It might worth thinking a little further. We have discussed this before that we want to avoid holding all the group metadata in memory.
I am thinking about the following end state: 1. Enable compression on the offset topic. 2. Instead of holding the entire group metadata in memory on the brokers, each broker only keeps a [group -> Offset] map, the offset points to the message in the offset topic which holds the latest metadata of the group. 3. DescribeGroupResponse will read from the offset topic directly like a normal consumption, except that only exactly one message will be returned. 4. SyncGroupResponse will read the message, extract the assignment part and send back the partition assignment. We can compress the partition assignment before sends it out if we want. Jiangjie (Becket) Qin On Mon, May 23, 2016 at 5:08 PM, Jason Gustafson <ja...@confluent.io> wrote: > > > > Jason, doesn't gzip (or other compression) basically do this? If the > topic > > is a string and the topic is repeated throughout, won't compression > > basically replace all repeated instances of it with an index reference to > > the full string? > > > Hey James, yeah, that's probably true, but keep in mind that the > compression happens on the broker side. It would be nice to have a more > compact representation so that get some benefit over the wire as well. This > seems to be less of a concern here, so the bigger gains are probably from > reducing the number of partitions that need to be listed individually. > > -Jason > > On Mon, May 23, 2016 at 4:23 PM, Onur Karaman < > onurkaraman.apa...@gmail.com> > wrote: > > > When figuring out these optimizations, it's worth keeping in mind the > > improvements when the message is uncompressed vs when it's compressed. > > > > When uncompressed: > > Fixing the Assignment serialization to instead be a topic index into the > > corresponding member's subscription list would usually be a good thing. > > > > I think the proposal is only worse when the topic names are small. The > > Type.STRING we use in our protocol for the assignment's TOPIC_KEY_NAME is > > limited in length to Short.MAX_VALUE, so our strings are first prepended > > with 2 bytes to indicate the string size. > > > > The new proposal does worse when: > > 2 + utf_encoded_string_payload_size < index_type_size > > in other words when: > > utf_encoded_string_payload_size < index_type_size - 2 > > > > If the index type ends up being Type.INT32, then the proposal is worse > when > > the topic is length 1. > > If the index type ends up being Type.INT64, then the proposal is worse > when > > the topic is less than length 6. > > > > When compressed: > > As James Cheng brought up, I'm not sure how things change when > compression > > comes into the picture. This would be worth investigating. > > > > On Mon, May 23, 2016 at 4:05 PM, James Cheng <wushuja...@gmail.com> > wrote: > > > > > > > > > On May 23, 2016, at 10:59 AM, Jason Gustafson <ja...@confluent.io> > > > wrote: > > > > > > > > 2. Maybe there's a better way to lay out the assignment without > needing > > > to > > > > explicitly repeat the topic? For example, the leader could sort the > > > topics > > > > for each member and just use an integer to represent the index of > each > > > > topic within the sorted list (note this depends on the subscription > > > > including the full topic list). > > > > > > > > Assignment -> [TopicIndex [Partition]] > > > > > > > > > > Jason, doesn't gzip (or other compression) basically do this? If the > > topic > > > is a string and the topic is repeated throughout, won't compression > > > basically replace all repeated instances of it with an index reference > to > > > the full string? > > > > > > -James > > > > > > > You could even combine these two options so that you have only 3 > > integers > > > > for each topic assignment: > > > > > > > > Assignment -> [TopicIndex MinPartition MaxPartition] > > > > > > > > There may even be better options with a little more thought. All of > > this > > > is > > > > just part of the client-side protocol, so it wouldn't require any > > version > > > > bumps on the broker. What do you think? > > > > > > > > Thanks, > > > > Jason > > > > > > > > > > > > > > > > > > > > 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 > > > >> > > > > > > > > >
