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From sebb <seb...@gmail.com>
Subject Re: Bug 56119 - dealing with idle connection timeout and dropped connection
Date Sun, 23 Feb 2014 04:38:08 GMT
On 19 February 2014 21:51, sebb <sebbaz@gmail.com> wrote:
> On 19 February 2014 21:03, Philippe Mouawad <philippe.mouawad@gmail.com> wrote:
>> On Wed, Feb 19, 2014 at 9:58 PM, sebb <sebbaz@gmail.com> wrote:
>>
>>> On 19 February 2014 18:49, sebb <sebbaz@gmail.com> wrote:
>>> > It looks as though the problem reported in Bug 56119 was due to the
>>> > server dropping connections that have been idle too long.
>>> >
>>> > There may also be servers that only allow a connection to be reused a
>>> > certain number of times (this does not seem to have been the case
>>> > here).
>>> >
>>> > This email is to discuss what JMeter could perhaps do to make it
>>> > easier to test such servers.
>>> >
>>> > The two existing work-rounds are:
>>> > - disable Keep-Alive
>>> > - enable staleCheck
>>> >
>>> > Neither is ideal; the first is awkward to use, and staleCheck can
>>> > generate unnecessary additional traffic (which is why it was disabled
>>> > in 2.11).
>>> >
>>> > I can think of two possible approaches:
>>> >
>>> > 1) proactively shut connections. This would be easy for servers that
>>> > limit reuse.
>>> > Just count reuses and turn off keep-alive when a specified limit is
>>> reached.
>>> > Not so easy for idle timeouts; one cannot retroactively disable
>>> keep-alive.
>>> >
>>> > 2) Deal with the disconnects when they occur.
>>> > The code needs to distinguish which errors are retriable, and may need
>>> > to distinguish at what point the failure occurs. For example, even a
>>> > POST ought to be retriable if JMeter is unable to send any data on the
>>> > connection.
>>> >
>>> > Also need to consider how one might report retries.
>>> > I think the tester needs to be able to find out if additional requests
>>> > have been made by JMeter.
>>>
>>> Further testing against the ASF servers shows that HC 4.2.x does
>>> handle idle timeouts without needing to use the staleCheck option.
>>> This relies on the server sending a header of the form:
>>>
>>> Keep-Alive: timeout=5, max=100
>>>
>>> In this case, the connection is automatically recreated if necessary
>>> when the next sampler runs.
>>> If the server fails to send the header, then the connection may be
>>> dropped unexpectedly (which is what was happening with Bug 56119).
>>> So another approach might be to allow an optional keep-alive timeout
>>> in case the server does not provide one.
>>>
>>> Or we could take the view that there is nothing to fix in JMeter.
>>> The Keep-Alive header is there for a reason, it tells the client when
>>> it is safe to reuse the connectiion.
>>> If the server fails to send it, then it is broken, and so the failed
>>> samples are to be expected.
>>>
>>
>> I think we need to make something at least for servers like Amazon S3 which
>> close connections after number of uses.
>> Did you check to see if this kind of server send a keep alive header ?
>
> I just tested again with jmeter.a.o.
> It returns headers of the form:
>
> Keep-Alive: timeout=5, max=100
> Connection: Keep-Alive
> ...
> Keep-Alive: timeout=5, max=99
> Connection: Keep-Alive
> ...
> etc
> ...
> Keep-Alive: timeout=5, max=1
> Connection: Keep-Alive
> ...
> Connection: close
>
> So the HC connection manager does not need to keep track of the
> remaining re-use count; the server disconnects at the end of the last
> request.
> Nice and simple.
>
> I assume S3 does the same as jmeter.a.o if it is well-behaved.

I asked on the HC dev list, and found out that the Keep-Alive header
is optional.
So servers that don't send it are not misconfigured - though of course
it helps the client if they send the header.

Also, the HC default retry processing does handle the disconnect case,
i.e. where the server disconnects whilst starting to send the next
request.
Setting retry count to 1 allows the request to be retried.

JMeter previously used to enable retries, but we found this had
adverse effects, as it could generate extra requests.
It turns out that the default retry handler always retries failed
idempotent requests (e.g. GET), even if they were successfully sent.
This is perhaps what caused the additional traffic.

So I think we need an amended version of the retry handler which only
retries failed sends, if the retry count is 0.
We can use the HC version for retry counts > 0.

>> Anyway on my side I think what has been changed in 2.11 should not be
>> reverted, because for servers correctly configured you don't get these
>> errors, I made 3 campaigns on different servers  with 2.11 and never got
>> this kind of issues.
>
> Agreed, no need to revert.
>
>> But maybe we should document it better somewhere.
>
> Yes, the error and likely cause should be documented.
> Probably easiest to start as a Wiki page.
>

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