On 23 February 2014 04:38, sebb <[email protected]> wrote: > On 19 February 2014 21:51, sebb <[email protected]> wrote: >> On 19 February 2014 21:03, Philippe Mouawad <[email protected]> >> wrote: >>> On Wed, Feb 19, 2014 at 9:58 PM, sebb <[email protected]> wrote: >>> >>>> On 19 February 2014 18:49, sebb <[email protected]> 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.
Unfortunately this does not work, as the disconnect is only detected once the request has been sent. However, it is possible to specify an idle timeout to deal with a missing Keep-Alive header. I've tried that and it works, but we need some decisions on how to implement it. I'll send a separate e-mail for that. It may also be possible to code a conditional stale connection check which is not applied every single time. e.g. it could be done after N requests or when the connection has been idle for T seconds. I'll look into this a bit more and report back in another mail. >>> 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. >>
