I've just updated the wiki with most of your comments hoping to make it more clear. I stole much of Joel's comment so all the credit goes to him if it works! The line between documenting the code and describing it is always thin. My intention was not to make anything official at first either, hence the disclaimers and email quotes :)
I agree a graph would be great but didn't take the time to do this just yet. For the CSV reporter, I can easily reproduce the problem with PurgatorySize and NumDelayedRequests metrics. My workaround was to delete the files a first time after starting the brokers, it would cure me from having error logs but I was probably missing metrics from one of the purgatories. I'll open start email thread for that. I used it extensively because it gives me better accuracy with metrics that the system we use and I can easily generate simple graphs from it using Python's matplotlib. On Sun, Nov 10, 2013 at 12:15 PM, Priya Matpadi <[email protected] > wrote: > Marc, thanks much for documenting the guts! > There is one correction for Fetch Request handling: > > When is it satisfied? > > The fetch size requested is reached - ie. the amount of data the consumer > wishes to receive in one response > > Consumer configuration: *fetch.message.max.bytes* > As per the code: > > /** > * A holding pen for fetch requests waiting to be satisfied > */ > class FetchRequestPurgatory(requestChannel: RequestChannel, > purgeInterval: Int) > extends RequestPurgatory[DelayedFetch, Int](brokerId, > purgeInterval) { > this.logIdent = "[FetchRequestPurgatory-%d] ".format(brokerId) > > /** > * A fetch request is satisfied when it has accumulated enough data to > meet the min_bytes field > */ > def checkSatisfied(messageSizeInBytes: Int, delayedFetch: > DelayedFetch): Boolean = { > val accumulatedSize = > delayedFetch.bytesAccumulated.addAndGet(messageSizeInBytes) > accumulatedSize >= delayedFetch.fetch.minBytes > } > > > > On Fri, Nov 8, 2013 at 1:01 PM, Joel Koshy <[email protected]> wrote: > > > Marc - thanks again for doing this. Couple of suggestions: > > > > - I would suggest removing the disclaimer and email quotes since this > > can become a stand-alone clean document on what the purgatory is and > > how it works. > > - A diagram would be helpful - it could say, show the watcher map and > > the expiration queue, and it will be especially useful if it can > > show the flow of producer/fetch requests through the purgatory. That > > would also help cut down a lot of the text in the doc. > > - I think it would be preferrable to have just high-level details in > > this document. Internal details (such as the purge interval > > settings) can either be removed or moved (to say, a short faq or > > config section at the end). > > - In the overview may want to comment on why we added it: i.e., it is > > the primary data structure we use for supporting long poll of > > producer/fetch requests. E.g., if we don't do this consumers would > > have to keep issuing fetch requests if there's no data yet - as > > opposed to just saying "respond when 'n' bytes of data are available > > or when 't' millisecs have elapsed, whichever is earlier." > > - WRT your question on PurgatorySize - we added that just to keep a > > tab on how many requests are sitting in purgatory (including both > > watchers map and expiration queue) as a rough gauge of memory usage. > > Also the fetch/producer request gauges should not collide - the > > KafkaMetricsGroup class takes care of this. The CSV reporter might > > run into issues though - I thought we had fixed that but could be > > wrong. > > > > Joel > > > > On Thu, Nov 07, 2013 at 11:01:06PM -0800, Joel Koshy wrote: > > > Excellent - thanks for putting that together! Will review it more > > > carefully tomorrow and suggest some minor edits if required. > > > > > > On Thu, Nov 07, 2013 at 10:45:40PM -0500, Marc Labbe wrote: > > > > 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 <[email protected]> > > wrote: > > > > > > > > > To edit the Wiki you need to send an ICLA > > > > > http://www.apache.org/licenses/#clas to Apache and then once that > > is done > > > > > an email to [email protected] (or to me and I will copy > > private) > > > > > with your Wiki username and that you sent the ICLA to Apache. > > > > > > > > > > Then, I can add you to edit the Wiki. > > > > > > > > > > /******************************************* > > > > > Joe Stein > > > > > Founder, Principal Consultant > > > > > Big Data Open Source Security LLC > > > > > http://www.stealth.ly > > > > > Twitter: @allthingshadoop <http://www.twitter.com/allthingshadoop > > > > > > > ********************************************/ > > > > > > > > > > > > > > > On Fri, Nov 1, 2013 at 9:08 PM, Marc Labbe <[email protected]> > > 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 <[email protected]> > > 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 <[email protected] > > > > 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 > > > > > > > > <[email protected]>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 < > > [email protected]> > > > > > > > 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 < > > [email protected]> > > > > > > > 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 < > > > > > > > >> > > [email protected] > > > > > > > >> > > > 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 > > > > > > > >> > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > > > >
