Hi David, Thanks for the KIP.
I had some questions/suggestions : It would be great if you can explain with an example about how the min offset for all the consumers will be calculated, in the KIP. What I meant was, it would be great to understand with a pseudo code/workflow if possible, how each broker knows all the consumers for the given topic-partition and how the min is calculated. Also it would be good to understand what happens if we start a console consumer which would actually start reading from the beginning offset and commit and crash immediately. How will the segments get deleted? Will it delete all the log segments if all the consumers have read till latest? If Yes, would we be able to handle a scenario were we say that user can rewind its offset to reprocess the data since log.retention.ms might not has reached. Thanks, Mayuresh On Mon, Oct 17, 2016 at 12:27 AM, Becket Qin <becket....@gmail.com> wrote: > Hey David, > > Thanks for replies to the questions. > > I think one major thing still not clear at this point is that whether the > brokers will only apply the consumed log retention to a specific set of > interested consumer groups, or it does not have such a set of consumer > groups. > > For example, for topic T, assume we know that there will be two downstream > consumer groups CG1 and CG2 consuming data from topic T. Will we add a > topic configurations such as > "log.retention.commitoffset.interested.groups=CG1,CG2" to topic T so that > the brokers only care about CG1 and CG2. The committed offsets of other > groups are not interested and won't have any impact on the committed offset > based log retention. > > It seems the current proposal does not have an "interested consumer group > set" configuration, so that means any random consumer group may affect the > committed offset based log retention. > > I think the committed offset based log retention seems more useful in cases > where we already know which consumer groups will be consuming from this > topic, so we will only wait for those consumer groups but ignore the > others. If a group will be consumed by many unknown or unpredictable > consumer groups, it seems the existing time based log retention is much > simple and clear enough. So I would argue we don't need to address the case > that some groups may come later in the committed offset based retention. > > That said, there may still be value to keep the data for some time even > after all the interested consumer groups have consumed the messages. For > example, in a pipelined stream processing DAG, we may want to keep the data > of an intermediate topic for some time in case the job fails. So we can > resume from a previously succeeded stage instead of restart the entire > pipeline. Or we can use the intermediate topic for some debugging work. > > Thanks, > > Jiangjie (Becket) Qin > > > > On Sun, Oct 16, 2016 at 2:15 AM, 东方甲乙 <254479...@qq.com> wrote: > > > Hi Dong, > > The KIP is used to solve both these 2 cases, we specify a small > > consumed log retention time to deleted the consumed data and avoid losing > > un-consumed data. > > And the specify a large force log retention time used as higher bound for > > the data. I will update the KIP for this info. > > Another solution I think may be ok is to support an API to delete the > > inactive group? If the group is in inactive, but it's commit offset is > > also in the __commit_offsets topic and > > stay in the offset cache, we can delete it via this API. > > > > > > Thanks, > > David > > > > > > ------------------ 原始邮件 ------------------ > > 发件人: "Dong Lin";<lindon...@gmail.com>; > > 发送时间: 2016年10月14日(星期五) 凌晨5:01 > > 收件人: "dev"<dev@kafka.apache.org>; > > > > 主题: Re: [DISCUSS] KIP-68 Add a consumed log retention before log > retention > > > > > > > > Hi David, > > > > As explained in the motivation section of the KIP, the problem is that if > > log retention is too small, we may lose data; and if log retention is too > > large, then we waste disk space. Therefore, we need to solve one if the > two > > problems -- allow data to be persisted longer for consumption if log > > retention is set too small, or allow data to be expired earlier if log > > retention is too large. I think the KIP probably needs to make this clear > > and explain which one is rejected and why. Note that the choice of the > two > > affects the solution -- if we want to address the first problem then > > log.retention.ms should be used as lower bound on the actual retention > > time, and if we want to address the second problem then the > > log.retention.ms > > should be used as higher bound on the actual retention time. > > > > In both cases, we probably need to figure out a way to determine "active > > consumer group". Maybe we can compare the time-since-last-commit against > a > > threshold to determine this. In addition, the threshold can be overridden > > either per-topic or per-groupId. If we go along this route, the rejected > > solution (per-topic vs. per-groupId) should probably be explained in the > > KIP. > > > > > > Thanks, > > Dong > > > > > > > > On Thu, Oct 13, 2016 at 10:23 AM, Dong Lin <lindon...@gmail.com> wrote: > > > > > Hi David, > > > > > > Thanks for your explanation. There still seems to be issue with this > > > solution. Please see my comment inline. > > > > > > > > > On Thu, Oct 13, 2016 at 8:46 AM, 东方甲乙 <254479...@qq.com> wrote: > > > > > >> Hi Dong, > > >> Sorry for the delay, here are the comments: > > >> 1.I think we should distinguish these two cases: > > >> (1) group has no member, but has commit offset : In this case we > should > > >> consider its commit offset > > >> (2) group has no member, no commit offset: Skip this group > > >> Is it ok? > > >> > > >> > > >> ListGroup API can list the groups, but this API only show the Online > > >> Group, so we should enhance the listGroup API to list those groups in > > the > > >> case (1) > > >> > > >> Say some user starts a consumer to consume topic A with > > > enable.auto.commit = true. Later they change the group name in the > > config. > > > Then the proposed solution will never execute consumed log retention > for > > > the topic A, right? I think group name change is pretty common and we > > > should take care of this issue. One possible solution is to add a > config > > to > > > specify the maximum time since last offset commit before we consider a > > > group is inactive. > > > > > > > > >> > > >> 2. Because every consumer group may appear in different time, say, > group > > >> 1 start to consume in day 1, group 2 start to consume in day 2. If we > > >> delete the log segment right away, > > >> group 2 can not consume these message. So we hope the messages can > hold > > >> for a specified time. I think many use-cases will need there configs, > > if > > >> there are many consumer groups. > > >> > > >> > > > If we want to take care of group 2, can we simply disable consumed log > > > retention for the topic and set log retention to 1 day? Can you explain > > the > > > benefit of enabling consumed log retention and set consumed log > retention > > > to 1 day? > > > > > > Currently the flow graph in the KIP suggests that we delete data iff > > > (consumed log retention is triggered OR forced log retention is > > triggered). > > > And alternative solution is to delete data iff ( (consumed log > retention > > is > > > disabled OR consumed log retention is triggered) AND forced log > retention > > > is triggered). I would argue that the 2nd scheme is better. Say the > > > consumed log retention is enabled. The 1st scheme basically interprets > > > forced log retention as the upper bound of the time the data can stay > in > > > Kafka. The 2nd scheme interprets forced log retention as the lower > bound > > of > > > the time the data can stay in Kafka, which is more consistent with the > > > purpose of having this forced log retention (to save disk space). And > if > > we > > > adopt the 2nd solution, the use-case you suggested can be easily > > addressed > > > by setting forced log retention to 1 day and enable consumed log > > retention. > > > What do you think? > > > > > > > > > > > >> > > >> Thanks, > > >> David > > >> > > >> > > >> > > >> > > >> ------------------ 原始邮件 ------------------ > > >> 发件人: "Dong Lin";<lindon...@gmail.com>; > > >> 发送时间: 2016年10月10日(星期一) 下午4:05 > > >> 收件人: "dev"<dev@kafka.apache.org>; > > >> > > >> 主题: Re: [DISCUSS] KIP-68 Add a consumed log retention before log > > retention > > >> > > >> > > >> > > >> Hey David, > > >> > > >> Thanks for reply. Please see comment inline. > > >> > > >> On Mon, Oct 10, 2016 at 12:40 AM, Pengwei (L) <pengwei...@huawei.com> > > >> wrote: > > >> > > >> > Hi Dong > > >> > Thanks for the questions: > > >> > > > >> > 1. Now we don't distinguish inactive or active groups. Because in > > some > > >> > case maybe inactive group will become active again, and using the > > >> previous > > >> > commit offset. > > >> > > > >> > So we will not delete the log segment in the consumer retention if > > there > > >> > are some groups consume but not commit, but the log segment can be > > >> delete by > > >> > the force retention. > > >> > > > >> > > >> So in the example I provided, the consumed log retention will be > > >> effectively disabled, right? This seems to be a real problem in > > operation > > >> -- we don't want log retention to be un-intentionally disabled simply > > >> because someone start a tool to consume from that topic. Either this > KIP > > >> should provide a way to handle this, or there should be a way for > > operator > > >> to be aware of such case and be able to re-eanble consumed log > retention > > >> for the topic. What do you think? > > >> > > >> > > >> > > >> > 2. These configs are used to determine the out of date time of the > > >> > consumed retention, like the parameters of the force retention > > >> > (log.retention.hours, log.retention.minutes, log.retention.ms). For > > >> > example, users want the save the log for 3 days, after 3 days, kafka > > >> will > > >> > delete the log segments which are > > >> > > > >> > consumed by all the consumer group. The log retention thread need > > these > > >> > parameters. > > >> > > > >> > It makes sense to have configs such as log.retention.ms -- it is > used > > >> to > > >> make data available for up to a configured amount of time before it is > > >> deleted. My question is what is the use-case for making log available > > for > > >> another e.g. 3 days after it has been consumed by all consumer groups. > > The > > >> purpose of this KIP is to allow log to be deleted right as long as all > > >> interested consumer groups have consumed it. Can you provide a > use-case > > >> for > > >> keeping log available for longer time after it has been consumed by > all > > >> groups? > > >> > > >> > > >> > > > >> > Thanks, > > >> > David > > >> > > > >> > > > >> > > Hey David, > > >> > > > > >> > > Thanks for the KIP. Can you help with the following two questions: > > >> > > > > >> > > 1) If someone start a consumer (e.g. kafka-console-consumer) to > > >> consume a > > >> > > topic for debug/validation purpose, a randome consumer group may > be > > >> > created > > >> > > and offset may be committed for this consumer group. If no offset > > >> commit > > >> > is > > >> > > made for this consumer group in the future, will this effectively > > >> > > disable consumed log retention for this topic? In other words, how > > do > > >> > this > > >> > > KIP distinguish active consumer group from inactive ones? > > >> > > > > >> > > 2) Why do we need new configs such as > log.retention.commitoffset.hou > > >> rs? > > >> > Can > > >> > >we simply delete log segments if consumed log retention is enabled > > for > > >> > this > > >> > > topic and all consumer groups have consumed messages in the log > > >> segment? > > >> > > > > >> > > Thanks, > > >> > > Dong > > >> > > > > >> > > > > >> > > > > >> > >On Sat, Oct 8, 2016 at 2:15 AM, Pengwei (L) <pengwei...@huawei.com > > > > >> > wrote: > > >> > > > > >> > > > Hi Becket, > > >> > > > > > >> > > > Thanks for the feedback: > > >> > > > 1. We use the simple consumer api to query the commit offset, > so > > we > > >> > don't > > >> > > > need to specify the consumer group. > > >> > > > 2. Every broker using the simple consumer api(OffsetFetchKey) > to > > >> query > > >> > > > the commit offset in the log retention process. The client can > > >> commit > > >> > > > offset or not. > > >> > > > 3. It does not need to distinguish the follower brokers or > leader > > >> > > > brokers, every brokers can query. > > >> > > > 4. We don't need to change the protocols, we mainly change the > > log > > >> > > > retention process in the log manager. > > >> > > > > > >> > > > One question is the query min offset need O(partitions * > groups) > > >> time > > >> > > > complexity, another alternative is to build an internal topic to > > >> save > > >> > every > > >> > > > partition's min offset, it can reduce to O(1). > > >> > > > I will update the wiki for more details. > > >> > > > > > >> > > > Thanks, > > >> > > > David > > >> > > > > > >> > > > > > >> > > > > Hi Pengwei, > > >> > > > > > > >> > > > > Thanks for the KIP proposal. It is a very useful KIP. At a > high > > >> > level, > > >> > > > the > > >> > > > > proposed behavior looks reasonable to me. > > >> > > > > > > >> > > > > However, it seems that some of the details are not mentioned > in > > >> the > > >> > KIP. > > >> > > > > For example, > > >> > > > > > > >> > > > > 1. How will the expected consumer group be specified? Is it > > >> through > > >> > a per > > >> > > > > topic dynamic configuration? > > >> > > > > 2. How do the brokers detect the consumer offsets? Is it > > required > > >> > for a > > >> > > > > consumer to commit offsets? > > >> > > > > 3. How do all the replicas know the about the committed > offsets? > > >> > e.g. 1) > > >> > > > > non-coordinator brokers which do not have the committed > offsets, > > >> 2) > > >> > > > > follower brokers which do not have consumers directly > consuming > > >> from > > >> > it. > > >> > > > > 4. Is there any other changes need to be made (e.g. new > > >> protocols) in > > >> > > > > addition to the configuration change? > > >> > > > > > > >> > > > > It would be great if you can update the wiki to have more > > details. > > >> > > > > > > >> > > > > Thanks, > > >> > > > > > > >> > > > > Jiangjie (Becket) Qin > > >> > > > > > > >> > > > > On Wed, Sep 7, 2016 at 2:26 AM, Pengwei (L) < > > >> pengwei...@huawei.com> > > >> > > > wrote: > > >> > > > > > > >> > > > > > Hi All, > > >> > > > > > I have made a KIP to enhance the log retention, details > as > > >> > follows: > > >> > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP- > > >> > > > > > 68+Add+a+consumed+log+retention+before+log+retention > > >> > > > > > Now start a discuss thread for this KIP , looking forward > > to > > >> the > > >> > > > > > feedback. > > >> > > > > > > > >> > > > > > Thanks, > > >> > > > > > David > > >> > > > > > > > >> > > > > > > > >> > > > >> > > > >> > > > > > > > > > -- -Regards, Mayuresh R. 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