+1 on Jun's idea. Even without the new consumer, currently we may still face this issue of record size too large in offset topic if user commits offsets with a big blob of metadata.
Topic pattern would help reduce the group metadata size. However some use cases may not be able to benefit from it. For example, one use case we have for mirror maker is dynamically changing the topics to consume based on the end user's requirement. In that case even if we use the regex it would probably become a topic list. Also, I think we should probably turn on compression for offset topic by default. The metadata of JoinGroupRequests are likely similar so the aggregated metadata should be highly compressible. Thanks, Jiangjie (Becket) Qin On Mon, May 23, 2016 at 9:17 AM, Guozhang Wang <wangg...@gmail.com> wrote: > The original concern is that regex may not be efficiently supported > across-languages, but if there is a neat workaround I would love to learn. > > Guozhang > > On Mon, May 23, 2016 at 5:31 AM, Ismael Juma <ism...@juma.me.uk> wrote: > > > +1 to Jun's suggestion. > > > > Having said that, as a general point, I think we should consider > supporting > > topic patterns in the wire protocol. It requires some thinking for > > cross-language support, but it seems surmountable and it could make > certain > > operations a lot more efficient (the fact that a basic regex subscription > > causes the consumer to request metadata for all topics is not great). > > > > Ismael > > > > On Sun, May 22, 2016 at 11:49 PM, Guozhang Wang <wangg...@gmail.com> > > wrote: > > > > > I like Jun's suggestion in changing the handling logics of single large > > > message on the consumer side. > > > > > > As for the case of "a single group subscribing to 3000 topics", with > 100 > > > consumers the 2.5Mb Gzip size is reasonable to me (when storing in ZK, > we > > > also have the znode limit which is set to 1Mb by default, though > > admittedly > > > it is only for one consumer). And if we do the change as Jun suggested, > > > 2.5Mb on follower's memory pressure is OK I think. > > > > > > > > > Guozhang > > > > > > > > > On Sat, May 21, 2016 at 12:51 PM, Onur Karaman < > > > onurkaraman.apa...@gmail.com > > > > wrote: > > > > > > > Results without compression: > > > > 1 consumer 292383 bytes > > > > 5 consumers 1079579 bytes * the tipping point > > > > 10 consumers 1855018 bytes > > > > 20 consumers 2780220 bytes > > > > 30 consumers 3705422 bytes > > > > 40 consumers 4630624 bytes > > > > 50 consumers 5555826 bytes > > > > 60 consumers 6480788 bytes > > > > 70 consumers 7405750 bytes > > > > 80 consumers 8330712 bytes > > > > 90 consumers 9255674 bytes > > > > 100 consumers 10180636 bytes > > > > > > > > So it looks like gzip compression shrinks the message size by 4x. > > > > > > > > On Sat, May 21, 2016 at 9:47 AM, Jun Rao <j...@confluent.io> wrote: > > > > > > > > > Onur, > > > > > > > > > > Thanks for the investigation. > > > > > > > > > > Another option is to just fix how we deal with the case when a > > message > > > is > > > > > larger than the fetch size. Today, if the fetch size is smaller > than > > > the > > > > > fetch size, the consumer will get stuck. Instead, we can simply > > return > > > > the > > > > > full message if it's larger than the fetch size w/o requiring the > > > > consumer > > > > > to manually adjust the fetch size. On the broker side, to serve a > > fetch > > > > > request, we already do an index lookup and then scan the log a bit > to > > > > find > > > > > the message with the requested offset. We can just check the size > of > > > that > > > > > message and return the full message if its size is larger than the > > > fetch > > > > > size. This way, fetch size is really for performance optimization, > > i.e. > > > > in > > > > > the common case, we will not return more bytes than fetch size, but > > if > > > > > there is a large message, we will return more bytes than the > > specified > > > > > fetch size. In practice, large messages are rare. So, it shouldn't > > > > increase > > > > > the memory consumption on the client too much. > > > > > > > > > > Jun > > > > > > > > > > On Sat, May 21, 2016 at 3:34 AM, Onur Karaman < > > > > > onurkaraman.apa...@gmail.com> > > > > > wrote: > > > > > > > > > > > Hey everyone. So I started doing some tests on the new > > > > > consumer/coordinator > > > > > > to see if it could handle more strenuous use cases like mirroring > > > > > clusters > > > > > > with thousands of topics and thought I'd share whatever I have so > > > far. > > > > > > > > > > > > The scalability limit: the amount of group metadata we can fit > into > > > one > > > > > > message > > > > > > > > > > > > Some background: > > > > > > Client-side assignment is implemented in two phases > > > > > > 1. a PreparingRebalance phase that identifies members of the > group > > > and > > > > > > aggregates member subscriptions. > > > > > > 2. an AwaitingSync phase that waits for the group leader to > decide > > > > member > > > > > > assignments based on the member subscriptions across the group. > > > > > > - The leader announces this decision with a SyncGroupRequest. > The > > > > > > GroupCoordinator handles SyncGroupRequests by appending all group > > > state > > > > > > into a single message under the __consumer_offsets topic. This > > > message > > > > is > > > > > > keyed on the group id and contains each member subscription as > well > > > as > > > > > the > > > > > > decided assignment for each member. > > > > > > > > > > > > The environment: > > > > > > - one broker > > > > > > - one __consumer_offsets partition > > > > > > - offsets.topic.compression.codec=1 // this is gzip > > > > > > - broker has my pending KAFKA-3718 patch that actually makes use > of > > > > > > offsets.topic.compression.codec: > > > > > https://github.com/apache/kafka/pull/1394 > > > > > > - around 3000 topics. This is an actual subset of topics from one > > of > > > > our > > > > > > clusters. > > > > > > - topics have 8 partitions > > > > > > - topics are 25 characters long on average > > > > > > - one group with a varying number of consumers each hardcoded > with > > > all > > > > > the > > > > > > topics just to make the tests more consistent. wildcarding with > .* > > > > should > > > > > > have the same effect once the subscription hits the coordinator > as > > > the > > > > > > subscription has already been fully expanded out to the list of > > > topics > > > > by > > > > > > the consumers. > > > > > > - I added some log messages to Log.scala to print out the message > > > sizes > > > > > > after compression > > > > > > - there are no producers at all and auto commits are disabled. > The > > > only > > > > > > topic with messages getting added is the __consumer_offsets topic > > and > > > > > > they're only from storing group metadata while processing > > > > > > SyncGroupRequests. > > > > > > > > > > > > Results: > > > > > > The results below show that we exceed the 1000012 byte > > > > > > KafkaConfig.messageMaxBytes limit relatively quickly (between > 30-40 > > > > > > consumers): > > > > > > 1 consumer 54739 bytes > > > > > > 5 consumers 261524 bytes > > > > > > 10 consumers 459804 bytes > > > > > > 20 consumers 702499 bytes > > > > > > 30 consumers 930525 bytes > > > > > > 40 consumers 1115657 bytes * the tipping point > > > > > > 50 consumers 1363112 bytes > > > > > > 60 consumers 1598621 bytes > > > > > > 70 consumers 1837359 bytes > > > > > > 80 consumers 2066934 bytes > > > > > > 90 consumers 2310970 bytes > > > > > > 100 consumers 2542735 bytes > > > > > > > > > > > > Note that the growth itself is pretty gradual. Plotting the > points > > > > makes > > > > > it > > > > > > look roughly linear w.r.t the number of consumers: > > > > > > > > > > > > > > > > > > > > > > > > > > > https://www.wolframalpha.com/input/?i=(1,+54739),+(5,+261524),+(10,+459804),+(20,+702499),+(30,+930525),+(40,+1115657),+(50,+1363112),+(60,+1598621),+(70,+1837359),+(80,+2066934),+(90,+2310970),+(100,+2542735) > > > > > > > > > > > > Also note that these numbers aren't averages or medians or > anything > > > > like > > > > > > that. It's just the byte size from a given run. I did run them a > > few > > > > > times > > > > > > and saw similar results. > > > > > > > > > > > > Impact: > > > > > > Even after adding gzip to the __consumer_offsets topic with my > > > pending > > > > > > KAFKA-3718 patch, the AwaitingSync phase of the group fails with > > > > > > RecordTooLargeException. This means the combined size of each > > > member's > > > > > > subscriptions and assignments exceeded the > > > KafkaConfig.messageMaxBytes > > > > of > > > > > > 1000012 bytes. The group ends up dying. > > > > > > > > > > > > Options: > > > > > > 1. Config change: reduce the number of consumers in the group. > This > > > > isn't > > > > > > always a realistic answer in more strenuous use cases like > > > MirrorMaker > > > > > > clusters or for auditing. > > > > > > 2. Config change: split the group into smaller groups which > > together > > > > will > > > > > > get full coverage of the topics. This gives each group member a > > > smaller > > > > > > subscription.(ex: g1 has topics starting with a-m while g2 has > > topics > > > > > > starting ith n-z). This would be operationally painful to manage. > > > > > > 3. Config change: split the topics among members of the group. > > Again > > > > this > > > > > > gives each group member a smaller subscription. This would also > be > > > > > > operationally painful to manage. > > > > > > 4. Config change: bump up KafkaConfig.messageMaxBytes (a > > topic-level > > > > > > config) and KafkaConfig.replicaFetchMaxBytes (a broker-level > > config). > > > > > > Applying messageMaxBytes to just the __consumer_offsets topic > seems > > > > > > relatively harmless, but bumping up the broker-level > > > > replicaFetchMaxBytes > > > > > > would probably need more attention. > > > > > > 5. Config change: try different compression codecs. Based on 2 > > > minutes > > > > of > > > > > > googling, it seems like lz4 and snappy are faster than gzip but > > have > > > > > worse > > > > > > compression, so this probably won't help. > > > > > > 6. Implementation change: support sending the regex over the wire > > > > instead > > > > > > of the fully expanded topic subscriptions. I think people said in > > the > > > > > past > > > > > > that different languages have subtle differences in regex, so > this > > > > > doesn't > > > > > > play nicely with cross-language groups. > > > > > > 7. Implementation change: maybe we can reverse the mapping? > Instead > > > of > > > > > > mapping from member to subscriptions, we can map a subscription > to > > a > > > > list > > > > > > of members. > > > > > > 8. Implementation change: maybe we can try to break apart the > > > > > subscription > > > > > > and assignments from the same SyncGroupRequest into multiple > > records? > > > > > They > > > > > > can still go to the same message set and get appended together. > > This > > > > way > > > > > > the limit become the segment size, which shouldn't be a problem. > > This > > > > can > > > > > > be tricky to get right because we're currently keying these > > messages > > > on > > > > > the > > > > > > group, so I think records from the same rebalance might > > accidentally > > > > > > compact one another, but my understanding of compaction isn't > that > > > > great. > > > > > > > > > > > > Todo: > > > > > > It would be interesting to rerun the tests with no compression > just > > > to > > > > > see > > > > > > how much gzip is helping but it's getting late. Maybe tomorrow? > > > > > > > > > > > > - Onur > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > -- Guozhang > > > > > > > > > -- > -- Guozhang >
