Hi PoAn,
Thanks for the update. I haven't read the updated KIP yet.
DJ02: I am not sure about using Guava as a dependency. I mentioned it more
as an inspiration/reference. I suppose that we could use it on the server
but we should definitely not use it on the client. I am not sure how others
feel about it.
Best,
David
On Mon, Jan 6, 2025 at 5:21 AM PoAn Yang <yangp...@gmail.com> wrote:
> Hi Chia-Ping / David / Lucas,
>
> Happy new year and thanks for the review.
>
> DJ02: Thanks for the suggestion. I updated the PR to use Guava.
>
> DJ03: Yes, I updated the description to mention ISR change,
> add altering partition reassignment case, and mention that
> non-related topic change doesn’t trigger a rebalance.
> DJ03.1: Yes, I will keep using ModernGroup#requestMetadataRefresh
> to notify group.
>
> DJ06: Updated the PR to use Guava Hashing#combineUnordered
> function to combine topic hash.
>
> DJ07: Renamed it to MetadataHash.
>
> DJ08: Added a sample hash function to the KIP and use first byte as magic
> byte. This is also included in latest PR.
>
> DJ09: Added two paragraphs about upgraded and downgraded.
>
> DJ10: According to Lucas’s comment, I add StreamsGroupMetadataValue update
> to this KIP.
>
> Thanks,
> PoAn
>
>
> > On Dec 20, 2024, at 3:58 PM, Chia-Ping Tsai <chia7...@gmail.com> wrote:
> >
> >> because assignors are sticky.
> >
> > I forgot about that spec again :(
> >
> >
> >
> >
> > David Jacot <david.ja...@gmail.com> 於 2024年12月20日 週五 下午3:41寫道：
> >
> >> Hi Chia-Ping,
> >>
> >> DJ08: In my opinion, changing the format will be rare so it is
> >> acceptable if rebalances are triggered in this case on
> >> upgrade/downgrade. It is also what will happen if a cluster is
> >> downgraded from 4.1 (with this KIP) to 4.0. The rebalance won't change
> >> anything if the topology of the group is the same because assignors
> >> are sticky. The default ones are and we recommend custom ones to also
> >> be.
> >>
> >> Best,
> >> David
> >>
> >> On Fri, Dec 20, 2024 at 2:11 AM Chia-Ping Tsai <chia7...@apache.org>
> >> wrote:
> >>>
> >>> ummm, it does not work for downgrade as the old coordinator has no idea
> >> about new format :(
> >>>
> >>>
> >>> On 2024/12/20 00:57:27 Chia-Ping Tsai wrote:
> >>>> hi David
> >>>>
> >>>>> DJ08:
> >>>>
> >>>> That's a good question. If the "hash" lacks version control, it could
> >> trigger a series of unnecessary rebalances. However, adding additional
> >> information ("magic") to the hash does not help the upgraded coordinator
> >> determine the "version." This means that the upgraded coordinator would
> >> still trigger unnecessary rebalances because it has no way to know which
> >> format to use when comparing the hash.
> >>>>
> >>>> Perhaps we can add a new field to ConsumerGroupMetadataValue to
> >> indicate the version of the "hash." This would allow the coordinator,
> when
> >> handling subscription metadata, to compute the old hash and determine
> >> whether an epoch bump is necessary. Additionally, the coordinator can
> >> generate a new record to upgrade the hash without requiring an epoch
> bump.
> >>>>
> >>>> Another issue is whether the coordinator should cache all versions of
> >> the hash. I believe this is necessary; otherwise, during an upgrade,
> there
> >> would be extensive recomputing of old hashes.
> >>>>
> >>>> I believe this idea should also work for downgrades, and that's just
> >> my two cents.
> >>>>
> >>>> Best,
> >>>> Chia-Ping
> >>>>
> >>>>
> >>>> On 2024/12/19 14:39:41 David Jacot wrote:
> >>>>> Hi PoAn and Chia-Ping,
> >>>>>
> >>>>> Thanks for your responses.
> >>>>>
> >>>>> DJ02: Sorry, I was not clear. I was wondering whether we could
> >> compute the
> >>>>> hash without having to convert to bytes before. Guava has a nice
> >> interface
> >>>>> for this allowing to incrementally add primitive types to the hash.
> >> We can
> >>>>> discuss this in the PR as it is an implementation detail.
> >>>>>
> >>>>> DJ03: Thanks. I don't think that the replicas are updated when a
> >> broker
> >>>>> shuts down. What you said applies to the ISR. I suppose that we can
> >> rely on
> >>>>> the ISR changes to trigger updates. It is also worth noting
> >>>>> that TopicsDelta#changedTopics is updated for every change (e.g. ISR
> >>>>> change, leader change, replicas change, etc.). I suppose that it is
> >> OK but
> >>>>> it seems that it will trigger refreshes which are not necessary.
> >> However, a
> >>>>> rebalance won't be triggered because the hash won't change.
> >>>>> DJ03.1: I suppose that we will continue to rely on
> >>>>> ModernGroup#requestMetadataRefresh to notify groups that must
> >> refresh their
> >>>>> hashes. Is my understanding correct?
> >>>>>
> >>>>> DJ05: Fair enough.
> >>>>>
> >>>>> DJ06: You mention in two places that you would like to combine
> >> hashes by
> >>>>> additioning them. I wonder if this is a good practice. Intuitively,
> >> I would
> >>>>> have used XOR or hashed the hashed. Guava has a method for combining
> >>>>> hashes. It may be worth looking into the algorithm used.
> >>>>>
> >>>>> DJ07: I would rename "AllTopicHash" to "MetadataHash" in order to be
> >> more
> >>>>> generic.
> >>>>>
> >>>>> DJ08: Regarding the per topic hash, I wonder whether we should
> >> precise in
> >>>>> the KIP how we will compute it. I had the following in mind:
> >>>>> hash(topicName; numPartitions; [partitionId;sorted racks]). We could
> >> also
> >>>>> add a magic byte at the first element as a sort of version. I am not
> >> sure
> >>>>> whether it is needed though. I was thinking about this while
> >> imagining how
> >>>>> we would handle changing the format in the future.
> >>>>>
> >>>>> DJ09: It would be great if we could provide more details about
> >> backward
> >>>>> compatibility. What happens when the cluster is upgraded or
> >> downgraded?
> >>>>>
> >>>>> DJ10: We should update KIP-1071. It may be worth pigging them in the
> >>>>> discussion thread of KIP-1071.
> >>>>>
> >>>>> Best,
> >>>>> David
> >>>>>
> >>>>> On Tue, Dec 17, 2024 at 9:25 AM PoAn Yang <yangp...@gmail.com>
> >> wrote:
> >>>>>
> >>>>>> Hi Chia-Ping / David / Andrew,
> >>>>>>
> >>>>>> Thanks for the review and suggestions.
> >>>>>>
> >>>>>> DJ01: Removed all implementation details.
> >>>>>>
> >>>>>> DJ02: Does the “incrementally” mean that we only calculate the
> >> difference
> >>>>>> parts?
> >>>>>> For example, if the number of partition change, we only calculate
> >> the hash
> >>>>>> of number of partition and reconstruct it to the topic hash.
> >>>>>> IMO, we only calculate topic hash one time. With cache mechanism,
> >> the
> >>>>>> value can be reused in different groups on a same broker.
> >>>>>> The CPU usage for this part is not very high.
> >>>>>>
> >>>>>> DJ03: Added the update path to KIP for both cases.
> >>>>>>
> >>>>>> DJ04: Yes, it’s a good idea. With cache mechanism and single hash
> >> per
> >>>>>> group, we can balance cpu and disk usage.
> >>>>>>
> >>>>>> DJ05: Currently, the topic hash is only used in coordinator.
> >> However, the
> >>>>>> metadata image is used in many different places.
> >>>>>> How about we move the hash to metadata image when we find more use
> >> cases?
> >>>>>>
> >>>>>> AS1, AS2: Thanks for the reminder. I will simply delete
> >>>>>> ShareGroupPartitionMetadataKey/Value and add a new field to
> >>>>>> ShareGroupMetadataValue.
> >>>>>>
> >>>>>> Best,
> >>>>>> PoAn
> >>>>>>
> >>>>>>> On Dec 17, 2024, at 5:50 AM, Andrew Schofield <
> >>>>>> andrew_schofield_j...@outlook.com> wrote:
> >>>>>>>
> >>>>>>> Hi PoAn,
> >>>>>>> Thanks for the KIP.
> >>>>>>>
> >>>>>>> AS1: From the point of view of share groups, the API and record
> >> schema
> >>>>>>> definitions are unstable in AK 4.0. In AK 4.1, we will start
> >> supporting
> >>>>>> proper
> >>>>>>> versioning. As a result, I think you do not need to deprecate
> >> the fields
> >>>>>> in the
> >>>>>>> ShareGroupPartitionMetadataValue. Just include the schema for
> >> the fields
> >>>>>>> which are actually needed, and I'll update the schema in the
> >> code when
> >>>>>>> the KIP is implemented.
> >>>>>>>
> >>>>>>> AS2: In the event that DJ04 actually removes the need for
> >>>>>>> ConsumerGroupPartitionMetadataKey/Value entirely, I would simply
> >>>>>>> delete ShareGroupPartitionMetadataKey/Value, assuming that it is
> >>>>>>> accepted in time for AK 4.1.
> >>>>>>>
> >>>>>>>
> >>>>>>> Thanks,
> >>>>>>> Andrew
> >>>>>>> ________________________________________
> >>>>>>> From: Chia-Ping Tsai <chia7...@apache.org>
> >>>>>>> Sent: 16 December 2024 16:27
> >>>>>>> To: dev@kafka.apache.org <dev@kafka.apache.org>
> >>>>>>> Subject: Re: [DISCUSS] KIP-1101: Trigger rebalance on rack
> >> topology
> >>>>>> changes
> >>>>>>>
> >>>>>>> hi David
> >>>>>>>
> >>>>>>>> DJ05
> >>>>>>>
> >>>>>>> One of the benefits of having a single hash per group (DJ04) is
> >> the
> >>>>>> reduction in the size of stored data. Additionally, the cost of
> >>>>>> re-computing can be minimized thanks to caching. So I'm + 1 to
> >> DJ04.
> >>>>>> However, the advantage of storing the topic cache in the metadata
> >> image is
> >>>>>> somewhat unclear to me. Could you please provide more details on
> >> what you
> >>>>>> mean by "tight"?Furthermore, since the metadata image is a
> >> thread-safe
> >>>>>> object, we need to ensure that the lazy initialization is also
> >> thread-safe.
> >>>>>> If no other components require the cache, it would be better to
> >> keep the
> >>>>>> caches within the coordinator.
> >>>>>>>
> >>>>>>> Best,
> >>>>>>> Chia-Ping
> >>>>>>>
> >>>>>>> On 2024/12/16 14:01:35 David Jacot wrote:
> >>>>>>>> Hi PoAn,
> >>>>>>>>
> >>>>>>>> Thanks for the KIP. I have some comments about it.
> >>>>>>>>
> >>>>>>>> DJ01: Please, remove all the code from the KIP. We only care
> >> about
> >>>>>> public
> >>>>>>>> interface changes, not about implementation details.
> >>>>>>>> DJ02: Regarding the hash computation, I agree that we should use
> >>>>>> Murmur3.
> >>>>>>>> However, I don't quite like the implementation that you shared.
> >> I
> >>>>>> wonder if
> >>>>>>>> we could make it work incrementally instead of computing a hash
> >> of
> >>>>>>>> everything and combining them.
> >>>>>>>> DJ03: Regarding the cache, my understanding is that the cache is
> >>>>>> populated
> >>>>>>>> when a topic without hash is seen in a HB request and the cache
> >> is
> >>>>>> cleaned
> >>>>>>>> up when topics are deleted based on the metadata image.
> >> However, the
> >>>>>> update
> >>>>>>>> path is not clear. Let's say that a partition is added to a
> >> topic, how
> >>>>>> does
> >>>>>>>> it detect it? Let's also imagine that the racks of a partition
> >> have
> >>>>>>>> changed, how does it detect it? In the KIP, it would be nice to
> >> be clear
> >>>>>>>> about those.
> >>>>>>>> DJ04: I wonder whether we should go with a single hash per
> >> group. Your
> >>>>>>>> argument against it is that it would require to re-compute the
> >> hash of
> >>>>>> all
> >>>>>>>> the topics when it needs to be computed. In my opinion, we could
> >>>>>> leverage
> >>>>>>>> the cached hash per topic to compute the hash of all the
> >> subscribed
> >>>>>> ones.
> >>>>>>>> We could basically combine all the hashes without having to
> >> compute all
> >>>>>> of
> >>>>>>>> them. This approach has a few benefits. 1) We could get rid of
> >>>>>>>> the ConsumerGroupPartitionMetadata record as we could store the
> >> hash
> >>>>>> with
> >>>>>>>> the group epoch. 2) We could get rid of the Map that we keep in
> >> each
> >>>>>> group
> >>>>>>>> to store the hashed corresponding to the subscribed topics.
> >>>>>>>> DJ05: Regarding the cache again, I wonder if we should actually
> >> store
> >>>>>> the
> >>>>>>>> hash in the metadata image instead of maintaining it somewhere
> >> else. We
> >>>>>>>> could still lazily compute it. The benefit is that the value
> >> would be
> >>>>>> tight
> >>>>>>>> to the topic. I have not really looked into it. Would it be an
> >> option?
> >>>>>>>>
> >>>>>>>> I'll be away for two weeks starting from Saturday. I kindly ask
> >> you to
> >>>>>> wait
> >>>>>>>> on me if we cannot conclude this week.
> >>>>>>>>
> >>>>>>>> Best,
> >>>>>>>> David
> >>>>>>>>
> >>>>>>>> On Tue, Nov 5, 2024 at 1:43 PM Frank Yang <yangp...@gmail.com>
> >> wrote:
> >>>>>>>>
> >>>>>>>>> Hi Chia-Ping,
> >>>>>>>>>
> >>>>>>>>> Thanks for the review and suggestions.
> >>>>>>>>>
> >>>>>>>>> Q0: Add how rack change and how it affects topic partition.
> >>>>>>>>>
> >>>>>>>>> Q1: Add why we need a balance algorithm to Motivation section.
> >>>>>>>>>
> >>>>>>>>> Q2: After checking again, we don’t need to update cache when
> >> we replay
> >>>>>>>>> records. We only need to renew it in consumer heartbeat.
> >>>>>>>>>
> >>>>>>>>> Q3: Add a new section “Topic Hash Function”.
> >>>>>>>>>
> >>>>>>>>> Thanks.
> >>>>>>>>> PoAn
> >>>>>>>>>
> >>>>>>>>>> On Nov 1, 2024, at 4:39 PM, Chia-Ping Tsai <
> >> chia7...@gmail.com>
> >>>>>> wrote:
> >>>>>>>>>>
> >>>>>>>>>> hi PoAn
> >>>>>>>>>>
> >>>>>>>>>> Thanks for for this KIP!
> >>>>>>>>>>
> >>>>>>>>>> Q0: Could you add more details about `A topic partition has
> >> rack
> >>>>>> change`?
> >>>>>>>>>> IIRC, the "rack change" includes both follower and leader,
> >> right?
> >>>>>>>>>>
> >>>>>>>>>> Q1: Could you please add the 'concerns' we discussed to the
> >> Motivation
> >>>>>>>>>> section? This should include topics like 'computations' and
> >> 'space
> >>>>>>>>> usage'.
> >>>>>>>>>>
> >>>>>>>>>> Q2: `The group coordinator can leverage it to add a new topic
> >>>>>> hash.`This
> >>>>>>>>>> description seems a bit off to me. Why do we need to update
> >> the cache
> >>>>>> at
> >>>>>>>>>> this phase? The cache is intended to prevent duplicate
> >> computations
> >>>>>>>>> caused
> >>>>>>>>>> by heartbeat requests that occur between two metadata change
> >> events.
> >>>>>>>>>> Therefore, we could even remove the changed topics from
> >> caches on a
> >>>>>>>>>> metadata change, as the first heartbeat request would update
> >> the
> >>>>>> caches
> >>>>>>>>> for
> >>>>>>>>>> all changed topics.
> >>>>>>>>>>
> >>>>>>>>>> Q3: Could you please include a section about the choice of
> >> hash
> >>>>>>>>>> implementation? The hash implementation must be consistent
> >> across
> >>>>>>>>> different
> >>>>>>>>>> JDKs, so we use Murmur3 to generate the hash value.
> >>>>>>>>>>
> >>>>>>>>>> Best,
> >>>>>>>>>> Chia-Ping
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> Frank Yang <yangp...@gmail.com> 於 2024年11月1日 週五 下午3:57寫道：
> >>>>>>>>>>
> >>>>>>>>>>> Hi all,
> >>>>>>>>>>>
> >>>>>>>>>>> I would like to start a discussion thread on KIP-1101.
> >> Trigger
> >>>>>> rebalance
> >>>>>>>>>>> on rack topology changes. In this KIP, we aim to use less
> >> memory /
> >>>>>> disk
> >>>>>>>>>>> resources to detect rack changes in the new coordinator.
> >>>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>
> >>>>>>
> >>
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-1101%3A+Trigger+rebalance+on+rack+topology+changes
> >>>>>>>>>>>
> >>>>>>>>>>> Please take a look and feel free to share any thoughts.
> >>>>>>>>>>>
> >>>>>>>>>>> Thanks.
> >>>>>>>>>>> PoAn
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>
> >>>>>>
> >>>>>>
> >>>>>
> >>>>
> >>
>
>
