Gwen, Joel: That's correct. The protocol does allow us to give an assignor its own assignment schema, but I think this will require a couple internal changes to the consumer to make use of the full generality.
One thing I'm a little uncertain about is whether we should use a different protocol type. For a little context, the group membership protocol allows the client to provide a "protocol type" when joining the group to ensure that all members have some basic semantic compatibility. For example, the consumer uses "consumer" and Kafka Connect uses "connect." Currently all assignors using the "consumer" protocol share a common schema for representing subscriptions and assignment. This is convenient for tools (like consumer-groups.sh) since they just need to know how to parse the "consumer" protocol type without knowing anything about the assignors. So introducing another schema would break that assumption and we'd need those tools to do assignor-specific parsing. Maybe this is OK? Alternatively, we could use a separate protocol type (e.g. "compact-consumer"), but that seems less than desirable. Thanks, Jason On Wed, May 25, 2016 at 11:00 AM, Gwen Shapira <g...@confluent.io> wrote: > ah, right - we can add as many strategies as we want. > > On Wed, May 25, 2016 at 10:54 AM, Joel Koshy <jjkosh...@gmail.com> wrote: > > > > Yes it would be a protocol bump. > > > > > > > Sorry - I'm officially confused. I think it may not be required - since > the > > more compact format would be associated with a new assignment strategy - > > right? > > > > > > > smaller than the plaintext PAL, but the post-compressed binary PAL is > > just > > > 25% smaller than the post-compressed plaintext PAL. IOW using a symbol > > > table helps a lot but further compression on that already compact > format > > > would yield only marginal return. > > > > > > > > So basically I feel we could get pretty far with a more compact field > > > format for assignment and if we do that then we would potentially not > > even > > > want to do any compression. > > > > > > > Also just wanted to add that this compression on the binary PAL did help > > but the compression ratio was obviously not as high as plaintext > > compression. > > > > > > > > > > On Tue, May 24, 2016 at 4:19 PM, Gwen Shapira <g...@confluent.io> > wrote: > > > > > >> Regarding the change to the assignment field. It would be a protocol > > bump, > > >> otherwise consumers will not know how to parse the bytes the broker is > > >> returning, right? > > >> Or did I misunderstand the suggestion? > > >> > > >> On Tue, May 24, 2016 at 2:52 PM, Guozhang Wang <wangg...@gmail.com> > > >> wrote: > > >> > > >> > I think for just solving issue 1), Jun's suggestion is sufficient > and > > >> > simple. So I'd prefer that approach. > > >> > > > >> > In addition, Jason's optimization on the assignment field would be > > good > > >> for > > >> > 2) and 3) as well, and I like that optimization for its simplicity > and > > >> no > > >> > format change as well. And in the future I'm in favor of considering > > to > > >> > change the in-memory cache format as Jiangjie suggested. > > >> > > > >> > Guozhang > > >> > > > >> > > > >> > On Tue, May 24, 2016 at 12:42 PM, Becket Qin <becket....@gmail.com> > > >> wrote: > > >> > > > >> > > Hi Jason, > > >> > > > > >> > > There are a few problems we want to solve here: > > >> > > 1. The group metadata is too big to be appended to the log. > > >> > > 2. Reduce the memory footprint on the broker > > >> > > 3. Reduce the bytes transferred over the wire. > > >> > > > > >> > > To solve (1), I like your idea of having separate messages per > > member. > > >> > The > > >> > > proposal (Onur's option 8) is to break metadata into small records > > in > > >> the > > >> > > same uncompressed message set so each record is small. I agree it > > >> would > > >> > be > > >> > > ideal if we are able to store the metadata separately for each > > >> member. I > > >> > > was also thinking about storing the metadata into multiple > messages, > > >> too. > > >> > > What concerns me was that having multiple messages seems breaking > > the > > >> > > atomicity. I am not sure how we are going to deal with the > potential > > >> > > issues. For example, What if group metadata is replicated but the > > >> member > > >> > > metadata is not? It might be fine depending on the implementation > > >> though, > > >> > > but I am not sure. > > >> > > > > >> > > For (2) we want to store the metadata onto the disk, which is what > > we > > >> > have > > >> > > to do anyway. The only question is in what format should we store > > >> them. > > >> > > > > >> > > To address (3) we want to have the metadata to be compressed, > which > > is > > >> > > contradict to the the above solution of (1). > > >> > > > > >> > > I think Jun's suggestion is probably still the simplest. To avoid > > >> > changing > > >> > > the behavior for consumers, maybe we can do that only for > > >> offset_topic, > > >> > > i.e, if the max fetch bytes of the fetch request is smaller than > the > > >> > > message size on the offset topic, we always return at least one > full > > >> > > message. This should avoid the unexpected problem on the client > side > > >> > > because supposedly only tools and brokers will fetch from the the > > >> > internal > > >> > > topics, > > >> > > > > >> > > As a modification to what you suggested, one solution I was > thinking > > >> was > > >> > to > > >> > > have multiple messages in a single compressed message. That means > > for > > >> > > SyncGroupResponse we still need to read the entire compressed > > messages > > >> > and > > >> > > extract the inner messages, which seems not quite different from > > >> having a > > >> > > single message containing everything. But let me just put it here > > and > > >> see > > >> > > if that makes sense. > > >> > > > > >> > > We can have a map of GroupMetadataKey -> GroupMetadataValueOffset. > > >> > > > > >> > > The GroupMetadataValue is stored in a compressed message. The > inner > > >> > > messages are the following: > > >> > > > > >> > > Inner Message 0: Version GroupId Generation > > >> > > > > >> > > Inner Message 1: MemberId MemberMetadata_1 (we can compress the > > bytes > > >> > here) > > >> > > > > >> > > Inner Message 2: MemberId MemberMetadata_2 > > >> > > .... > > >> > > Inner Message N: MemberId MemberMetadata_N > > >> > > > > >> > > The MemberMetadata format is the following: > > >> > > MemberMetadata => Version Generation ClientId Host Subscription > > >> > > Assignment > > >> > > > > >> > > So DescribeGroupResponse will just return the entire compressed > > >> > > GroupMetadataMessage. SyncGroupResponse will return the > > corresponding > > >> > inner > > >> > > message. > > >> > > > > >> > > Thanks, > > >> > > > > >> > > Jiangjie (Becket) Qin > > >> > > > > >> > > > > >> > > > > >> > > On Tue, May 24, 2016 at 9:14 AM, Jason Gustafson < > > ja...@confluent.io> > > >> > > wrote: > > >> > > > > >> > > > Hey Becket, > > >> > > > > > >> > > > I like your idea to store only the offset for the group metadata > > in > > >> > > memory. > > >> > > > I think it would be safe to keep it in memory for a short time > > after > > >> > the > > >> > > > rebalance completes, but after that, it's only real purpose is > to > > >> > answer > > >> > > > DescribeGroup requests, so your proposal makes a lot of sense to > > me. > > >> > > > > > >> > > > As for the specific problem with the size of the group metadata > > >> message > > >> > > for > > >> > > > the MM case, if we cannot succeed in reducing the size of the > > >> > > > subscription/assignment (which I think is still probably the > best > > >> > > > alternative if it can work), then I think there are some options > > for > > >> > > > changing the message format (option #8 in Onur's initial > e-mail). > > >> > > > Currently, the key used for storing the group metadata is this: > > >> > > > > > >> > > > GroupMetadataKey => Version GroupId > > >> > > > > > >> > > > And the value is something like this (some details elided): > > >> > > > > > >> > > > GroupMetadataValue => Version GroupId Generation > [MemberMetadata] > > >> > > > MemberMetadata => ClientId Host Subscription Assignment > > >> > > > > > >> > > > I don't think we can change the key without a lot of pain, but > it > > >> seems > > >> > > > like we can change the value format. Maybe we can take the > > >> > > > subscription/assignment payloads out of the value and introduce > a > > >> new > > >> > > > "MemberMetadata" message for each member in the group. For > > example: > > >> > > > > > >> > > > MemberMetadataKey => Version GroupId MemberId > > >> > > > > > >> > > > MemberMetadataValue => Version Generation ClientId Host > > Subscription > > >> > > > Assignment > > >> > > > > > >> > > > When a new generation is created, we would first write the group > > >> > metadata > > >> > > > message which includes the generation and all of the memberIds, > > and > > >> > then > > >> > > > we'd write the member metadata messages. To answer the > > DescribeGroup > > >> > > > request, we'd read the group metadata at the cached offset and, > > >> > depending > > >> > > > on the version, all of the following member metadata. This would > > be > > >> > more > > >> > > > complex to maintain, but it seems doable if it comes to it. > > >> > > > > > >> > > > Thanks, > > >> > > > Jason > > >> > > > > > >> > > > On Mon, May 23, 2016 at 6:15 PM, Becket Qin < > becket....@gmail.com > > > > > >> > > wrote: > > >> > > > > > >> > > > > 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 > > >> > > > > > > > >> > > >> > > > > > > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > > > >> > > > > > > >> > > > > > >> > > > > >> > > > >> > > > >> > > > >> > -- > > >> > -- Guozhang > > >> > > > >> > > > > > > > > >
