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Re: Review of new HTTPbis text for 303 See Other

From: Pat Hayes <phayes@ihmc.us>
Date: Sun, 12 Jul 2009 18:31:56 -0500
Cc: "Roy T. Fielding" <fielding@gbiv.com>, Jonathan Rees <jar@creativecommons.org>, Julian Reschke <julian.reschke@gmx.de>, HTTP Working Group <ietf-http-wg@w3.org>, www-tag@w3.org
Message-Id: <41101FBA-0B1E-4A3E-B30D-F2AD9EFBFFB1@ihmc.us>
To: Richard Cyganiak <richard@cyganiak.de>

On Jul 11, 2009, at 5:27 AM, Richard Cyganiak wrote:

> Pat,
>
> On 10 Jul 2009, at 01:32, Pat Hayes wrote:
>>> If the server has a transferable representation, it would
>>> respond to the GET with the appropriate status code (200 or 304).
>>
>> Well, yes, IF it were driven solely by what one might call rational  
>> HTTP architectural principles. BUt surely the whole issue about  
>> httprange14 is that it introduces new principles which on their  
>> face have nothing to do with http architecture as such, but to do  
>> with denotation and naming.
>
> Not as far as HTTP is concerned. HTTP is just a transfer protocol.  
> The HTTP world is really simple:
>
> 1. There are URIs. URIs are thought to identify things called  
> resources.

OK, stop there and tell me what you mean here by "identifies".  
Because...

> As far as HTTP is concerned, it does not matter much what the  
> resource actually is -- a document, a file on a server, a person,  
> whatever.

... in the usual sense of 'identifies' that one might expect to be use  
in the context of a network transfer protocol, which is similar to the  
use one might expect when talking about programming language  
identifiers and what they identify, it most certainly does matter. In  
particular, it simply does not make sense to speak, using that normal  
terminology, of 'identifying' a person (or a galaxy or a sodium atom,  
etc.); in fact, it does not make sense to talk of identifying anything  
much beyond some kind of data structure or data object. So if HTML  
claims to be able to make sense of talking of 'identifying' people  
(for example), it must be in a wholly different space than all  
previous computationally based notational systems, and be using the  
word "identify" in a wholly different sense. And, to repeat, can you  
tell me what that sense is?

>
> 2. Resources (whatever they are) are thought to have things called  
> representations. As far as HTTP is concerned, it is totally up to  
> the server owner to decide what's a representation of what. After  
> the server owner has made their decision, a resource either has a  
> representation or not.

Really? OK, I will take you at your word. I am a server owner, and I  
will decide that a certain resource, to wit, me, has a thing called a  
representation of me. This representation of me is in fact a portrait,  
painted using acrylic paints on a piece of masonite approximately 30  
cm square almost exactly a month ago: but let us not go into details,  
as you tell me that such details are none of HTTP's business. Still,  
the representation exists, and the resource has it. OK, let us proceed.

>
> 3. If a resource has a representation, then a GET to its URI should  
> be answered by 200. If not, then 303, 404 or 410 would be fine  
> choices.

So, HTTP must reply to a GET on my URI with a 200. OK, what should it  
put as the payload of this 200 response, attached to the code  
information? HOw do I get acrylic-coated masonite into an http  
response? There is no representation which can be transmitted in bits.  
You did not mention this aspect in your above summary: was that an  
omission?

>
> I repeat: For the operation of the HTTP protocol, IT DOES NOT MATTER  
> what exactly a resource is and what the exact relationship between  
> resources and representations is.

As you can see, I took advantage of this freedom in my example.

> All these matters of denotation, information resources and so on are  
> introduced by higher layers of the architecture.

Wrong. Denotation is not introduced by a higher level, and even if it  
were, it would not be higher in an architectural sense. You, in this  
very message, in fact brought denotation into the picture, by telling  
me that a URI can "identify" a person. URIs are symbols strings, and  
the ONLY POSSIBLE SEMANTIC RELATIONSHIP between ANY symbol and a  
physical object, is denotation. Sorry to shout there a little, but the  
point needs to be made strongly. That is what "denotation" means: it  
is all that is left of "identifying" when you take away the actual  
network machinery, the computational byte-transferring. And you have  
to take this away when you start claiming to talk of relationships  
between names (of any kind) and non-computational entities such as  
people (or indeed of any kind), simply because computational byte- 
transfer talk is COMPLETELY IRRELEVANT to semantic relationships (such  
as "identification") between symbols (of any kind) and non- 
computational entities. The fact, if it is a fact, that this word is  
not in your technical vocabulary is entirely irrelevant. By claiming  
that your symbols "identify" non-computational entities such as people  
or books (or the weather in Oaxacala, to take another random  
example) , you are no longer playing in the network-architectural  
sandbox, precisely because these kind of things simply are not  
connected to networks in the same functional sense that things like  
web servers are. Either HTTP is a computational notion or it isn't. If  
it is, then it is indeed quite simple. And I would be delighted if the  
HTTP literature simply restricted itself to the computational world.  
But it does not, and never has: HTTP has ALWAYS had these claims to  
semantic grandeur: it has ALWAYS claimed to be not just about web  
sites and web servers and files and documents, but about the whole  
grand span of symbol usage to refer to absolutely anything in any  
possible universe. And if indeed that is what HTTP is claiming to be  
able to talk about, then it is about denotation, right out of the box.

>
> Yes, it would be useful to provide guidance to publishers about how  
> best to model their information space as resources and  
> representations. But this is out of scope for the HTTP protocol.

See above. If indeed it is out of scope, so is any talk of URIs  
"identifying" people. You can't have it both ways. Either you are  
doing real semantics or you aren't. If you aren't, then don't make  
ridiculous claims about "identifying" things that have no possible  
connection to any physical network, or of "representations" that  
cannot be sent in a byte stream.

> The HTTP protocol kicks in AFTER the publisher has made up their  
> mind about what resources they have and wether they have  
> representations or not.

OK, please tell me how to use HTTP to send my piece of masonite  
attached to a 200 code. I've made up MY mind: over to you.

>
> Now, different subcommunities have different opinions on how to  
> model resources and representations. That's not a good thing, and it  
> would be good for interoperability if everyone agreed. However, this  
> is pretty much orthogonal to any discussion of the HTTP protocol. As  
> long as the subcommunities subscribe to the basic "URI-identifies- 
> resource-which-can-have-representations" model, HTTP can accomodate  
> them.
>
> Now let me take off my RDF hat for a bit.
>
> The suggested change for the 303 text came about because one  
> subcommunity had the funny idea that some resources SHOULD have URIs  
> but NO representations and it should STILL be possible to get  
> information about them via HTTP.

No, that is not the primary reason. Http-range-14 is not about  
resources, it is about URIs and what they denote. The dilemma is that  
people want 'normal' URIs to denote what it that HTTP thinks of them  
as identifying, the "information resource" (not that that matters).  
Which would be fine, except that there are some URIs which people want  
to denote something else. And still, actually for different ('linked  
data', Timblish) reasons, people want a GET on those URIs to finish  
up, one way or another, with useful information being returned. This  
is a problem. It would be ugly to have two 'kinds' of URI, and  
impossible to change the millions of 'normal' URIs in any way at all.  
The decision allows the few non-normal URIs to take part in a slightly  
irrational HTTP dance which allows everyone to say: look, since it  
didn't return a 200 code, its not 'normal', and HTTP says it doesn't  
identify anything at all; so the 'normal' assumptions about what it  
denotes are cancelled. And that cancellation is the entire content of  
the decision: it has no other purpose. The nature of the entity which  
handles the GET, and the presence or absence of 'representations' of  
it, are irrelevant.

> It beats me why anyone would want to do that

The reason is that there are, believe it or not, entities in the  
universe other than web servers; and people want to refer to them  
using URIs.

> ; but if we can make them happy with a minimal tweak to the language  
> of an existing status code, then why not. HTTP is for everyone.
>
>> If the URI in the GET request is not intended to denote the  
>> resource to which the GET is directed, then that resource must  
>> issue a 303 redirection, and must not return a representation using  
>> a 200 status code.
>
> There is no such thing as denotation in HTTP. The only relation  
> between URIs and resources in HTTP is "identifies".

Which, if i means anything at all when used between a symbol and a non- 
computational entity, means 'denote' (or, if you prefer, 'refers to'  
or 'is a name for'; they are all equivalent usages.) And again, I  
challenge you (or anyone else) to tall me what "identifies" can  
possibly mean, in thee circumstances, other than this.

> If you care about other relations, you have to figure out how to  
> translate them into the "URI-identifies-resource-which-can-have- 
> representations" model of HTTP.

That model is either (1) already about denotation, or (2) utterly  
broken, or (3) meaningless as stated.

>
>> That has nothing to do with the existence or not of such a  
>> representation. Even if the representation exists and the server  
>> has access to it, it cannot return it with a 200 code when the URI  
>> is intended to denote some other thing, in particular a non- 
>> information resource of some kind.
>
> Wether a representation exists or not for a particular kind of  
> resource is entirely up to the server owner, as far as HTTP is  
> concerned. If you subscribe to a religion that says, "Thou shall not  
> make a representation of me, for I am not an information resource",  
> then that's great, and let me shake your hand brother, but this has  
> no effect on HTTP.

But thats the easy case. The hard case, for you, is when I use that  
very handy English word "representation" is one of its normal senses,  
not when I refuse to use it at all. There are many,  many kinds of  
representations of things, and only a miniscule proportion of them  
have anything even remotely to do with computers or network transfer  
protocols.

>
>> If we follow your rule, above, and also httprange14, then a server  
>> can be placed in an impossible position. If it has a representation  
>> of itself which  could be put into a 200-code response, and it  
>> receives a GET request with a URI which it knows (somehow, perhaps  
>> by some externally agreed convention) is being used to denote a non- 
>> information resource; what should it do? HTTPrange14 requires it to  
>> not deliver a 200-coded reply, but your criterion requires that it  
>> must. This is why I think the wording should make absilutely  
>> minimal assumptions about what exactly the 303 means.
>
> (RDF hat back on) Any sensible definition of "non-information  
> resource" obviously MUST entail "does not have representations in  
> the HTTP sense". In fact, that IS the definition of "non-information  
> resource", in my book.
>

Of course, but that is completely irrelevant to my point. The server,  
in my example, is not the non-information resource that the URI refers  
to; that is precisely why httprange14 requires it, the server, to emit  
a 303 code rather than a 200 code. It is merely the servant whose job  
it is to emit the appropriate code to make everything work properly.  
But it is AN information resource, and it may well have a  
representation (in the http sense) of itself. Its just a different  
resource than the one the URI denotes/refers to.

> Wrapping up:
>
> For the function of the HTTP transfer protocol, it does not matter  
> what exactly the nature of the things identified by URIs is.

Oh, but it does. Because HTTP talks about information transfer between  
entities which can transfer information, but it talks of  
'identification' of ANY THINGS WHATSOEVER, whether they can or even  
possibly could transfer information. For example, a numeral identifies  
a number, and also is a representation of it. So HTTP should apply to  
this case as well, according to what you say here. I should be able to  
send a GET request to the number seventeen and expect to get sent back  
a 200-coded response with a suitable numeral in its body, say "17". I  
know that is ridiculous: but it FOLLOWS FROM WHAT YOU ARE SAYING;  
ergo, what you are saying is ridiculous.  So you ought to modify what  
you are saying, so that it makes more sense.

>
> For the function of the HTTP transfer protocol, it does not matter  
> wether the things you serve as representations on your server make  
> particularly good representations of the resources.
>
> There are different schools of thought that try to clarify the  
> nature of the "identifies" and "has representation" relationships,  
> and this is critically important if we want to use HTTP URIs as  
> identifiers for things that exist outside of the Web. But the HTTP  
> protocol itself is and should be agnostic with regard to your  
> position in these debates. That's layering.

No, it is a poisonous combination of semantic (or maybe philosophical  
or semiotic) ignorance, and hubris. You want http to be universal, but  
you are claiming a kind of universality which goes way beyond anything  
to do with network architecture, and so you can't escape the  
consequences by appealing to network design principles.  Maybe you  
don't intend to be doing this, but it is being done by what you (and I  
should cast this in a kind of anonymous plural, as the excellent  
southern phrase y'all, as I don't intend this rant to be directed at  
you in particular) are actually saying.

Best wishes

Pat

>
> Best,
> Richard
>
>
>
>>
>> Pat
>>
>>> ....Roy
>>>
>>>
>>>
>>
>> ------------------------------------------------------------
>> IHMC                                     (850)434 8903 or (650)494  
>> 3973
>> 40 South Alcaniz St.           (850)202 4416   office
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>> phayesAT-SIGNihmc.us       http://www.ihmc.us/users/phayes
>>
>>
>>
>>
>>
>>
>
>
>

------------------------------------------------------------
IHMC                                     (850)434 8903 or (650)494 3973
40 South Alcaniz St.           (850)202 4416   office
Pensacola                            (850)202 4440   fax
FL 32502                              (850)291 0667   mobile
phayesAT-SIGNihmc.us       http://www.ihmc.us/users/phayes
Received on Sunday, 12 July 2009 23:33:24 GMT

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