On 15/09/2008, at 11:13 PM, Julian Reschke wrote:
Mark Nottingham wrote:
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.
Of course POST can do that. Did anybody argue something else?
No, just laying the groundwork...
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?
If I understand correctly, in the first example the server
immediately assigns the URI for the resource-to-be-created, let's
the client know it, and lets it transfer the remaining bytes to that
resource. In this case, the server would need to make sure that this
resource is only available to the client until the transfer is
completed.
... or use ETags on the requests to differentiate them.
In the second case, the server assigns a temporary URI which is just
used to complete the transfer. Once that's done, the "final"
resource is being created. This looks similar to what Roy proposed
in <http://lists.w3.org/Archives/Public/ietf-http-wg/2008AprJun/0082.html
>.
I think the second approach is more versatile, because it also
covers cases where the server wouldn't create a new URI upon POST.
... as would using ETags to differentiate requests. :) Understand
I'm not pushing that solution strongly (yet), It's just that they're
functionally equivalent so far.
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).
Yes.
BTW: it wouldn't necessarily be PUT; it could be anything that
allows "appending", such as POST or PATCH.
Good point.
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.).
Well, not entirely.
Let's say you've got three concurrent resumable transfers started
to /a, and the server has assigned the etags "C1-1", "C2-1" and
"C3-1" to the three clients.
Client 1 starts its upload:
REQ: POST /a
REQ: If-Match: "C1-1"
REQ: Content-Range: 0-50/100
REQ: [bytes]
RES: 200 OK
RES: ETag: "C1-2"
...what I'm concerned if is that we're essentially introduce ETag-
based variant-selection here.
So what do these ETags represent in requests *other* than resumable
uploads, such as in:
REQ: GET/a
REQ: If-Match: "C1-1"
?
Will it have an effect?
So just avoiding new URIs may look simpler first, but it also
requires additional specification work.
It will require additional spec work if this use of ETags is
incompatible with existing ones. If it is, they shouldn't be used for
this purpose at all, but I don't *think* they are (happy to be proven
wrong, as always).
I.e., yes, if-match will work as specified; what else needs to be said?
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
That assumes that PUT with Content-Range can be used today, which
really isn't the case, unless the client can be confident that the
server actually understands PUT with ranges.
That seems to be a problem with all the approaches on the table,
according to the flows in the current document. By the letter of the
law, if the server doesn't understand a Content-* header on a PUT
request, it should refuse it, but we already have an open issue or two
(#79, #102) on that...
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.
I agree with that part; the URI assigned for the upload really
should be temporary.
With that, the approaches are almost identical; in both cases unique
identifiers are minted (ETags or URIs), the server needs to deal
with house keeping, and the impact of other methods must be
understood and specified.
BR, Julian
BTW, we should really be talking about an Internet-Draft at this
point, rather than a wiki page. Google guys, when will that happen?
--
Mark Nottingham [EMAIL PROTECTED]
