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Re: Concurrent non-error response disallowed. ("clarification of 7.2.2. Monitoring Connections for Error Status Messages")

From: Wenbo Zhu <wenboz@google.com>
Date: Fri, 14 May 2010 15:16:35 -0700
Message-ID: <AANLkTinikDrexPp6ngpxTHBQdWyZ6PN9g52_WqdaD-dD@mail.gmail.com>
To: Henrik Nordström <henrik@henriknordstrom.net>
Cc: Jamie Lokier <jamie@shareable.org>, ietf-http-wg@w3.org
On Fri, May 14, 2010 at 11:09 AM, Henrik Nordström
<henrik@henriknordstrom.net> wrote:
> fre 2010-05-14 klockan 18:33 +0100 skrev Jamie Lokier:
>
>> It is possible to implement servers where the response is an
>> incremental function of the request, and if it worked reliably, that
>> would actually be useful.
>
> There is applications operating in this manner, but the HTTP transport
> isn't well suited for this as you have no control over any buffering
> done by intermediary servers or even clients http frameworks operate.
>
>> As far as I can tell, it would be within the HTTP specs if a new
>> client was deliberately made which supported bidirectional streaming
>> POSTs over a single connection, and a new server took advantage of
>> that when it knew the client supported it.
>
> Yes, and such clients do exists, even if most select to either do it
> over an SSL connection or by using two parallell requests.
>
>> Then the sticking point would be what proxies do.
>
> Depends on the purpose of the proxy.
>
>> That requirement says nothing about *non-error* codes.  What part of
>> accepting those while continuing to send the request is broken?  Or
>> alternatively of not accepting those?
>
> None, other than me reading a bit too quickly.
>
>> Do you think the above idea, of a client supporting it and a server
>> making use of it if it knows the particular client supports it, would
>> work now?  That would halve the number of connections needed for
>> comet-style interactions.
>
> It will work in most environments, but you will find networks where
> intermediary proxies do buffer considerable amount of data or which may
> even serialize it down to request->response pattern.  HTTP does not
> provide any guarantee that full duplex communication will work.
True.

Then the question remains: 1) the behavior is disallowed by the spec
as it's not reliable - due to proxies (if not ideologically); or 2)
the behavior is supported, with all the implementation caveats and
related best-effort/fall-back concerns.
I am less concerned about breaking existing clients, as such a
behavior is completely controlled by the server, and specially enabled
for custom applications.

>
>> I mostly agree with Henrik, but if it's already common practice for
>> clients to abort when they see part of a non-error response, then I'm
>> in favour of the spec reflecting reality, instead of ignoring it.
>
> Depends on where you draw the boundary for the client. If you include
> the end user behavior in the client then most users do not wait for
> their browser to finish sending something if they have already got the
> response they are expecting.
>
> Most if not all clients if left alone by their user will happily
> continue sending the request without interruption provided the server
> does not close the connection on them.
>
> But some clients or intermediaries MAY get confused and think they are
> done when the server response is completed. This is however not
> something which is supported by the specifications and the cases I have
> seen has been pure bugs and not intentional.
>
> Regards
> Henrik
>
>
Received on Friday, 14 May 2010 22:17:31 GMT

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