery adjective "semantic", which
> you're never going to pin down in a manner that is both rigorous and
> general, but you could legitimately ask someone to list some positive
> and negative examples and situations where differences between
> representations might or might not matter to users and/or
> applications.
> 
> Jonathan
> 
> On Thu, Feb 12, 2009 at 7:29 AM, Michael Hausenblas
> <michael.hausenblas@deri.org> wrote:
>> 
>> Dear TAG members, dear subscribers,
>> 
>> I would like to ask you about your opinion on the following scenario. Please
>> note that (1) though I'm a member of the W3C Media Fragments WG I speak only
>> for myself, and (2) that all URIs used in the following are dereferenceable
>> and made out of 100% recycled electrons.
>> 
>> Given three URIs, namely,
>> 
>> <http://sw-app.org/sandbox/house>
>> 
>> <http://sw-app.org/sandbox/house.png>
>> 
>> <http://sw-app.org/sandbox/house.ttl>
>> 
>> is it 'allowed' (that is, does it break the Web architecture) if one does
>> the following:
>> 
>> $curl -I -H "Accept: image/png" http://sw-app.org/sandbox/house
>> HTTP/1.1 200 OK
>> Date: Thu, 12 Feb 2009 12:12:39 GMT
>> Server: Apache/2.2.3 (CentOS)
>> Content-Location: house.png
>> Vary: negotiate,accept
>> TCN: choice
>> Last-Modified: Thu, 12 Feb 2009 11:54:07 GMT
>> ETag: "5c0fd-2deb-462b760a7f5c0;462b77ce8a040"
>> Accept-Ranges: bytes
>> Content-Length: 11755
>> Connection: close
>> Content-Type: image/png
>> 
>> $ curl -I -H "Accept: text/turtle" http://sw-app.org/sandbox/house
>> HTTP/1.1 200 OK
>> Date: Thu, 12 Feb 2009 12:13:01 GMT
>> Server: Apache/2.2.3 (CentOS)
>> Content-Location: house.ttl
>> Vary: negotiate,accept
>> TCN: choice
>> Last-Modified: Thu, 12 Feb 2009 11:54:06 GMT
>> ETag: "5c0fc-173-462b76098b380;462b77ce8a040"
>> Accept-Ranges: bytes
>> Content-Length: 371
>> Connection: close
>> Content-Type: text/turtle
>> 
>> Please note that I don't ask if this works. It does. Obviously. The
>> question, to put it in other words, is: is the PNG *representation* derived
>> via conneg from the generic resource <http://sw-app.org/sandbox/house>
>> equivalent to the RDF in Turtle?
>> 
>> If not, why not? If it is, can you please point me to a finding, note, a
>> specification, etc. that 'normatively' defines what 'equivalency' really is?
>> 
>> Cheers,
>>      Michael
>> 
>> --
>> Dr. Michael Hausenblas
>> DERI - Digital Enterprise Research Institute
>> National University of Ireland, Lower Dangan,
>> Galway, Ireland, Europe
>> Tel. +353 91 495730
>> http://sw-app.org/about.html
Received on Wednesday, 18 February 2009 19:01:07 GMT

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