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RE: introducing URIs [was: 13 Aug Arch Doc...]

From: Williams, Stuart <skw@hplb.hpl.hp.com>
Date: Fri, 16 Aug 2002 16:59:05 +0100
Message-ID: <5E13A1874524D411A876006008CD059F04A06FCE@0-mail-1.hpl.hp.com>
To: "'Christopher B Ferris'" <chrisfer@us.ibm.com>
Cc: "'Dan Connolly'" <connolly@w3.org>, "Ian B. Jacobs" <ij@w3.org>, www-tag@w3.org

Kind of depends on whether the purpose of the URI is to identify "the engine
in Chris's car" or a "particular engine". Anyway the point of the discussion
of car, engine and piston was not to suggest that this was the best approach
to the assignment of URI's in some vehicle and vehicle parts tracking
system, but, as it turned out, to become more comfortable with the notion
that the things identified by URI+frag could also be resources.

It is also the case that any 'part-of' relationship between a car and an
engine that might be inferred from http://example.com/myCar/engine#piston1
can only be asserted by the assignment authority. It cannot in general be
inferred from the URI (which should be opaque).

Stuart
--

-----Original Message-----
From: Christopher B Ferris [mailto:chrisfer@us.ibm.com]
Sent: 16 August 2002 16:22
To: Williams, Stuart
Cc: 'Dan Connolly'; Ian B. Jacobs; www-tag@w3.org; www-tag-request@w3.org
Subject: RE: introducing URIs [was: 13 Aug Arch Doc...]



And what happens when you remove the engine from http://example.com/myCar
and install it into 
another car? It's the same engine. Seems like it might be a "cool URI" that
shouldn't change. Yet, if 
its URI includes the fragment, then its URI would have to change, breaking
all the assertions 
and references to it. You can't redirect a fragment (the engine has moved to
Chris' car). 

Cheers, 

Christopher Ferris
Architect, Emerging e-business Industry Architecture
email: chrisfer@us.ibm.com
phone: +1 508 234 3624 

www-tag-request@w3.org wrote on 08/16/2002 10:43:37 AM:

> 
> Hi Dan,
> 
> In [1] you asked me:
> 
> > Please take a look at my "introducing URIs" message to see
> > if it really looks like folly.
> 
> Well... the short answer is yes... I continue to think that redefinition
of
> the terminology around URI would be folly (more at the very end below).
> 
> On the plus side (I think)... I believe I have come to be comfortable with
> the notion that URI composed with fragment id can indeed be used to
identify
> Resources. That is elaborated below - I'd appreciate your thoughts,
because
> the reasoning below may be flawed, and I would like to be agreeing on the
> reasoning as well as the conclusion. I think I also came close to
> understanding TBL's position on httpRange-14 and trip at the last hurdle.
> 
> regards
> 
> Stuart
> [1] http://lists.w3.org/Archives/Public/www-tag/2002Aug/0149.html
> 
> > -----Original Message-----
> > From: Dan Connolly [mailto:connolly@w3.org]
> > Sent: 13 August 2002 22:55
> > To: Ian B. Jacobs
> > Cc: www-tag@w3.org
> > Subject: introducing URIs [was: 13 Aug Arch Doc...]
> > 
> > 
> > 
> > On Tue, 2002-08-13 at 15:29, Ian B. Jacobs wrote:
> > [...]
> > > [1] http://www.w3.org/2001/tag/2002/0813-archdoc
> > 
> > "1.1 Use of terms URI and URI reference in this document
> > 
> > RFC 2396 divides the world ..."
> > 
> > Hmm... that starts from a perspective that we intend to
> > obsolete. How about starting from the other perspective:
> > 
> > =======
> > 
> > Chapter 1: Identifiers and resources
> > 
> > The Web is a universe of resources; resources are a generalization
> > over documents, files, menu items, machines, and services, as
> > well as people, organizations, concepts, etc. Web architecture
> > starts with a uniform syntax of identifiers for resources, so
> > that we can refer to them, access them, describe them, share
> > them, etc. The syntax employs an extensible set of schemes. Several of
> > the schemes incorporate established identification mechanisms
> > into this syntax:
> > 
> >    mailto:nobody@example.org
> >       mailbox names (including DNS domain names)
> >    ftp://example.org/aDirectory/aFile
> >       ftp file names (including DNS domain names)
> >    news:comp.infosystems.www
> >       newsgroup names
> >    tel:+1-816-555-1212
> >       telephone numbers
> >    urn:uuid:@@look-up-syntax
> >       UUIDs, from Apollo/DCE/COM
> > 
> > and others incorporate new naming schemes, including those
> > introduced as a consequence of new protocols:
> > 
> >    http://www.example.org/something?with=arg1;and=arg2
> >       HTTP resources
> >    ldap:@@look-up-ldap-syntax
> >       LDAP entries
> >    urn:oasis:SAML:1.0 (@@double-check)
> >       a namespace from an Oasis specification
> 
> No problem up to here... really nice.
> 
> > Indentifiers in any of these schemes can be composed with
>    ^sp
> > a fragment identifier to yield an identifier for a
> > resource that is a part of, or view on, another resource:
> 
> This begins to wriggle (maybe)... at the very least this suggests that the
> sort of thing identified by a URI composed with a fragment identifier is a
> specialisation of the sort of thing that a plain URI identifies. The
former
> is 'constrained' to identify a resource that is either "part of" another
> resource or a resource which is a "view on" another resource. For there to
> be no specialisation here, for the resource identified by the URI#frag
> composition to be fully a first class resource, all resources would have
to
> be "part of" or a "view on" to some other resource (and maybe that is also
> the case... up to some 'Top').
> 
> I tried to discuss some of this with a colleague locally we got round to
> using Cars as an example to discuss this stuff to see where we get to...
so
> bear with me... I'm not sure where this will conclude...
> 
> Maybe we can think of the #1 piston of the engine of Dan's car. I've
chosen
> "Dan's" car because we can perhaps also explore in the in the context of
> httpRange-14 which question whether or not it is legitimate to identify a
> car with an absolute HTTP URI. We can consider both cases below. A car is
> also interesting because we can think of it in real-world terms with VINs
on
> chasis, engine numbers on engine blocks, and maybe batch and serial
markings
> on individual pistons.
> 
> Case 1
> ------
> http://example.com/myCar       identifies a particular Car.
> 
> http://example.com/myCar#engine   identifies the engine in the car
identified
> by http://example.com/myCar.
> 
> http://example.com/myCar#piston1   identifies the #1 piston (in the
> engine) in the car identified by http://example.com/myCar.
> 
> 
> That the piston and the engine are part of the car is evident from their
> respective identifiers. That the piston is also part of the engine is not
> evident from the identifiers... but I could 'mint' another identifier for
> the engine, and identify the piston as part of the engine. Hence:
> 
> http://example.com/myCar/engine   also identifies the engine in the car
> identified by http://example.com/myCar.
> 
> http://example.com/myCar/engine#piston1
>                   also identifies the #1
> piston in the engine in the car identified by http://example.com/myCar.
> 
> 
> Similarly, if there were parts of a piston to speak of then we could
> 'promote' the piston to having a URI such that:
> 
> http://example.com/myCar/engine/piston1
>                   also identifies the #1
> piston in the engine in the car identified by http://example.com/myCar.
> 
> Note that the 'equivalence' of the multiple identifiers of the piston is
> only asserted by the authority that assigns the URI. It is not a general
> property of URI syntax that replacement of a '#' with a '/' results in two
> identifiers that identify the same resource, but it is a convention used
in
> this example.
> 
> Case 2
> ------
> http://example.com#myCar
>    identifies a particular Car.
> 
> Now I'm stuck...! How do name the engine in a way that reflects that it is
> part of the Car? So... scratch the above and go postfix (shouldn't be hard
> for some one from HP).... Start again:
> 
> http://example.com/myCar#
>    identifies a particular Car.
> 
> http://example.com/myCar#engine
>    identifies the engine in the car identified 
>    by http://example.com/myCar#
> 
> http://example.com/myCar#piston1
>    identifies the #1 piston (in the engine) in 
>    the car identified by http://example.com/myCar#.
> 
> 
> And as before we can 'prompt' fragments into a URI so that we can further
> fragment the fragment, hence:
> 
> http://example.com/myCar/engine#
>    also identifies the engine.
> 
> http://example.com/myCar/piston1#
>    also identifies the #1 piston.
> 
> and
> 
> http://example.com/myCar/engine/piston1#
>    also identifies the #1 piston.
> 
> The 'equivalence' of various of these identifiers again is assured by the
> assigment authority and *not* the URI syntax.
> 
> This also provides scope to ask... so what to http://example.com/myCar,
> http://example.com/myCar/engine etc. identify... and the assignment
> authority might reply... well that's easy... they identify documents that
> describe the various things identified by extending each identifier with a
> '#'. 
> 
> However, this does then leaves the ambiguity about whether
> http://example.com/myCar#engine identifies a part of a car or a part of a
> document. Hmmm I think that the balloon just bulged somewhere else.
> 
> Conclusion
> ----------
> With case 2 I was beinging to feel that I understand where TBL is coming
> from on httpRange-14... however I was disappointed to end in an ambiguity.
> 
> With case 1, a part, SP, of a part, P, of a resource, R, can realily be
> identified as a part of R. But to refer to SP as a part of P requires that
P
> also be identified by an independent URI.  The equivalence of the URI of P
> and the compostion of the URI of R with the fragment identifier of P is
> assured only by the assignment authority and cannot be deduced from the
URI
> syntax of the identifiers.... which are opaque in general (by design).
> 
> --
> 
> Sorry that was so long... I found it useful... apologies if you do not.
I'd
> be interested in your thoughts. Clearly a car, its engine and a piston
> within that engine are all resources... case 1 does seem to work... and
the
> simple conclusion is that in order to identify a part of something as a
part
> of that something the URI reference, the something needs to be identified
> with URI (no fragment).
> 
> I would have liked case 2 to have worked... it was doing fine until I
answer
> the question of what the plain URI (might) identify. Maybe there is a
> different answer eg. they don't identify anything which makes case 2 work.
> But I think it then makes the two cases equivalent with infix and postfix
> use of the '#' and a need to adminsitratively assign distinct URI to
> resources and documents (also resources) that describe those resource.
> 
> --
> 
> Ok... real bottom line is that I have convinced myself that a URI+fragment
> can reference a resource (case 1). Queue the counter arguments.... :-)
> 
> >    ftp://example.org/aDirectory/aDocument#section1
> >    http://www.example.org/aList#item1
> >    http://www.example.org/states#texas
> > 
> > Note that while this composition is syntactically fully general,
> > many cases such as mailto:nobody@example.org#abc
> > don't make much sense to any deployed software or
> > specifications.
> > 
> > To summarize, a <dfn>Uniform Resource Identifier</dfn>, or
> > <dfn>URI</dfn>, is a
> > character sequence starting with a scheme name, followed by
> > a number of scheme-specific fields, optionally
> > followed by a fragment identifier.
> 
> Personnally I continue to prefer that we align our terminology with
RFC2396
> and use that terminology precisely and carefully. And... despite the above
I
> do believe that it would be folly to redefine the term URI in a way that
is
> inconsitent with RFC 2396 or its successor. *IF* a successor to RFC2396
were
> to make such a terminological change, I would be 'ok'ish for the TAG's
> Architecture to follow suit... BUT I think that such a change would be a
> huge diservice those who in the past have been careful in their use of the
> URI/URI reference (and same-document reference) terms. I think the likely
> maintenance impact on existing specifications that have carefully got it
> right should be objectively evaluated by those who advocate such a change
(I
> was going to say doesn't bear thinking about, but that's more emotional
than
> rational).
> 
> So... yes I continue to think such a change in the use of tersm is indeed
> folly.
> 
> > This URI syntax is accompanied by a shorthand
> > <dfn>URI reference</dfn> syntax.
> > A URI reference is an abbreviation of a URI that can be expanded
> > by combining it with a base URI. For example, in a document
> > whose base URI is http://example/dir1/dir2/file1 ,
> > the URI reference ../file2 abbreviates http://example/dir1/file2
> > and the URI reference #abc
> > abbreviates http://example/dir1/dir2/file1#abc.
> 
> The latter I think is wrong, #abc is a same document reference is is *not*
> evaluated with respect to a base URI [RFC2396 see Sections 4.2 and 5.2].
> 
> Again, I would request the use of the RFC2396 terms "Relative URI" and
"Same
> Document Reference"... yes the identifiers in your example are URI
> References too... but is the "Absolute URI" that you used as a base URI.
> Mixing these up going forward will, IMO, add confusion rather than
> clarification.
> 
> > [[NOTE: The current URI specification, RFC2396, uses a more
> > constrained definition of the term URI; by that definition,
> > identifiers that include fragment identifiers are not URIs.
> > The TAG intends to request a revision to RFC 2396 to adopt
> > the less constrained definition used here.]]
> > 
> > =======
> > 
> > then continue with 1.2 Resources and URIs.
> > 
> > 
> > > [2] http://www.w3.org/2001/tag/#tag-attn
> > > -- 
> > > Ian Jacobs (ij@w3.org)   http://www.w3.org/People/Jacobs
> > > Tel:                     +1 718 260-9447
> > -- 
> > Dan Connolly, W3C http://www.w3.org/People/Connolly/
> 
> 
> regards
> 
> Stuart
> 
Received on Friday, 16 August 2002 12:02:35 GMT

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