Notes on Conneg for TAG f2f
Henry S. Thompson
2 March 2009
1. Conneg.
1.1. Review of the thread
Hausenblas asks
"is the PNG *representation* derived via conneg from the generic
resource <http://sw-app.org/sandbox/house> equivalent to the RDF in
Turtle?"
Two possible answers:
- Yes, because you own the resource and by doing the conneg
that way you are asserting that they are equivalent for your
purposes
- But you are being unhelpful in asserting that equivalence,
as it is unhelpful to users. The png response implies the resource
is an IR, i.e. that the generic URI identifies "an image of a
house". The RDF response (modulo the lack of redirection) implies
the resource is not an IR, i.e. that the generic URI identifies a
house.
The overloading of conneg to attempt UAM is bad for web architecture,
but the correct response is to provide a clean UAM story, which should
relieve the pressure on conneg.
WebArch itself talks mostly about 'consistency', including the
intriguing line "Improper use of content negotiation can lead to
inconsistent representations." C.f. JAR's 0170, I think webarch's
comments on CN are all contextualised by an assumption that we're
talking about png vs. jpg or 1200x1600 vs 600x800.
Xiaoshu's 0105 is interesting, in that it raises well the issue of
what the resources are that are named by the URIs returned as
Content-locations: in responses to conneg. Do we have an answer to
this one? In Raman's alternatives discovery finding? Hmmm, yes, he
talks about 'generic resources' and 'specific resources' as if they
were distinct. . . Language conneg, UA-type conneg (VARY?) and
Content-type conneg are possibly different in this regard?
My top-of-my-head response is that all three URIs identify the
same resource. I think we're forced to that conclusion by the fact
that the owner asserts that both representations are representations
of the resource identified by the original URI. . . Can the current
state of AWWSW prove this conclusion?
JAR's narrative in his 0172 makes me uncomfortable, and what it is
that makes me uncomfortable is precisely the possibility of two things
which are so different as PNG and RDF being served as
alternatives. . . I am inclined to reject even the "PNG of the graph"
or "graph of the pixels" 2nd-order thought experiments, because what's
crucial wrt Media Types is that applications which understand that
media type will produce consistent effects. In none of the three
cases is that true.
Last-minute thread
Axiom 1 is true with or without frag-ids. . .
I don't think Axiom 2 is correct. . .
1.2. Some thoughts of my own
Conneg only appies to IRs [i.e. you must serve them all with 200, and
if you lost one, you'd serve it with 200][there might be a case in
which you serve them all with 303---didn't we look at that in Cool
URIs for the SemWeb?]
One possible exercise would be to enumerate application types (as close
to media types as possible, but allowing for e.g. application/xml+svg
to be in with image/jpg) and make assertions by cases. Here's a straw
man: with certain listed exceptions (e.g. svg, application/xml plus a
stylesheet) conneg SHOULD involve the same primary media type.
Here's a completely different straw man: conneg should only ever
involve two Manifestations of the same _expression_.
Note that
"Translations from one language to another, musical
transcriptions and arrangements, and dubbed or subtitled versions of a
film are also considered simply as different expressions of the same
original work. [emphasis in the original]"
Score and performance are different expressions!
"w1 Franz Schubert's Trout quintet
e1 the composer's score
e2 a performance by the Amadeus Quartet and
Hephzibah Menuhin on piano
e3 a performance by the Cleveland Quartet and
Yo-Yo Ma on the cello" (ibid., 3.2.2)
"The boundaries between one manifestation and another are drawn
on the basis of both intellectual content and physical form."
"Changes in physical form include changes affecting display
characteristics (e.g., a change in typeface, size of font, page
layout, etc.),. . . "
Photographs and paintings are works [for paintings I guess
_expression_ and manifestation are typically 1-1]
In FRBR, attributes are associated with specific entities, and one
of the attributes of manifestation include "File Characteristics
(Electronic Resource) File characteristics for an electronic resource
include standards or schemes used to encode the file. . .and other
characteristics that have a bearing on how the file can be processed."
Which of course brings me back to my favourite candidate for a
'definition' of IR: It's an _expression_. That is, an IR is anything
which a) fits within the FRBR ontology and b) qualifies as an
_expression_. (Works are not IRs, they're abstract; whether manifestations
are IRs depends on where we go on the 'specific resource' issue).
|
