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 <jjkosh...@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 <mrla...@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.matp...@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 <wangg...@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 <mrla...@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.matp...@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 > >> > > >> >
