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Re: [google-gears-eng] Re: Deploying new expectation-extensions

From: Mark Nottingham <mnot@yahoo-inc.com>
Date: Mon, 15 Sep 2008 12:05:59 +1000
Cc: Charles Fry <fry@google.com>, gears-eng@googlegroups.com, Alex Rousskov <rousskov@measurement-factory.com>, HTTP Working Group <ietf-http-wg@w3.org>
Message-Id: <58B2E71C-80A2-45CB-AEBD-DF14CC1575C6@yahoo-inc.com>
To: Julian Reschke <julian.reschke@gmx.de>


On 12/09/2008, at 5:25 PM, Julian Reschke wrote:

>>> As far as I can tell, if you use the ETag based approach, and  
>>> multiple clients try to post to the same collection (POST URI),  
>>> then you'll have to disambiguate the requests. That problem would  
>>> go away if each of them would use a different URL.
>> I read the doc as saying that the server would provide unique ETags  
>> somehow...
>
> Disambiguating by ETag probably would work, but that doesn't feel  
> right to me. If multiple resumable transfers can be in progress at  
> the same point of time, then this really sounds like multiple  
> resources (thus multiple URIs), not multiple variants of the same  
> resource to me.


Huh. That's very revealing, I think (if unintentional :) POST can  
already create a new resource with a new, server-selected URI, and the  
pattern for doing so is already described with POST, 201 and Location.

Question: If I want to make this sort of request resumeable, do I do  
this?

REQ: POST /a
REQ: Content-Range: bytes */100
RES: 308 Resume Incomplete
RES: Location: /b

REQ: POST /b
REQ: Content-Range: 0-100/100
REQ: [bytes]
RES: 200 OK

or this?

REQ: POST /a
REQ: Content-Range: bytes */100
RES: 308 Resume Incomplete
RES: Location: /z

REQ: POST /z
REQ: Content-Range: 0-100/100
REQ: [bytes]
RES: 201 Created
RES: Location: /b

?

The important part here is: is this protocol defining a "temporary"  
resource (with a very specific interface) for the Location in a 308  
refers to, or is the Location in a 308 referring to a "regular"  
resource that's used for more than that?

It's interesting to note that the second approach (with the temp  
resource) preserves the 201 status code in the interchange, while in  
the former approach, it's not there (308 usurps it).

Now look at it with PUT (to a not-yet-existent resource);

REQ: PUT /a
REQ: Content-Range: bytes */100
RES: 308 Resume Incomplete
RES: Location: /b

REQ: PUT /b
REQ: Content-Range: 0-100/100
REQ: [bytes]
RES: 201 Created
RES: Location: /a

Here, if we use URIs, /b *has* to be a "temporary" resource with a  
very specifically defined behaviour; it accepts PUTs and has a side  
effect of having its bytes copied to /a (presumably when the final 201  
is sent).

My point here is that there are actually some pretty deep differences  
between the URI approach and the ETag approach; the URI approach is  
much more intrusive and needs to be specified in a different way  
(e.g., talking about what methods to use, the nature of the resource  
created, etc.).

Back to your comment;

> Disambiguating by ETag probably would work, but that doesn't feel  
> right to me. If multiple resumable transfers can be in progress at  
> the same point of time, then this really sounds like multiple  
> resources (thus multiple URIs), not multiple variants of the same  
> resource to me.

I don't know that I agree; with PUT, it's very natural to use ETags  
(you avoid creating the temporary resource, and have the option of  
409'ing any concurrent PUTs after the first), whereas with POST,  
you're just pushing the assignment of a final identifier for the  
created resource until the entire request entity is received (which is  
the case with the URI-based approach anyway, unless you're arguing  
that POST is a special case and *doesn't* create a temporary resource,  
unlike PUT), and you still have the option of not assigning it any  
identity (just as many POST processors do today).

So, I'm firmly leaning in the direction of the ETags-only approach  
now; I think the selection of a URI for created resources is  
separable, and should be separate.

Cheers,

--
Mark Nottingham       mnot@yahoo-inc.com
Received on Monday, 15 September 2008 02:07:32 GMT

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