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From Joe Stein <joe.st...@stealth.ly>
Subject Re: Purgatory
Date Sat, 02 Nov 2013 01:43:48 GMT
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Then, I can add you to edit the Wiki.

/*******************************************
 Joe Stein
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On Fri, Nov 1, 2013 at 9:08 PM, Marc Labbe <mrlabbe@gmail.com> wrote:

> Hi Joel,
>
> I used to have edit to the wiki, I made a few additions to it a while ago
> but it's seem I don't have it anymore. It might have been lost in the
> confluence update. I would be glad to add what I have written if I get it
> back. Otherwise, feel free to paste my words in one of the pages, I don't
> intend on asking for copyrights for this :).
>
> marc
>
>
> On Fri, Nov 1, 2013 at 4:32 PM, Joel Koshy <jjkoshy.w@gmail.com> wrote:
>
> > Marc, thanks for writing that up. I think it is worth adding some
> > details on the request-purgatory on a wiki (Jay had started a wiki
> > page for kafka internals [1] a while ago, but we have not had time to
> > add much to it since.) Your write-up could be reviewed and added
> > there. Do you have edit permissions on the wiki?
> >
> > As for the purge interval config - yes the documentation can be
> > improved a bit. It's one of those "internal" configs that generally
> > don't need to be modified by users. The reason we added that was as
> > follows:
> > - We found that for low-volume topics, replica fetch requests were
> > getting expired but sitting around in purgatory
> > - This was because we were expiring them from the delay queue (used to
> > track when requests should expire), but they were still sitting in the
> > watcherFor map - i.e., they would get purged when the next producer
> > request to that topic/partition arrived, but for low volume topics
> > this could be a long time (or never in the worst case) and we would
> > eventually run into an OOME.
> > - So we needed to periodically go through the entire watcherFor map
> > and explicitly remove those requests that had expired.
> > - More details on this are in KAFKA-664.
> >
> > Thanks,
> >
> > Joel
> >
> > [1] https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Internals
> >
> > On Fri, Nov 1, 2013 at 12:33 PM, Marc Labbe <mrlabbe@gmail.com> wrote:
> > > Guozhang,
> > >
> > > I have to agree with Priya the doc isn't very clear. Although the
> > > configuration is documented, it is simply rewording the name of the
> > config,
> > > which isn't particularly useful if you want more information about what
> > the
> > > purgatory is. I searched the whole wiki and doc and could not find
> > anything
> > > very useful as opposed looking a the code. In this case,
> > > kafka.server.KafkaApis and kafka.server.RequestPurgatory will be your
> > > friends.
> > >
> > > I'll try to add to Joe's answer here, mostly just reporting what's
> > > available in the Scala doc from the project. I am doing this to
> > understand
> > > the mechanics myself btw.
> > >
> > > As Joe said, messages are not dropped by the purgatory but simply
> removed
> > > from the purgatory when they are satisfied. Satisfaction conditions are
> > > different for both fetch and produce requests and this is implemented
> in
> > > their respective DelayedRequest implementation (DelayedFetch and
> > > DelayedProduce).
> > >
> > > Requests purgatories are defined as follow in the code:
> > >  - ProducerRequestPurgatory: A holding pen for produce requests waiting
> > to
> > > be satisfied.
> > >  - FetchRequestPurgatory: A holding pen for fetch requests waiting to
> be
> > > satisfied
> > >
> > > Each request purgatory runs a thread (ExpiredRequestReaper). This
> thread
> > > will first try to find an expired delayed request. When one if found,
> it
> > > will run the purgatory's expire method to handle the delayed request
> > > expiration. In both produce and fetch cases, it sends a response to the
> > > client. An expired request will be a satisfied request. The next step
> of
> > > the thread's loop is when it checks for the configuration parameters
> you
> > > asked for initially (purgatory.purge.interval.requests). When the
> number
> > of
> > > delayed requests given to watch by the purgatory reaches this value, it
> > > goes through all previously queued requests and removes those which are
> > > marked as satisfied. Because of that, it is really an interval more
> than
> > it
> > > is a threshold since it doesn't really care about the amount of
> satisfied
> > > requests or the size of the queue.
> > >
> > > Producer request
> > > - When is it added to purgatory (delayed)?:
> > >   * when it uses ack=-1 (actually, the code tells me anything but 0 or
> > 1);
> > > Producer config: request.required.acks
> > >   * partitions have more than one replica (in this case, ack=-1 isn't
> > > different to ack=1 and it doesn't make much sense to use a delayed
> > request)
> > >   * not all partitions are in error
> > > - When does it expire? when it reaches the timeout defined in the
> produce
> > > request (ackTimeoutMs). Translates from producer config
> > request.timeout.ms.
> > > - What happens (on the broker) when it expires? Sends a response to the
> > > client. Response content depends on the request of course.
> > > - When is it satisfied? I didn't find the courage to dig into the
> details
> > > of this one :(  ... but mainly when all the follower have also
> > acknowledge
> > > the produce request for their replica
> > >
> > > Fetch request
> > > - When is it added to purgatory (delayed)? 2 parameters of the requests
> > are
> > > mainly useful here: max wait time and fetch size
> > >   * if max wait is greater than 0; otherwise, it is a blocking call by
> > the
> > > consumer
> > >   * if fetch size is greater than the current size of data available to
> > > fulfil the request
> > > - When does it expire?
> > >   * wait time: the amount of time the consumer is willing to wait for
> > data;
> > > Consumer config: fetch.wait.max.ms
> > > - When is it satisfied? the fetch size requested is reached - ie. the
> > > amount of data the consumer wishes to receive in one response (from
> > > consumer config: fetch.message.max.bytes)
> > >
> > > ******
> > >
> > > It would be useful to add some information about the metrics associated
> > > with this.
> > >
> > > Of course, I am all for being corrected if I said anything wrong here.
> > The
> > > truth is always the code :-)
> > >
> > > marc
> > > - mrtheb -
> > >
> > >
> > > On Fri, Nov 1, 2013 at 2:45 AM, Priya Matpadi
> > > <priya.matpadi@ecofactor.com>wrote:
> > >
> > >> Guozhang,
> > >> The documentation is not very clear.
> > >> Marc's response for producer purgatory makes sense.
> > >> I am not entirely clear on fetch purgatory.
> > >> How does broker use purgatory? Is it a temporary holding area? What
> > happens
> > >> to the messages if purge interval is exceeded in case of either/both
> > >> producer and consumer? Are messages dropped in this case?
> > >> Thanks,
> > >> Priya
> > >>
> > >>
> > >> On Thu, Oct 31, 2013 at 2:47 PM, Guozhang Wang <wangguoz@gmail.com>
> > wrote:
> > >>
> > >> > Hello Priya,
> > >> >
> > >> > You can find the definitions of these two configs here:
> > >> >
> > >> > http://kafka.apache.org/documentation.html#brokerconfigs
> > >> >
> > >> > Guozhang
> > >> >
> > >> >
> > >> > On Thu, Oct 31, 2013 at 11:20 AM, Marc Labbe <mrlabbe@gmail.com>
> > wrote:
> > >> >
> > >> > > Hi Priya
> > >> > >
> > >> > > my understanding is producer requests will be delayed (and put
in
> > >> request
> > >> > > purgatory) only if your producer uses ack=-1. It will be in the
> > >> purgatory
> > >> > > (delayed) until all brokers have acknowledged the messages to
be
> > >> > > replicated. The documentation suggests to monitor the
> > >> > > ProducerRequestPurgatory size metrics , but it only applies if
> > you're
> > >> > using
> > >> > > ack=-1, otherwise, this value will always be 0.
> > >> > >
> > >> > > For consumer requests, they'll be in purgatory (delayed) until
the
> > max
> > >> > > allowed time to respond has been reached, unless it has enough
> > messages
> > >> > to
> > >> > > fill the buffer before that. The request will not end up in the
> > >> purgatory
> > >> > > if you're making a blocking request (max wait <= 0).
> > >> > >
> > >> > > Not sure about the configuration interval though.
> > >> > >
> > >> > > marc
> > >> > >
> > >> > >
> > >> > > On Thu, Oct 31, 2013 at 12:41 PM, Priya Matpadi <
> > >> > > priya.matpadi@ecofactor.com
> > >> > > > wrote:
> > >> > >
> > >> > > > Hello,
> > >> > > > What is purgatory? I believe the following two properties
relate
> > to
> > >> > > > consumer and producer respectively.
> > >> > > > Could someone please explain the significance of these?
> > >> > > > fetch.purgatory.purge.interval.requests=100
> > >> > > > producer.purgatory.purge.interval.requests=100
> > >> > > >
> > >> > > > Thanks,
> > >> > > > Priya
> > >> > > >
> > >> > >
> > >> >
> > >> >
> > >> >
> > >> > --
> > >> > -- Guozhang
> > >> >
> > >>
> >
>

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