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Re: 303 for paging; was Re: 2NN Contents Of Related (303 Shortcut)

From: Roy T. Fielding <fielding@gbiv.com>
Date: Sun, 7 Sep 2014 10:30:02 -0700
Message-Id: <0454F561-F91E-4ECE-9A95-9CBCD582088B@gbiv.com>
Cc: Martin Thomson <martin.thomson@gmail.com>, Eric Prud'hommeaux <eric@w3.org>, Mark Nottingham <mnot@mnot.net>, HTTP Working Group <ietf-http-wg@w3.org>, "Julian F. Reschke" <julian.reschke@gmx.de>
To: Sandro Hawke <sandro@w3.org>
> On Sep 7, 2014, at 7:43 AM, Sandro Hawke <sandro@w3.org> wrote:
> 
>> On 09/05/2014 01:54 PM, Roy T. Fielding wrote:
>>> > The actual driving force behind this I-D is not about using 303s to deal with httpRange-14, it's to deal with paging.  That is, the client does a GET on A, including these request headers:
>>> > 
>>> >   Prefer: contents-of-related
>>> >   Prefer: return=representation; max-triple-count="100"
>>> > 
>>> > and now the server can directly provide the first hundred triples, via a representation of B, which is that the first "page" of A.
>> The first hundred triples is a representation of resource A.
>> There is no requirement, anywhere, that representations be complete.
>> Prefer in this case is just another form of content negotiation and
>> the response is 200.  Responding 303 in this case would be wrong,
>> as would 2NN.
>> 
> 
> This is a crucial point.    If even responding 303 is wrong, we have a bigger problem.
> 
> It sounds to me like you're saying that a chapter of a book is the same thing as the entire book.   

No, you are assuming it is a book.

> Let's say we have an online textbook available at:
> http://example.org/WebDesign
> and each of its 40 chapters is available as a separate web page, with chapter number CC being available at
> http://example.org/WebDesign?chapter={CC}
> 
> Now imagine a client does GET http://example.org/WebDesign, with a prefer header saying it's fine to just send the first chapter if the book is too big.

Then that resource is more than just a book. It provides views of the resource state, which is no more or less than what content negotiation provides in HTTP.


> I think you're saying it would be fine for the server to respond 200 OK, Content-Location: http://example.org/WebDesign?chapter=1, and give the content of that first chapter.

The server can respond however they like. It is only the consistency of those responses that defines what the resource might be, to any extent that matters.

> To my understanding, that's wrong, because it violates the semantics of 200 OK and Content-Location.  Specifically, since a book is not the same thing as its first chapter, http://example.org/WebDesign and http://example.org/WebDesign?chapter=1 are distinct resources.  If they are distinct we can't use 200 OK+CL to respond to one with the other.

But the first URI is not a book. You cannot define it as one thing and then say its behavior doesn't fit that definition. It is not that thing.

> I think I hear you saying that resources being distinct doesn't matter, that the notion of "representation" is much fuzzier than that.  I think you're saying that even though a chapter of a book and a book are different, it's fine to response 200 OK and give the text of the chapter as a representation of the book (assuming there was some negotiation licensing such behavior).

No, I am saying that the responses define the resource, not the other way around. If you have distinct URIs with decidedly different behavior, like your chapters, then they are of course distinct resources. So is the resource that always responds with a complete book. It is defined by what it does.

A successful response to GET is a representation of the target resource. Range, for example, does not alter the representation; it simply provides a range of that representation in the payload of a 206. Conneg doesn't alter the representation either. It merely selects one of the available representations.

So, if 2NN is a success to GET, it has to convey a selected representation of that resource.  Not some other resource, though it is fine for a single representation to represent multiple resources. 3xx, in contrast, says the request was not successful but this other thing might be just as good.

There is nothing wrong with having a representation of a resource that is a paged view. The only need is that something in the representation data or metadata indicates that view, preferably with prev, this, and next links. CL is a "this" link. The resource is then partially defined by the fact that it offers views of the overall state. All 200 responses. No big deal.

> If the HTTP WG really has consensus on that idea, I guess I can live with it, even though it's counterintuitive to me.
> 
> But how far does this go?    When is 200+CL not okay?    Could the first sentence of the book be a representation of it?   How about the the first letter?   How about the 10th letter?    How about the 13th, 7th, and 22nd letters, in that order?    How about the first word of a different book?   How about the first sentence of a different book?   How about the entire contents of a different book?    I don't see how you can draw a line here, with this way of thinking about it.

There are no lines. There is no need for any.

> To me, and the LDP WG, it's made a lot more sense to think of a chapter as simply being a different resource than the book, so if the server's going to give back a representation of the chapter, it can't use 200 OK.

The server is in control of what it's resources mean. If a resource is not limited to being an entire book, then it isn't an entire book. There's no reason to pretend otherwise.

....Roy.
Received on Sunday, 7 September 2014 17:30:27 UTC

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