Hi Andrew, I agree with having a single isolation.level for the whole group, it makes sense. As for: "b) The default isolation level for a share group is read_committed, in which case the SPSO and SPEO cannot move past the LSO."
With this limitation (SPEO not moving beyond LSO), are you trying to avoid handling the complexity of some kind of a "pending" state for the uncommitted in-flight messages? Thanks, Daniel Andrew Schofield <andrew_schofield_j...@outlook.com> ezt írta (időpont: 2023. jún. 7., Sze, 16:52): > HI Daniel, > I’ve been thinking about this question and I think this area is a bit > tricky. > > If there are some consumers in a share group with isolation level > read_uncommitted > and other consumers with read_committed, they have different expectations > with > regards to which messages are visible when EOS comes into the picture. > It seems to me that this is not necessarily a good thing. > > One option would be to support just read_committed in KIP-932. This means > it is unambiguous which records are in-flight, because they’re only > committed > ones. > > Another option would be to have the entire share group have an isolation > level, > which again gives an unambiguous set of in-flight records but without the > restriction of permitting just read_committed behaviour. > > So, my preference is for the following: > a) A share group has an isolation level that applies to all consumers in > the group. > b) The default isolation level for a share group is read_committed, in > which case > the SPSO and SPEO cannot move past the LSO. > c) For a share group with read_uncommited isolation level, the SPSO and > SPEO > can move past the LSO. > d) The kafka_configs.sh tool or the AdminClient can be used to set a > non-default > value for the isolation level for a share group. The value is applied when > the first > member joins. > > Thanks, > Andrew > > > On 2 Jun 2023, at 10:02, Dániel Urbán <urb.dani...@gmail.com> wrote: > > > > Hi Andrew, > > Thank you for the clarification. One follow-up to read_committed mode: > > Taking the change in message ordering guarantees into account, does this > > mean that in queues, share-group consumers will be able to consume > > committed records AFTER the LSO? > > Thanks, > > Daniel > > > > Andrew Schofield <andrew_schofield_j...@outlook.com> ezt írta (időpont: > > 2023. jún. 2., P, 10:39): > > > >> Hi Daniel, > >> Thanks for your questions. > >> > >> 1) Yes, read_committed fetch will still be possible. > >> > >> 2) You weren’t wrong that this is a broad question :) > >> > >> Broadly speaking, I can see two ways of managing the in-flight records: > >> the share-partition leader does it, or the share-group coordinator does > it. > >> I want to choose what works best, and I happen to have started with > trying > >> the share-partition leader doing it. This is just a whiteboard exercise > at > >> the > >> moment, looking at the potential protocol flows and how well it all > hangs > >> together. When I have something coherent and understandable and worth > >> reviewing, I’ll update the KIP with a proposal. > >> > >> I think it’s probably worth doing a similar exercise for the share-group > >> coordinator way too. There are bound to be pros and cons, and I don’t > >> really > >> mind which way prevails. > >> > >> If the share-group coordinator does it, I already have experience of > >> efficient > >> storage of in-flight record state in a way that scales and is > >> space-efficient. > >> If the share-partition leader does it, storage of in-flight state is a > bit > >> more > >> tricky. > >> > >> I think it’s worth thinking ahead to how EOS will work and also another > >> couple of enhancements (key-based ordering and acquisition lock > >> extension) so it’s somewhat future-proof. > >> > >> Thanks, > >> Andrew > >> > >>> On 1 Jun 2023, at 11:51, Dániel Urbán <urb.dani...@gmail.com> wrote: > >>> > >>> Hi Andrew, > >>> > >>> Thank you for the KIP, exciting work you are doing :) > >>> I have 2 questions: > >>> 1. I understand that EOS won't be supported for share-groups (yet), but > >>> read_committed fetch will still be possible, correct? > >>> > >>> 2. I have a very broad question about the proposed solution: why not > let > >>> the share-group coordinator manage the states of the in-flight records? > >>> I'm asking this because it seems to me that using the same pattern as > the > >>> existing group coordinator would > >>> a, solve the durability of the message state storage (same method as > the > >>> one used by the current group coordinator) > >>> > >>> b, pave the way for EOS with share-groups (same method as the one used > by > >>> the current group coordinator) > >>> > >>> c, allow follower-fetching > >>> I saw your point about this: "FFF gives freedom to fetch records from a > >>> nearby broker, but it does not also give the ability to commit offsets > >> to a > >>> nearby broker" > >>> But does it matter if message acknowledgement is not "local"? > Supposedly, > >>> fetching is the actual hard work which benefits from follower fetching, > >> not > >>> the group related requests. > >>> > >>> The only problem I see with the share-group coordinator managing the > >>> in-flight message state is that the coordinator is not aware of the > exact > >>> available offsets of a partition, nor how the messages are batched. For > >>> this problem, maybe the share group coordinator could use some form of > >>> "logical" addresses, such as "the next 2 batches after offset X", or > >> "after > >>> offset X, skip 2 batches, fetch next 2". Acknowledgements always > contain > >>> the exact offset, but for the "unknown" sections of a partition, these > >>> logical addresses would be used. The coordinator could keep track of > >>> message states with a mix of offsets and these batch based addresses. > The > >>> partition leader could support "skip X, fetch Y batches" fetch > requests. > >>> This solution would need changes in the Fetch API to allow such batch > >> based > >>> addresses, but I assume that fetch protocol changes will be needed > >>> regardless of the specific solution. > >>> > >>> Thanks, > >>> Daniel > >>> > >>> Andrew Schofield <andrew_schofi...@live.com> ezt írta (időpont: 2023. > >> máj. > >>> 30., K, 18:15): > >>> > >>>> Yes, that’s it. I imagine something similar to KIP-848 for managing > the > >>>> share group > >>>> membership, and consumers that fetch records from their assigned > >>>> partitions and > >>>> acknowledge when delivery completes. > >>>> > >>>> Thanks, > >>>> Andrew > >>>> > >>>>> On 30 May 2023, at 16:52, Adam Warski <a...@warski.org> wrote: > >>>>> > >>>>> Thanks for the explanation! > >>>>> > >>>>> So effectively, a share group is subscribed to each partition - but > the > >>>> data is not pushed to the consumer, but only sent on demand. And when > >>>> demand is signalled, a batch of messages is sent? > >>>>> Hence it would be up to the consumer to prefetch a sufficient number > of > >>>> batches to ensure, that it will never be "bored"? > >>>>> > >>>>> Adam > >>>>> > >>>>>> On 30 May 2023, at 15:25, Andrew Schofield < > andrew_schofi...@live.com > >>> > >>>> wrote: > >>>>>> > >>>>>> Hi Adam, > >>>>>> Thanks for your question. > >>>>>> > >>>>>> With a share group, each fetch is able to grab available records > from > >>>> any partition. So, it alleviates > >>>>>> the “head-of-line” blocking problem where a slow consumer gets in > the > >>>> way. There’s no actual > >>>>>> stealing from a slow consumer, but it can be overtaken and must > >>>> complete its processing within > >>>>>> the timeout. > >>>>>> > >>>>>> The way I see this working is that when a consumer joins a share > >> group, > >>>> it receives a set of > >>>>>> assigned share-partitions. To start with, every consumer will be > >>>> assigned all partitions. We > >>>>>> can be smarter than that, but I think that’s really a question of > >>>> writing a smarter assignor > >>>>>> just as has occurred over the years with consumer groups. > >>>>>> > >>>>>> Only a small proportion of Kafka workloads are super high > throughput. > >>>> Share groups would > >>>>>> struggle with those I’m sure. Share groups do not diminish the value > >> of > >>>> consumer groups > >>>>>> for streaming. They just give another option for situations where a > >>>> different style of > >>>>>> consumption is more appropriate. > >>>>>> > >>>>>> Thanks, > >>>>>> Andrew > >>>>>> > >>>>>>> On 29 May 2023, at 17:18, Adam Warski <a...@warski.org> wrote: > >>>>>>> > >>>>>>> Hello, > >>>>>>> > >>>>>>> thank you for the proposal! A very interesting read. > >>>>>>> > >>>>>>> I do have one question, though. When you subscribe to a topic using > >>>> consumer groups, it might happen that one consumer has processed all > >>>> messages from its partitions, while another one still has a lot of > work > >> to > >>>> do (this might be due to unbalanced partitioning, long processing > times > >>>> etc.). In a message-queue approach, it would be great to solve this > >> problem > >>>> - so that a consumer that is free can steal work from other consumers. > >> Is > >>>> this somehow covered by share groups? > >>>>>>> > >>>>>>> Maybe this is planned as "further work", as indicated here: > >>>>>>> > >>>>>>> " > >>>>>>> It manages the topic-partition assignments for the share-group > >>>> members. An initial, trivial implementation would be to give each > member > >>>> the list of all topic-partitions which matches its subscriptions and > >> then > >>>> use the pull-based protocol to fetch records from all partitions. A > more > >>>> sophisticated implementation could use topic-partition load and lag > >> metrics > >>>> to distribute partitions among the consumers as a kind of autonomous, > >>>> self-balancing partition assignment, steering more consumers to busier > >>>> partitions, for example. Alternatively, a push-based fetching scheme > >> could > >>>> be used. Protocol details will follow later. > >>>>>>> " > >>>>>>> > >>>>>>> but I’m not sure if I understand this correctly. A fully-connected > >>>> graph seems like a lot of connections, and I’m not sure if this would > >> play > >>>> well with streaming. > >>>>>>> > >>>>>>> This also seems as one of the central problems - a key > differentiator > >>>> between share and consumer groups (the other one being persisting > state > >> of > >>>> messages). And maybe the exact way we’d want to approach this would, > to > >> a > >>>> certain degree, dictate the design of the queueing system? > >>>>>>> > >>>>>>> Best, > >>>>>>> Adam Warski > >>>>>>> > >>>>>>> On 2023/05/15 11:55:14 Andrew Schofield wrote: > >>>>>>>> Hi, > >>>>>>>> I would like to start a discussion thread on KIP-932: Queues for > >>>> Kafka. This KIP proposes an alternative to consumer groups to enable > >>>> cooperative consumption by consumers without partition assignment. You > >> end > >>>> up with queue semantics on top of regular Kafka topics, with > per-message > >>>> acknowledgement and automatic handling of messages which repeatedly > >> fail to > >>>> be processed. > >>>>>>>> > >>>>>>>> > >>>> > >> > https://cwiki.apache.org/confluence/display/KAFKA/KIP-932%3A+Queues+for+Kafka > >>>>>>>> > >>>>>>>> Please take a look and let me know what you think. > >>>>>>>> > >>>>>>>> Thanks. > >>>>>>>> Andrew > >>>>>>> > >>>>>> > >>>>> > >>>>> -- > >>>>> Adam Warski > >>>>> > >>>>> https://www.softwaremill.com/ > >>>>> https://twitter.com/adamwarski > > >
