W3C home > Mailing lists > Public > www-tag@w3.org > August 2003

Re: httpRange-14

From: Tim Berners-Lee <timbl@w3.org>
Date: Fri, 1 Aug 2003 16:05:44 -0400
Cc: Norman Walsh <Norman.Walsh@Sun.COM>, Public W3C <www-tag@w3.org>
To: "Roy T. Fielding" <fielding@apache.org>
Message-Id: <88D1CA5C-C45B-11D7-AB0E-000393914268@w3.org>

On Tuesday, Jul 29, 2003, at 04:33 US/Eastern, Roy T. Fielding wrote:

>>> You have left out all of the HTTP-based services that are not
>>> documents, by any stretch of the imagination, and yet still are
>>> identified by "http" URIs.  "almost all" is not ALL.
>>
>> You are of course right that there are services,
>> but those still are not people. Norm's argument stands.
>
> How can it stand?  If you point at one person and observe that they
> are tall, does it stand to logic that all people are tall?  Or even
> most?  No.  Assertions about an entire set are false if they are
> found to be false for any member of that set.
>

Yes, but if you read my message you will see that his concept
of  documents (which is the 99% case) can be refined to be
a set which you would agree to.

>> You are formally right, but with respect to Norm's argument,
>> you are splitting hairs.
>> If we call the things which are there for GET "documents"
>> and the things which are there for POST, "services",
>> and the things which are  just there for HEAD "hopefulls",
>> then all these classes of thing (of which we talk about documents
>> mostly, as Norm did in his simplification) are still not people.
>>
>> If you take the union of all these things, it does not include people.
>
> How do you know?  Do we need to turn this into a Turing test?

We are writing the HTTP specs.  We define the properties of
the things which take part in the protocol.
I know we talk at cross-purposes here because  you
insist on using the word "representation" in a way which
allows you to try to define HTTP in a way which denies us
the right to identify what it actually invariant about the
URI.

> You are claiming that the identifier places restrictions on what
> can be named because it can be used as an identifier in an anchor
> (or submit, "Location" dialog, etc.) and activated.

No, I am not.

>   That is the
> basis of "http" means "document" claim of httpRange-14.

No, it is not.

> I am saying that the information system matches identifiers to
> representations according to a set of rules completely unknown
> to the client.  Given any URI of any scheme, I can place that URI
> into a correctly implemented user agent and it will be acted upon
> as if it were what you call an "information resource".  That is
> because the ONLY thing that makes it an information resource is
> the context in which the URI was used: the retrieval context of
> an information retrieval system.

Maybe we should talk about that context, then, as a way of avoiding
the "but it can be a service too" argument.

> If we take the same identifier and place it in a non-retrieval
> context, such as an xmlns attribute or an RDF assertion, then
> it no longer acts like an "information resource."

Sorry, but the semantic web architecture absolutely needs the idea
of information resources.  The RDF identifier foo#bar is
used by dereferencing foo and parsing it a  get information.

>   The URI does
> not change, nor does the resource, so any claim that the scheme
> causes the resource to fall into one category or another is false.

The scheme is a scheme for identifying things in a certain class.

> That holds true for all URI schemes, without exception.

This is totally wrong, Roy.  It isn't the web as I designed it.
The range of different identifiers are specifically different
classes in the architecture.

> There must not be any exceptions, since it is an axiom of the current
> Web architecture that identification is orthogonal to interaction.

That is a vaguely-stated rule of thumb which is good when comparing
different schemes for the same kind of thing.
But to say that one should not be able to make up a schme which has a 
particular range is architectual mistake.  For example, one must be 
able to define  ISBN number scheme and a car license number scheme such 
that their ranges are distinct, so that you can't put an ISBN number on 
a car licence plate for example.

Especially when the role of a new software concept is quite different 
from the roles of existing ones - that they just do not support the 
same operations.

>> It is still important for somebody who visits Mark Baker's web page
>> to be able to make comments about it as a work without having
>> to call him to find out whether the URI is being used for him or
>> his dog or a galaxy somewhere.
>
> It is important for somebody to be able to make comments about the
> resource, what might be obtained from that resource, who manages
> that resource, and how all of that might vary over time.

Indeed.

> That stuff
> is rarely defined by the scheme ("data" being one exception).

Agreed. only the class of thing is, and then inly in certain cases.

All I am saying is that the mapping the URI identifies that which is 
common among the set of representations, and that that is not the 
subject, but the information content. And the web actually depends on 
that invariance to be useful.

The assertion that the URI "identifies" anything other the consistency 
among set of representations returned (when they are returned, and just 
focussing on the information space at the moment, not services)


>> I am sorry, while there are two distinct things which are meant
>> by a single URI (say, Mark and his web page), then the system
>> is unusable for semantic web purposes until we can resolve which is 
>> used.
>
> I don't believe that is true.  Use an explicit assertion; that is more
> reliable than a false assumption, regardless of the system being 
> defined.

(a)  In the proposed architecture the assumption would be true, not 
false.
   when you argue the demerits of the architecture, you have to use its 
rules.
   So in my architecture, the assumption is that the URI can be used
to refer to the web page by anyone who has come across the web page.
(b)  In your alternative architecture, where does one get the explict 
information from - which allows one to refer to a web page by its URI 
without meaning bridge or a person?


[...]


>>>   The scheme is irrelevant to such assumptions.  The right
>>> solution is to fix the Semantic Web so that it doesn't throw away
>>> method semantics, as it does currently by assuming a URI denotes
>>> what is obtained by a response to GET.
>>
>> It does NOT assume that it denotes what you get in response.


> Then you can't say anything about how the true nature of the
> resource might differ from the pages you get interacting with it.

(I assume by "web page" here you mean "representations". My concept of 
the w"eb page" is an abstract thing whose existence you won't admit to, 
and which is the core of this argument. So when you use te term 'web 
page" I have to guess what you mean in your architecture.)

I don't know in these case whether you are trying to use my vocabulary 
or yours.  When as sentence begins "Then" after mine, I would assume 
you were trying to use mine and prove inconsistency.

> The claimed ambiguity was on the basis of retrieval, which means
> GET.

It is connected with GET, but not defined by GET.
A picture of a car is for me a concept itself.

It isn't defined by GET in that if no GET happens it still exists.

>> Please don't put words in people's mouths, incorrect ones.
>> This is explained in painstaking detail in
>> http://www.w3.org/DesignIssues/HTTP-URI.html,
>>  which was presented at the TAG F2F in vancouver in 2002-09.
>
> Which has been responded to, in painstaking detail.  For that
> matter, I would appreciate it if you would stop bringing out the
> car example

Excuse me, a solid example is a good basis for understanding each 
other's point of view.

>  and claiming things about what I think when I have
> already told you FIVE times now they are not what I think.

Could you point to one place where I have misrepresented what you think?
Specifically, if it is in the car example page I would like to correct 
it.

> I am sorry, Tim, but on this issue you have consistently refused
> to listen to any of my comments and those of the rest of the TAG --

I have tried.  I have found yours the most difficult.

> you can't even distinguish between our comments and those of
> Mark Baker.

In which case?
Well, I have heard you and he share the belief that an HTTP URI without 
a hash can identify a person.  I have not heard you go along with his 
idea that he is both a person the web page are in fact the same thing.  
  I hope I have not mis-attributed comments, but if I have I apologize. 
Again, if it has happened in an editable page, point me to it and I 
will fix it.

> You made up your mind a year ago and haven't heard
> a word since.

Ouch.
We could both say that.  I have listened to a lot of arguments.

> In my considered opinion, your "desirable distinction" for SW is
> undesirable for the deployed Web, fatal for Web Services, and
> not considered necessary by anyone else I've talked to that
> are currently working on SW.

See Pat Hayes
http://lists.w3.org/Archives/Public/www-tag/2003Jul/0129.html

"We need to get clear on this issue, or else we will continue to be  
mired in confusion.
[...]
The *source* of a representation and what might be
called the *topic or content* of the representation need have very 
little to do with one another"

>  I am not giving you that opinion
> because I like to wear out my fingers on the keyboard or spend
> oodles of money traveling to face-to-face meetings.

When, it is *my* considered opinion that the distinction is one on
which the web relies at the moment. Ignoring it is not fatal for the
web (which makes it implicitly) but will threaten new designs
which may explicitly and deliberately break the rule.

(The distinction is the fact that we expect a consistency in 
information content of dereferenced HTTP URIs, I assume)

> I have heard and understood every one of your comments, and I
> understand that you believe it is important for the Semantic Web
> to be able to distinguish these things.

People and their home pages? yes.

> However, the design
> principle of separation of concerns that led to the "orthogonal
> protocols deserve orthogonal specifications" constraint on the
> Web architecture is far more important than any of the perceived
> benefits you claim for SW.

It is a good rule of thumb, but when it blinds you to
a lack of orthogonality  which is in fact important,
or when it leads you to construct a logical system which is 
inconsistent, then
I would say it was being taken too far.

> ANY claim that a resource type
> distinction can always be determined by examining the URI scheme
> is false and always will be false, so if you build such an
> assumption into the Semantic Web then you are dooming that system
> to a rather dismal future.

This only comes up with the semantic web because
for the first time we have the tools and the need to formally
model both people and web pages in the same system.

The properties which I am observing are those of the web, though.
And properties on which it depends.

> Find another solution to your problem,
> preferably one that doesn't run counter to the established design
> principles that we have worked with for over a decade.

You have worked with these in theory, but
I don't believe that the software you have written  is actually is a 
test case, actually
has concepts of people and web pages and can really discuss the 
question of
whether they are the same thing.
You may have worked with the principle in theory that URIs denote 
people rather than web pages, but is there software you have built has 
actually dealt with both types of thing?

And I believe that you have quoted URIs many many times, expecting a 
consistency not in the subject portrayed but in the information content
of the representations.

Tim

> ....Roy
Received on Friday, 1 August 2003 16:05:50 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Thursday, 26 April 2012 12:47:20 GMT