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Re: Suggestion for NEW Issue: Pipelining problems

From: Mark Nottingham <mnot@mnot.net>
Date: Thu, 28 Feb 2008 11:00:55 +1100
Cc: "Roy T. Fielding" <fielding@gbiv.com>, "Jeffrey Mogul" <Jeff.Mogul@hp.com>, "Harald Tveit Alvestrand" <harald@alvestrand.no>, "Jamie Lokier" <jamie@shareable.org>, yngve@opera.com, "HTTP Working Group" <ietf-http-wg@w3.org>
Message-Id: <71584298-EAA6-43E0-BC19-EEFE35B7E96E@mnot.net>
To: Justin Erenkrantz <justin@erenkrantz.com>

There hasn't been any traffic on this thread for some time, and I  
neglected to open an issue for it. See: <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/99 
 >.

Based on the discussion so far, I think this is where we are WRT  
pipelining:

a) In the scope of HTTPBIS, the most we can do is insert some  
implementation advice that pipelining may be incorrectly implemented  
by origin servers and intermediaries (including "hidden" infrastructure)
b) However, we shouldn't deprecate it, as it is still useful (and  
actively used) in some cases
c) "fixing" the pipelining problems may involve a new transport  
protocol (see discussion of SCTP) or other mechanism, but is outside  
the scope of HTTPBIS.

To that end, I propose we add implementation advice as per (a) and  
close the issue. Thoughts?

If that sounds reasonable to people, I can trawl the archives for  
different issues that might be encountered as fodder for  
implementation advice.

Cheers,


On 09/08/2007, at 5:20 AM, Justin Erenkrantz wrote:

>
> On 7/23/07, Roy T. Fielding <fielding@gbiv.com> wrote:
>> of the network.  Likewise, pipelining has huge benefits for
>> specialized HTTP services, such as Subversion (when it uses it)
>> and content management systems.
>
> ra_serf (optional in Subversion 1.4+ and using the serf client
> library) uses pipelining.
>
> The pipelining problems I've seen in the real-world are:
>
> - Not knowing how many requests this connection will be able to serve.
> The default in httpd is 100 requests per connection, but
> svn.collab.net tuned it down to 10 - which caused all sort of fun when
> testing ra_serf against Subversion's own repository.  I eventually got
> them to up it back to the default.  serf tries to figure out the limit
> heuristically (i.e. write as much as possible, then count how many
> responses we got before the connection closed - that'll be the limit
> going forward).
>
> - Lost responses are, sadly, real.  Later versions of httpd 2.0.x
> re-introduced a lingering close bug where it won't wait for the last
> packet to be written; this is fixed in 2.2.x, but have to be accounted
> for.
>
> In general, what I settled upon is that serf remembers all in-flight
> requests so that if we don't get a response back we can re-send them
> if something happened (i.e. lost response or hitting the cap on the
> number of requests on that connection).  It's really the only reliable
> mechanism for dealing with pipelined requests.
>
> Out of order pipelining would be nice for things like WebDAV, but a
> bigger problem is that the WebDAV protocol requires specific methods
> to be executed in lock-step using the results from the prior response
> as input for the next request.  This is the largest protocol headache
> I have with WebDAV at the moment.  Subversion would ideally like to be
> able to use pipelining commits with WebDAV, but that'll likely require
> us forking the protocol in some sense to let the underlying methods be
> executed out-of-order as well.  -- justin
>


--
Mark Nottingham     http://www.mnot.net/
Received on Thursday, 28 February 2008 00:01:09 GMT

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