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From Marc Labbe <mrla...@gmail.com>
Subject Re: Purgatory
Date Fri, 08 Nov 2013 03:45:40 GMT
I've just added a page for purgatory, feel free to comment/modify at will.
I hope I didn't misinterpret too much of the code.

https://cwiki.apache.org/confluence/display/KAFKA/Request+Purgatory+(0.8)

I added a few questions of my own.


On Fri, Nov 1, 2013 at 9:43 PM, Joe Stein <joe.stein@stealth.ly> wrote:

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>
> Then, I can add you to edit the Wiki.
>
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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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