Hello John,
Your reasoning about non-logged state store looks good to me. But I think
it worth considering two cases differently: 1) a task's all state stores
are non-logged, 2) a task has non-logged state store but also have other
logged state store.
For 1), we should not have any standbys for such tasks at all, but it is
not the same as a stateless task, because the only active task could report
a lag of "0" while all others should logically report the lag as infinity
(of course, they would not encode such value).
For 2), it is sort of the same to a normal case: active task reporting a
lag of 0 while standby task reporting a lag summing all other logged state
stores, meaning that the non-logged state store does not matter in our
assignment semantics.
The rationale I had is to effectively favor stickiness for those tasks in
case 1) than arbitrarily reassign them even though it is true that for a
non-logged store there's no durability and hence restoration guarantees
anyways from Streams. Then algorithmically we may consider assigning tasks
with descending order of the number of instances reporting lags for it,
then for such tasks there's always one candidate reporting a lag of 0, and
assigning them to any instance does not actually change the cumulated total
lag either. At that time we can decide "if it is still within load balance
factor, then let's respect stickiness, otherwise it's free to move". WDYT?
Guozhang
On Wed, Sep 4, 2019 at 8:30 AM John Roesler <j...@confluent.io> wrote:
> Hey Bruno,
>
> Thanks for taking another look. Some quick responses:
>
> 1) It just means the number of offsets in the topic. E.g., the LSO is 100,
> but the first offset is 40 due to retention, so there are 60 offsets in the
> topic. Further, the lag on that topic would be considered to be 60 for any
> task that hadn't previously done any work on it.
>
> 2) This is undecidable in general. I.e., there's no way we can know whether
> the store is remote or not, and hence whether we can freely assign it to
> another instance, or whether we have to keep it on the same instance.
> However, there are a couple of reasons to go ahead and assume we have the
> freedom to move such tasks.
> * We know that nothing can prevent the loss of an instance in a cluster
> (I.e., this is true of all cloud environments, as well as any managed
> virtualized cluster like mesos or kubernetes), so any Streams program that
> makes use of non-remote, non-logged state is doomed to lose its state when
> it loses an instance.
> * If we take on a restriction that we cannot move such state between
> instances, we'd become overconstrained very quickly. Effectively, if you
> made use of non-logged stores, and we didn't assume freedom of movement,
> then we couldn't make use of any new instances in your cluster.
> * On the other hand, if we optimistically assume we can't move state, but
> only reassign it when we lose an instance, then we're supporting
> non-deterministic logic, because the program would produce different
> results, depending on whether you lost a node during the execution or not.
> 2b) That last point goes along with your side note. I'm not sure if we
> should bother dropping such state on every reassignment, though. It seems
> to be undefined territory enough that we can just do the simplest thing and
> assume people have made their own (external) provisions for durability.
> I.e., when we say "non-logged", we mean that it doesn't make use of _our_
> durability mechanism. I'm arguing that the only sane assumption is that
> such folks have opted to use their own durability measures, and we should
> just assume it works with no special considerations in the assignment
> algorithm.
>
> 3) Good catch! I've fixed it.
>
> Thanks again!
> -John
>
> On Wed, Sep 4, 2019 at 6:09 AM Bruno Cadonna <br...@confluent.io> wrote:
>
> > Hi,
> >
> > 1) What do you mean with "full set of offsets in the topic"? Is this
> > the sum of all offsets of the changelog partitions of the task?
> >
> > 2) I am not sure whether non-logged stateful tasks should be
> > effectively treated as stateless tasks during assignment. First we
> > need to decide whether a non-logged stateful task should preferably be
> > assigned to the same instance on which it just run in order to
> > continue to use its state or not.
> >
> > 3) In the example, you define stand-by tasks {S1, S2, ...} but never
> > use them, because below you use a dedicated row for stand-by tasks.
> >
> > As a side note to 2) since it is not directly related to this KIP: We
> > should decide if we want to avoid the possible non-determinism
> > introduced by non-logged stores or not. That is, if an instance hosts
> > a task with non-logged stores then we can have two cases after the
> > next rebalance: a) the task stays on the same instance and continues
> > to use the same state store as used so far or b) the task is assigned
> > to another instance and it starts an empty state store. The produced
> > results for these two cases might differ. To avoid the nondeterminism,
> > non-logged state stores would need to be wiped out before assignment.
> > Then the question arises, how the removal of non-logged state stores
> > before assignment would affect backward-compatibility.
> >
> > Best,
> > Bruno
> >
> > On Wed, Aug 21, 2019 at 11:40 PM John Roesler <j...@confluent.io> wrote:
> > >
> > > Hi Guozhang,
> > >
> > > > My impression from your previous email is that inside the algorithm
> > when
> > > we
> > > are "filling" them to instances some deterministic logic would be used
> to
> > > avoid the above case, is that correct?
> > >
> > > Yes, that was my plan, but I didn't formalize it. There was a
> requirement
> > > that the assignment algorithm must not produce a new assignment if the
> > > current assignment is already balanced, so at the least, any thrashing
> > > would be restricted to the "balancing" phase while tasks are moving
> > around
> > > the cluster.
> > >
> > > Anyway, I think it would be good to say that we'll "try to" produce
> > stable
> > > assignments, so I've added a "should" clause to the assignment spec:
> > >
> >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-441%3A+Smooth+Scaling+Out+for+Kafka+Streams#KIP-441:SmoothScalingOutforKafkaStreams-AssignmentAlgorithm
> > >
> > > For example, we would sort the stateless tasks and available instances
> > > before assigning them, so that the stateless task assignment would
> mostly
> > > stay stable between assignments, modulo the compute capacity of the
> > > instances changing a little as active stateful tasks get assigned in
> more
> > > balanced ways.
> > >
> > > Thanks,
> > > -John
> > >
> > >
> > > On Wed, Aug 21, 2019 at 1:55 PM Guozhang Wang <wangg...@gmail.com>
> > wrote:
> > >
> > > > Hello John,
> > > >
> > > > That sounds reasonable. Just double checked the code that with
> logging
> > > > disabled the corresponding checkpoint file would not contain any
> > values,
> > > > just like a stateless task. So I think treating them logically the
> > same is
> > > > fine.
> > > >
> > > > Guozhang
> > > >
> > > >
> > > > On Wed, Aug 21, 2019 at 11:41 AM John Roesler <j...@confluent.io>
> > wrote:
> > > >
> > > > > Hi again, Guozhang,
> > > > >
> > > > > While writing up the section on stateless tasks (
> > > > >
> > > > >
> > > >
> >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-441%3A+Smooth+Scaling+Out+for+Kafka+Streams#KIP-441:SmoothScalingOutforKafkaStreams-Statelesstasks
> > > > > ),
> > > > > I reconsidered whether stateful, but non-logged, tasks should
> > actually
> > > > > report a lag of zero, versus not reporting any lag. By the
> > definition of
> > > > > the "StatefulTasksToRankedCandidates" function, the leader would
> > compute
> > > > a
> > > > > lag of zero for these tasks anyway.
> > > > >
> > > > > Therefore, I think the same reasoning that I supplied you for
> > stateless
> > > > > tasks applies, since the member and leader will agree on a lag of
> > zero
> > > > > anyway, we can avoid adding them to the "Task Lags" map, and save
> > some
> > > > > bytes in the JoinGroup request. This would be especially beneficial
> > in an
> > > > > application that uses remote stores for _all_ its state stores, it
> > would
> > > > > have an extremely lightweight JoinGroup request, with no task lags
> at
> > > > all.
> > > > >
> > > > > WDYT?
> > > > > -John
> > > > >
> > > > > On Wed, Aug 21, 2019 at 1:17 PM John Roesler <j...@confluent.io>
> > wrote:
> > > > >
> > > > > > Thanks, Guozhang.
> > > > > >
> > > > > > (Side note: I noticed on another pass over the discussion that
> I'd
> > > > missed
> > > > > > addressing your comment about the potential race condition
> between
> > > > state
> > > > > > cleanup and lag-based assignment. I've added a solution to the
> > > > proposal:
> > > > > >
> > > > >
> > > >
> >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-441%3A+Smooth+Scaling+Out+for+Kafka+Streams#KIP-441:SmoothScalingOutforKafkaStreams-Racebetweenassignmentandstatecleanup
> > > > > > )
> > > > > >
> > > > > > In the JoinGroup (SubscriptionInfo) metadata, stateless tasks are
> > not
> > > > > > represented at all. This should save us some bytes in the request
> > > > > metadata.
> > > > > > If we treated them like non-logged stateful tasks and reported a
> > lag of
> > > > > 0,
> > > > > > the only difference is that the assignor would be able to tell
> > which
> > > > > > members previously hosted that stateless task.
> > > > > >
> > > > > > I'd like to make a simplifying assumption that stateless tasks
> can
> > just
> > > > > be
> > > > > > freely reassigned with no regard to stickiness at all, without
> > > > impacting
> > > > > > performance. This is almost true. In fact, while assigned a
> > stateless
> > > > > task,
> > > > > > a member fetches batches of records from the broker, so if we
> move
> > the
> > > > > > stateless task assignment, this buffered input is wasted and just
> > gets
> > > > > > dropped.
> > > > > >
> > > > > > However, we won't be moving the stateless tasks around all the
> time
> > > > (just
> > > > > > during rebalances), and we have the requirement that the
> assigment
> > > > > > algorithm must stabilize to guard against perpetually shuffling a
> > > > > stateless
> > > > > > task from one node to another. So, my hope is that this small
> > amount of
> > > > > > inefficiency would not be a performance-dominating factor. In
> > exchange,
> > > > > we
> > > > > > gain the opportunity for the assignment algorithm to use the
> > stateless
> > > > > > tasks as "filler" during unbalanced assignments. For example, if
> > there
> > > > > is a
> > > > > > node that is just warming up with several standby tasks, maybe
> the
> > > > > > assignment can give more stateless tasks to that node to balance
> > the
> > > > > > computational load across the cluster.
> > > > > >
> > > > > > It's worth noting that such an assignment would still not be
> > considered
> > > > > > "balanced", so the ultimately balanced final state of the
> > assignment
> > > > > (after
> > > > > > task movements) would still have the desired property that each
> > > > stateful
> > > > > > and stateless task is evenly spread across the cluster.
> > > > > >
> > > > > > Does that seem reasonable?
> > > > > >
> > > > > > Thanks,
> > > > > > -John
> > > > > >
> > > > > > On Wed, Aug 21, 2019 at 11:22 AM Guozhang Wang <
> wangg...@gmail.com
> > >
> > > > > wrote:
> > > > > >
> > > > > >> Hello John,
> > > > > >>
> > > > > >> I've made another pass on the wiki page again, overall LGTM. One
> > meta
> > > > > >> comment about the "stateless" tasks: how do we represent them in
> > the
> > > > > >> metadata? Are they just treated as stateful tasks with logging
> > > > disabled,
> > > > > >> or
> > > > > >> are specially handled? It is not very clear in the description.
> > > > > >>
> > > > > >>
> > > > > >> Guozhang
> > > > > >>
> > > > > >> On Wed, Aug 21, 2019 at 8:43 AM John Roesler <j...@confluent.io
> >
> > > > wrote:
> > > > > >>
> > > > > >> > I have also specifically called out that the assignment must
> > achieve
> > > > > >> both
> > > > > >> > "instance" and "task" balance:
> > > > > >> >
> > > > > >> >
> > > > > >>
> > > > >
> > > >
> >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-441%3A+Smooth+Scaling+Out+for+Kafka+Streams#KIP-441:SmoothScalingOutforKafkaStreams-Defining%22balance%22
> > > > > >> >
> > > > > >> > I've also addressed the problem of state stores with logging
> > > > disabled:
> > > > > >> >
> > > > > >> >
> > > > > >>
> > > > >
> > > >
> >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-441%3A+Smooth+Scaling+Out+for+Kafka+Streams#KIP-441:SmoothScalingOutforKafkaStreams-Statewithloggingdisabled
> > > > > >> >
> > > > > >> > I believe this addresses all the concerns that have been
> raised
> > to
> > > > > date.
> > > > > >> > Apologies if I've overlooked one of your concerns.
> > > > > >> >
> > > > > >> > Please give the KIP another read and let me know of any
> further
> > > > > >> thoughts!
> > > > > >> > Hopefully, we can start the voting on this KIP by the end of
> the
> > > > week.
> > > > > >> >
> > > > > >> > Thanks,
> > > > > >> > -John
> > > > > >> >
> > > > > >> > On Tue, Aug 20, 2019 at 5:16 PM John Roesler <
> j...@confluent.io
> > >
> > > > > wrote:
> > > > > >> >
> > > > > >> > > In response to Bruno's concern #2, I've also added that
> > section to
> > > > > the
> > > > > >> > > "Rejected Alternatives" section.
> > > > > >> > >
> > > > > >> > > Additionally, after reviewing some other assignment papers,
> > I've
> > > > > >> > developed
> > > > > >> > > the concern that specifying which "phases" the assignment
> > > > algorithm
> > > > > >> > should
> > > > > >> > > have, or indeed the logic of it at all, might be a mistake
> > that
> > > > > >> > > over-constrains our ability to write an optimal algorithm.
> > > > > Therefore,
> > > > > >> > I've
> > > > > >> > > also refactored the KIP to just describe the protocol, and
> > specify
> > > > > the
> > > > > >> > > requirements for the assignment algorithm, but not its exact
> > > > > behavior
> > > > > >> at
> > > > > >> > > all.
> > > > > >> > >
> > > > > >> > > Thanks,
> > > > > >> > > -John
> > > > > >> > >
> > > > > >> > > On Tue, Aug 20, 2019 at 5:13 PM John Roesler <
> > j...@confluent.io>
> > > > > >> wrote:
> > > > > >> > >
> > > > > >> > >> Hi All,
> > > > > >> > >>
> > > > > >> > >> Thanks for the discussion. I've been considering the idea
> of
> > > > giving
> > > > > >> the
> > > > > >> > >> "catching up" tasks a different name/role. I was in favor
> > > > > initially,
> > > > > >> but
> > > > > >> > >> after working though some details, I think it causes some
> > > > problems,
> > > > > >> > which
> > > > > >> > >> I've written up in the "rejected alternatives" part of the
> > KIP:
> > > > > >> > >>
> > > > > >> >
> > > > > >>
> > > > >
> > > >
> >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-441%3A+Smooth+Scaling+Out+for+Kafka+Streams#KIP-441:SmoothScalingOutforKafkaStreams-Addinganewkindoftask(%22moving%22,%22recovering%22,%22learner%22)fortaskmovements
> > > > > >> > >>
> > > > > >> > >> Please give it a read and let me know what you think.
> > > > > >> > >>
> > > > > >> > >> Thanks,
> > > > > >> > >> -John
> > > > > >> > >>
> > > > > >> > >> On Thu, Aug 8, 2019 at 7:57 PM Guozhang Wang <
> > wangg...@gmail.com
> > > > >
> > > > > >> > wrote:
> > > > > >> > >>
> > > > > >> > >>> I think I agree with you Sophie. My gut feeling is that 1)
> > it
> > > > > should
> > > > > >> > not
> > > > > >> > >>> be
> > > > > >> > >>> the major concern in assignor's algorithm for standby
> tasks
> > not
> > > > > >> > catching
> > > > > >> > >>> up, but rather be tackled in different modules, and 2) a
> > lot of
> > > > > >> > >>> optimization can be down at the stream thread itself, like
> > > > > dedicated
> > > > > >> > >>> threading and larger batching, or even complicated
> > scheduling
> > > > > >> > mechanisms
> > > > > >> > >>> between running, restoring and standby tasks. In anyways,
> I
> > > > think
> > > > > we
> > > > > >> > can
> > > > > >> > >>> take this out of the scope of KIP-441 for now.
> > > > > >> > >>>
> > > > > >> > >>>
> > > > > >> > >>> Guozhang
> > > > > >> > >>>
> > > > > >> > >>>
> > > > > >> > >>> On Thu, Aug 8, 2019 at 5:48 PM Sophie Blee-Goldman <
> > > > > >> > sop...@confluent.io>
> > > > > >> > >>> wrote:
> > > > > >> > >>>
> > > > > >> > >>> > > we may have other ways to not starving the standby
> > tasks,
> > > > for
> > > > > >> > >>> example, by
> > > > > >> > >>> > > using dedicate threads for standby tasks or even
> > consider
> > > > > having
> > > > > >> > >>> > *higher> priority for standby than active* so that we
> > always
> > > > try
> > > > > >> to
> > > > > >> > >>> caught
> > > > > >> > >>> > up standby
> > > > > >> > >>> > > first, then process active
> > > > > >> > >>> >
> > > > > >> > >>> > This is an interesting idea, but seems likely to get in
> > the
> > > > way
> > > > > of
> > > > > >> > the
> > > > > >> > >>> > original idea of this KIP
> > > > > >> > >>> > -- if we always process standby tasks first, then if we
> > are
> > > > > >> assigned
> > > > > >> > a
> > > > > >> > >>> new
> > > > > >> > >>> > standby task we
> > > > > >> > >>> > will have to wait for it to catch up completely before
> > > > > processing
> > > > > >> any
> > > > > >> > >>> > active tasks! That's
> > > > > >> > >>> > even worse than the situation this KIP is trying to help
> > with,
> > > > > >> since
> > > > > >> > a
> > > > > >> > >>> new
> > > > > >> > >>> > standby task has to
> > > > > >> > >>> > restore from 0 (whereas an active task at least can take
> > over
> > > > > from
> > > > > >> > >>> wherever
> > > > > >> > >>> > the standby was).
> > > > > >> > >>> >
> > > > > >> > >>> > During restoration -- while there exist any restoring
> > tasks
> > > > -- I
> > > > > >> > think
> > > > > >> > >>> it's
> > > > > >> > >>> > reasonable to de-prioritize the
> > > > > >> > >>> > standby tasks and just process restoring and active
> tasks
> > so
> > > > > both
> > > > > >> can
> > > > > >> > >>> make
> > > > > >> > >>> > progress. But we should
> > > > > >> > >>> > let them catch up afterwards somehow -- maybe we can
> apply
> > > > some
> > > > > >> kind
> > > > > >> > of
> > > > > >> > >>> > heuristic, like "if we haven't
> > > > > >> > >>> > processed standbys for X iterations, or Y milliseconds,
> > do so
> > > > > >> now."
> > > > > >> > >>> >
> > > > > >> > >>> > Actually, it might even be beneficial to avoid
> processing
> > > > > >> standbys a
> > > > > >> > >>> record
> > > > > >> > >>> > or two at a time and instead
> > > > > >> > >>> > wait for a large enough batch to build up for the
> RocksDB
> > > > > >> > bulk-loading
> > > > > >> > >>> > benefits.
> > > > > >> > >>> >
> > > > > >> > >>> > I think the "use dedicated threads for standby" is the
> > more
> > > > > >> promising
> > > > > >> > >>> end
> > > > > >> > >>> > goal, especially since
> > > > > >> > >>> > if we split restoration into "restoring tasks" then
> > active and
> > > > > >> > standbys
> > > > > >> > >>> > share almost nothing. But
> > > > > >> > >>> > that seems like follow-up work to the current KIP :)
> > > > > >> > >>> >
> > > > > >> > >>> > On Thu, Aug 8, 2019 at 5:31 PM Sophie Blee-Goldman <
> > > > > >> > >>> sop...@confluent.io>
> > > > > >> > >>> > wrote:
> > > > > >> > >>> >
> > > > > >> > >>> > > Stateful tasks with logging disabled seem to be an
> > > > interesting
> > > > > >> edge
> > > > > >> > >>> case.
> > > > > >> > >>> > > On the one hand,
> > > > > >> > >>> > > for balancing purposes they should be considered
> > stateful
> > > > > since
> > > > > >> as
> > > > > >> > >>> > > Guozhang pointed out
> > > > > >> > >>> > > they are still "heavy" in IO costs. But for "catching
> > up"
> > > > > >> purposes,
> > > > > >> > >>> ie
> > > > > >> > >>> > > when allocating standby
> > > > > >> > >>> > > tasks that will become active tasks, they should be
> > > > considered
> > > > > >> > >>> stateless
> > > > > >> > >>> > > as there is so
> > > > > >> > >>> > > meaningful sense of their lag. We should never
> allocate
> > > > > standby
> > > > > >> > >>> tasks for
> > > > > >> > >>> > > them during the
> > > > > >> > >>> > > first rebalance, but should ensure they are evenly
> > > > distributed
> > > > > >> > across
> > > > > >> > >>> > > instances. Maybe we
> > > > > >> > >>> > > should split these into a third category -- after we
> > assign
> > > > > all
> > > > > >> > >>> stateful
> > > > > >> > >>> > > tasks with logging, we
> > > > > >> > >>> > > then distribute the set of logging-disabled stateful
> > tasks
> > > > to
> > > > > >> > improve
> > > > > >> > >>> > > balance, before lastly
> > > > > >> > >>> > > distributing stateless tasks?
> > > > > >> > >>> > >
> > > > > >> > >>> > > This actually leads into what I was just thinking,
> > which is
> > > > > >> that we
> > > > > >> > >>> > really
> > > > > >> > >>> > > should distinguish the
> > > > > >> > >>> > > "catch-up" standbys from normal standbys as well as
> > > > > >> distinguishing
> > > > > >> > >>> > > actively processing tasks
> > > > > >> > >>> > > from active tasks that are still in the restore phase.
> > It's
> > > > > >> > somewhat
> > > > > >> > >>> > > awkward that today, some
> > > > > >> > >>> > > active tasks just start processing immediately while
> > others
> > > > > >> behave
> > > > > >> > >>> more
> > > > > >> > >>> > > like standby than active
> > > > > >> > >>> > > tasks for some time, before switching to real active.
> > They
> > > > > first
> > > > > >> > use
> > > > > >> > >>> the
> > > > > >> > >>> > > restoreConsumer, then
> > > > > >> > >>> > > later only the "normal" consumer.
> > > > > >> > >>> > >
> > > > > >> > >>> > > However, this restore period is still distinct from
> > normal
> > > > > >> standbys
> > > > > >> > >>> in a
> > > > > >> > >>> > > lot of ways -- the code path
> > > > > >> > >>> > > for restoring is different than for updating standbys,
> > for
> > > > > >> example
> > > > > >> > >>> in how
> > > > > >> > >>> > > long we block on #poll.
> > > > > >> > >>> > > So in addition to giving them their own name -- let's
> go
> > > > with
> > > > > >> > >>> restoring
> > > > > >> > >>> > > task for now -- they really
> > > > > >> > >>> > > do seem to deserve being their own distinct task. We
> can
> > > > > >> optimize
> > > > > >> > >>> them
> > > > > >> > >>> > for
> > > > > >> > >>> > > efficient conversion
> > > > > >> > >>> > > to active tasks since we know that's what they will
> be.
> > > > > >> > >>> > >
> > > > > >> > >>> > > This resolves some of the awkwardness of dealing with
> > the
> > > > > >> special
> > > > > >> > >>> case
> > > > > >> > >>> > > mentioned above: we
> > > > > >> > >>> > > find a balanced assignment of stateful and stateless
> > tasks,
> > > > > and
> > > > > >> > >>> create
> > > > > >> > >>> > > restoring tasks as needed.
> > > > > >> > >>> > > If logging is disabled, no restoring task is created.
> > > > > >> > >>> > >
> > > > > >> > >>> > >
> > > > > >> > >>> > > On Thu, Aug 8, 2019 at 3:44 PM Guozhang Wang <
> > > > > >> wangg...@gmail.com>
> > > > > >> > >>> wrote:
> > > > > >> > >>> > >
> > > > > >> > >>> > >> Regarding 3) above: I think for active task they
> should
> > > > still
> > > > > >> be
> > > > > >> > >>> > >> considered
> > > > > >> > >>> > >> stateful since the processor would still pay IO cost
> > > > > accessing
> > > > > >> the
> > > > > >> > >>> > store,
> > > > > >> > >>> > >> but they would not have standby tasks?
> > > > > >> > >>> > >>
> > > > > >> > >>> > >> On Thu, Aug 8, 2019 at 7:49 AM Bruno Cadonna <
> > > > > >> br...@confluent.io>
> > > > > >> > >>> > wrote:
> > > > > >> > >>> > >>
> > > > > >> > >>> > >> > Hi,
> > > > > >> > >>> > >> >
> > > > > >> > >>> > >> > Thank you for the KIP!
> > > > > >> > >>> > >> >
> > > > > >> > >>> > >> > Some questions/comments:
> > > > > >> > >>> > >> >
> > > > > >> > >>> > >> > 1. I am wondering if the "stand-by" tasks that
> catch
> > up
> > > > > state
> > > > > >> > >>> before
> > > > > >> > >>> > >> > the active task is switched deserve its own name in
> > this
> > > > > KIP
> > > > > >> and
> > > > > >> > >>> maybe
> > > > > >> > >>> > >> > in the code. We have already stated that they are
> not
> > > > true
> > > > > >> > >>> stand-by
> > > > > >> > >>> > >> > tasks, they are not configured through
> > > > > >> `num.standby.replicas`,
> > > > > >> > and
> > > > > >> > >>> > >> > maybe they have also other properties that
> > distinguish
> > > > them
> > > > > >> from
> > > > > >> > >>> true
> > > > > >> > >>> > >> > stand-by tasks of which we are not aware yet. For
> > > > example,
> > > > > >> they
> > > > > >> > >>> may be
> > > > > >> > >>> > >> > prioritized differently than other tasks.
> > Furthermore,
> > > > the
> > > > > >> name
> > > > > >> > >>> > >> > "stand-by" does not really fit with the planned
> > > > > >> functionality of
> > > > > >> > >>> those
> > > > > >> > >>> > >> > tasks. In the following, I will call them false
> > stand-by
> > > > > >> tasks.
> > > > > >> > >>> > >> >
> > > > > >> > >>> > >> > 2. Did you consider to trigger the probing
> > rebalances not
> > > > > at
> > > > > >> > >>> regular
> > > > > >> > >>> > >> > time intervals but when the false stand-by tasks
> > reach an
> > > > > >> > >>> acceptable
> > > > > >> > >>> > >> > lag? If you did consider, could you add a paragraph
> > why
> > > > you
> > > > > >> > >>> rejected
> > > > > >> > >>> > >> > this idea to the "Rejected Alternatives" section.
> > > > > >> > >>> > >> >
> > > > > >> > >>> > >> > 3. Are tasks that solely contain stores with
> disabled
> > > > > logging
> > > > > >> > >>> > >> > classified as stateful or stateless in the
> > algorithm? I
> > > > > would
> > > > > >> > >>> guess
> > > > > >> > >>> > >> > stateless, although if possible they should be
> > assigned
> > > > to
> > > > > >> the
> > > > > >> > >>> same
> > > > > >> > >>> > >> > instance they had run before the rebalance. As far
> > as I
> > > > can
> > > > > >> see
> > > > > >> > >>> this
> > > > > >> > >>> > >> > special case is not handled in the algorithm.
> > > > > >> > >>> > >> >
> > > > > >> > >>> > >> > Best,
> > > > > >> > >>> > >> > Bruno
> > > > > >> > >>> > >> >
> > > > > >> > >>> > >> >
> > > > > >> > >>> > >> >
> > > > > >> > >>> > >> > On Thu, Aug 8, 2019 at 8:24 AM Guozhang Wang <
> > > > > >> > wangg...@gmail.com>
> > > > > >> > >>> > >> wrote:
> > > > > >> > >>> > >> > >
> > > > > >> > >>> > >> > > 1. Sounds good, just wanted to clarify; and it
> may
> > > > worth
> > > > > >> > >>> documenting
> > > > > >> > >>> > >> it
> > > > > >> > >>> > >> > so
> > > > > >> > >>> > >> > > that users would not be surprised when monitoring
> > their
> > > > > >> > >>> footprint.
> > > > > >> > >>> > >> > >
> > > > > >> > >>> > >> > > 2. Hmm I see... I think the trade-off can be
> > described
> > > > as
> > > > > >> "how
> > > > > >> > >>> much
> > > > > >> > >>> > >> > > imbalance would bother you to be willing to pay
> > another
> > > > > >> > >>> rebalance,
> > > > > >> > >>> > >> along
> > > > > >> > >>> > >> > > with potentially more restoration lag", and the
> > current
> > > > > >> > >>> definition
> > > > > >> > >>> > of
> > > > > >> > >>> > >> > > rebalance_factor can be considered as a rough
> > > > measurement
> > > > > >> of
> > > > > >> > >>> that
> > > > > >> > >>> > >> > > imbalance. Of course one can argue that a finer
> > grained
> > > > > >> > >>> measurement
> > > > > >> > >>> > >> could
> > > > > >> > >>> > >> > > be "resource footprint" like CPU / storage of
> each
> > > > > instance
> > > > > >> > >>> like we
> > > > > >> > >>> > >> have
> > > > > >> > >>> > >> > in
> > > > > >> > >>> > >> > > Kafka broker auto balancing tools, but I'd prefer
> > not
> > > > > doing
> > > > > >> > >>> that as
> > > > > >> > >>> > >> part
> > > > > >> > >>> > >> > of
> > > > > >> > >>> > >> > > the library but more as an operational tool in
> the
> > > > > future.
> > > > > >> On
> > > > > >> > >>> the
> > > > > >> > >>> > >> other
> > > > > >> > >>> > >> > > hand, I've seen stateful and stateless tasks
> having
> > > > very
> > > > > >> > >>> different
> > > > > >> > >>> > >> load,
> > > > > >> > >>> > >> > > and sometimes the only bottleneck of a Streams
> app
> > is
> > > > > just
> > > > > >> one
> > > > > >> > >>> > >> stateful
> > > > > >> > >>> > >> > > sub-topology and whoever gets tasks of that
> > > > sub-topology
> > > > > >> > become
> > > > > >> > >>> > >> hotspot
> > > > > >> > >>> > >> > > (and that's why our algorithm tries to balance
> per
> > > > > >> > sub-topology
> > > > > >> > >>> as
> > > > > >> > >>> > >> well),
> > > > > >> > >>> > >> > > so maybe we can just consider stateful tasks when
> > > > > >> calculating
> > > > > >> > >>> this
> > > > > >> > >>> > >> factor
> > > > > >> > >>> > >> > > as a very brute force heuristic?
> > > > > >> > >>> > >> > >
> > > > > >> > >>> > >> > > 3.a. Thinking about this a bit more, maybe it's
> > better
> > > > > not
> > > > > >> try
> > > > > >> > >>> to
> > > > > >> > >>> > >> tackle
> > > > > >> > >>> > >> > an
> > > > > >> > >>> > >> > > unseen enemy just yet, and observe if it really
> > emerges
> > > > > >> later,
> > > > > >> > >>> and
> > > > > >> > >>> > by
> > > > > >> > >>> > >> > then
> > > > > >> > >>> > >> > > we may have other ways to not starving the
> standby
> > > > tasks,
> > > > > >> for
> > > > > >> > >>> > >> example, by
> > > > > >> > >>> > >> > > using dedicate threads for standby tasks or even
> > > > consider
> > > > > >> > having
> > > > > >> > >>> > >> higher
> > > > > >> > >>> > >> > > priority for standby than active so that we
> always
> > try
> > > > to
> > > > > >> > >>> caught up
> > > > > >> > >>> > >> > standby
> > > > > >> > >>> > >> > > first, then process active; and if active's
> lagging
> > > > > >> compared
> > > > > >> > to
> > > > > >> > >>> > >> > > log-end-offset is increasing then we should
> > increase
> > > > > >> capacity,
> > > > > >> > >>> etc
> > > > > >> > >>> > >> etc.
> > > > > >> > >>> > >> > >
> > > > > >> > >>> > >> > > 4. Actually with KIP-429 this may not be the
> case:
> > we
> > > > may
> > > > > >> not
> > > > > >> > >>> call
> > > > > >> > >>> > >> > > onPartitionsRevoked prior to rebalance any more
> so
> > > > would
> > > > > >> not
> > > > > >> > >>> transit
> > > > > >> > >>> > >> > state
> > > > > >> > >>> > >> > > to PARTITIONS_REVOKED, and hence not cause the
> > state of
> > > > > the
> > > > > >> > >>> instance
> > > > > >> > >>> > >> to
> > > > > >> > >>> > >> > be
> > > > > >> > >>> > >> > > REBALANCING. In other words, even if a instance
> is
> > > > > >> undergoing
> > > > > >> > a
> > > > > >> > >>> > >> rebalance
> > > > > >> > >>> > >> > > it's state may still be RUNNING and it may still
> be
> > > > > >> processing
> > > > > >> > >>> > >> records at
> > > > > >> > >>> > >> > > the same time.
> > > > > >> > >>> > >> > >
> > > > > >> > >>> > >> > >
> > > > > >> > >>> > >> > > On Wed, Aug 7, 2019 at 12:14 PM John Roesler <
> > > > > >> > j...@confluent.io
> > > > > >> > >>> >
> > > > > >> > >>> > >> wrote:
> > > > > >> > >>> > >> > >
> > > > > >> > >>> > >> > > > Hey Guozhang,
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > > > Thanks for the review!
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > > > 1. Yes, even with `num.standby.replicas := 0`,
> we
> > > > will
> > > > > >> still
> > > > > >> > >>> > >> > temporarily
> > > > > >> > >>> > >> > > > allocate standby tasks to accomplish a
> > no-downtime
> > > > task
> > > > > >> > >>> migration.
> > > > > >> > >>> > >> > > > Although, I'd argue that this doesn't really
> > violate
> > > > > the
> > > > > >> > >>> config,
> > > > > >> > >>> > as
> > > > > >> > >>> > >> the
> > > > > >> > >>> > >> > > > task isn't a true hot standby. As soon as it
> > catches
> > > > > up,
> > > > > >> > we'll
> > > > > >> > >>> > >> > rebalance
> > > > > >> > >>> > >> > > > again, that task will become active, and the
> > original
> > > > > >> > instance
> > > > > >> > >>> > that
> > > > > >> > >>> > >> > hosted
> > > > > >> > >>> > >> > > > the active task will no longer have the task
> > assigned
> > > > > at
> > > > > >> > all.
> > > > > >> > >>> Once
> > > > > >> > >>> > >> the
> > > > > >> > >>> > >> > > > stateDirCleaner kicks in, we'll free the disk
> > space
> > > > > from
> > > > > >> it,
> > > > > >> > >>> and
> > > > > >> > >>> > >> > return to
> > > > > >> > >>> > >> > > > the steady-state of having just one copy of the
> > task
> > > > in
> > > > > >> the
> > > > > >> > >>> > cluster.
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > > > We can of course do without this, but I feel
> the
> > > > > current
> > > > > >> > >>> proposal
> > > > > >> > >>> > is
> > > > > >> > >>> > >> > > > operationally preferable, since it doesn't make
> > > > > >> configuring
> > > > > >> > >>> > >> > hot-standbys a
> > > > > >> > >>> > >> > > > pre-requisite for fast rebalances.
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > > > 2. Yes, I think your interpretation is what we
> > > > > intended.
> > > > > >> The
> > > > > >> > >>> > default
> > > > > >> > >>> > >> > > > balance_factor would be 1, as it is implicitly
> > today.
> > > > > >> What
> > > > > >> > >>> this
> > > > > >> > >>> > >> does is
> > > > > >> > >>> > >> > > > allows operators to trade off less balanced
> > > > assignments
> > > > > >> > >>> against
> > > > > >> > >>> > >> fewer
> > > > > >> > >>> > >> > > > rebalances. If you have lots of space capacity
> in
> > > > your
> > > > > >> > >>> instances,
> > > > > >> > >>> > >> this
> > > > > >> > >>> > >> > may
> > > > > >> > >>> > >> > > > be a perfectly fine tradeoff, and you may
> prefer
> > for
> > > > > >> Streams
> > > > > >> > >>> not
> > > > > >> > >>> > to
> > > > > >> > >>> > >> > bother
> > > > > >> > >>> > >> > > > streaming GBs of data from the broker in
> pursuit
> > of
> > > > > >> perfect
> > > > > >> > >>> > balance.
> > > > > >> > >>> > >> > Not
> > > > > >> > >>> > >> > > > married to this configuration, though. It was
> > > > inspired
> > > > > by
> > > > > >> > the
> > > > > >> > >>> > >> related
> > > > > >> > >>> > >> > work
> > > > > >> > >>> > >> > > > research we did.
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > > > 3. I'll take a look
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > > > 3a. I think this is a good idea. I'd classify
> it
> > as a
> > > > > >> type
> > > > > >> > of
> > > > > >> > >>> grey
> > > > > >> > >>> > >> > failure
> > > > > >> > >>> > >> > > > detection. It may make more sense to tackle
> grey
> > > > > >> failures as
> > > > > >> > >>> part
> > > > > >> > >>> > of
> > > > > >> > >>> > >> > the
> > > > > >> > >>> > >> > > > heartbeat protocol (as I POCed here:
> > > > > >> > >>> > >> > > >
> https://github.com/apache/kafka/pull/7096/files
> > ).
> > > > > WDYT?
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > > > 4. Good catch! I didn't think about that
> before.
> > > > > Looking
> > > > > >> at
> > > > > >> > it
> > > > > >> > >>> > now,
> > > > > >> > >>> > >> > though,
> > > > > >> > >>> > >> > > > I wonder if we're actually protected already.
> The
> > > > > >> > >>> stateDirCleaner
> > > > > >> > >>> > >> > thread
> > > > > >> > >>> > >> > > > only executes if the instance is in RUNNING
> > state,
> > > > and
> > > > > >> > KIP-441
> > > > > >> > >>> > >> > proposes to
> > > > > >> > >>> > >> > > > use "probing rebalances" to report task lag.
> > Hence,
> > > > > >> during
> > > > > >> > the
> > > > > >> > >>> > >> window
> > > > > >> > >>> > >> > > > between when the instance reports a lag and the
> > > > > assignor
> > > > > >> > >>> makes a
> > > > > >> > >>> > >> > decision
> > > > > >> > >>> > >> > > > about it, the instance should remain in
> > REBALANCING
> > > > > >> state,
> > > > > >> > >>> right?
> > > > > >> > >>> > If
> > > > > >> > >>> > >> > so,
> > > > > >> > >>> > >> > > > then this should prevent the race condition. If
> > not,
> > > > > >> then we
> > > > > >> > >>> do
> > > > > >> > >>> > >> indeed
> > > > > >> > >>> > >> > need
> > > > > >> > >>> > >> > > > to do something about it.
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > > > 5. Good idea. I think that today, you can only
> > see
> > > > the
> > > > > >> > >>> consumer
> > > > > >> > >>> > lag,
> > > > > >> > >>> > >> > which
> > > > > >> > >>> > >> > > > is a poor substitute. I'll add some metrics to
> > the
> > > > > >> proposal.
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > > > Thanks again for the comments!
> > > > > >> > >>> > >> > > > -John
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > > > On Tue, Aug 6, 2019 at 4:27 PM Guozhang Wang <
> > > > > >> > >>> wangg...@gmail.com>
> > > > > >> > >>> > >> > wrote:
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > > > > Hello Sophie,
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > Thanks for the proposed KIP. I left some
> > comments
> > > > on
> > > > > >> the
> > > > > >> > >>> wiki
> > > > > >> > >>> > >> itself,
> > > > > >> > >>> > >> > > > and I
> > > > > >> > >>> > >> > > > > think I'm still not very clear on a couple or
> > > > those:
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > 1. With this proposal, does that mean with
> > > > > >> > >>> num.standby.replicas
> > > > > >> > >>> > ==
> > > > > >> > >>> > >> > 0, we
> > > > > >> > >>> > >> > > > > may sometimes still have some standby tasks
> > which
> > > > may
> > > > > >> > >>> violate
> > > > > >> > >>> > the
> > > > > >> > >>> > >> > config?
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > 2. I think I understand the rationale to
> > consider
> > > > > lags
> > > > > >> > that
> > > > > >> > >>> is
> > > > > >> > >>> > >> below
> > > > > >> > >>> > >> > the
> > > > > >> > >>> > >> > > > > specified threshold to be equal, rather than
> > still
> > > > > >> > >>> considering
> > > > > >> > >>> > >> 5000
> > > > > >> > >>> > >> > is
> > > > > >> > >>> > >> > > > > better than 5001 -- we do not want to
> > > > "over-optimize"
> > > > > >> and
> > > > > >> > >>> > >> potentially
> > > > > >> > >>> > >> > > > falls
> > > > > >> > >>> > >> > > > > into endless rebalances back and forth.
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > But I'm not clear about the rationale of the
> > second
> > > > > >> > >>> parameter of
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>>
> > constrainedBalancedAssignment(StatefulTasksToRankedCandidates,
> > > > > >> > >>> > >> > > > > balance_factor):
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > Does that mean, e.g. with balance_factor of
> 3,
> > we'd
> > > > > >> > >>> consider two
> > > > > >> > >>> > >> > > > > assignments one resulting balance_factor 0
> and
> > one
> > > > > >> > resulting
> > > > > >> > >>> > >> > > > balance_factor
> > > > > >> > >>> > >> > > > > 3 to be equally optimized assignment and
> > therefore
> > > > > may
> > > > > >> > "stop
> > > > > >> > >>> > >> early"?
> > > > > >> > >>> > >> > This
> > > > > >> > >>> > >> > > > > was not very convincing to me :P
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > 3. There are a couple of minor comments about
> > the
> > > > > >> > algorithm
> > > > > >> > >>> > >> itself,
> > > > > >> > >>> > >> > left
> > > > > >> > >>> > >> > > > on
> > > > > >> > >>> > >> > > > > the wiki page since it needs to refer to the
> > exact
> > > > > line
> > > > > >> > and
> > > > > >> > >>> > better
> > > > > >> > >>> > >> > > > > displayed there.
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > 3.a Another wild thought about the threshold
> > > > itself:
> > > > > >> today
> > > > > >> > >>> the
> > > > > >> > >>> > >> > assignment
> > > > > >> > >>> > >> > > > > itself is memoryless, so we would not know if
> > the
> > > > > >> reported
> > > > > >> > >>> > >> `TaskLag`
> > > > > >> > >>> > >> > > > itself
> > > > > >> > >>> > >> > > > > is increasing or decreasing even if the
> current
> > > > value
> > > > > >> is
> > > > > >> > >>> under
> > > > > >> > >>> > the
> > > > > >> > >>> > >> > > > > threshold. I wonder if it worthy to make it a
> > bit
> > > > > more
> > > > > >> > >>> > >> complicated to
> > > > > >> > >>> > >> > > > track
> > > > > >> > >>> > >> > > > > task lag trend at the assignor? Practically
> it
> > may
> > > > > not
> > > > > >> be
> > > > > >> > >>> very
> > > > > >> > >>> > >> > uncommon
> > > > > >> > >>> > >> > > > > that stand-by tasks are not keeping up due to
> > the
> > > > > fact
> > > > > >> > that
> > > > > >> > >>> > other
> > > > > >> > >>> > >> > active
> > > > > >> > >>> > >> > > > > tasks hosted on the same thread is starving
> the
> > > > > standby
> > > > > >> > >>> tasks.
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > 4. There's a potential race condition risk
> when
> > > > > >> reporting
> > > > > >> > >>> > >> `TaskLags`
> > > > > >> > >>> > >> > in
> > > > > >> > >>> > >> > > > the
> > > > > >> > >>> > >> > > > > subscription: right after reporting it to the
> > > > leader,
> > > > > >> the
> > > > > >> > >>> > cleanup
> > > > > >> > >>> > >> > thread
> > > > > >> > >>> > >> > > > > kicks in and deletes the state directory. If
> > the
> > > > task
> > > > > >> was
> > > > > >> > >>> > assigned
> > > > > >> > >>> > >> > to the
> > > > > >> > >>> > >> > > > > host it would cause it to restore from
> > beginning
> > > > and
> > > > > >> > >>> effectively
> > > > > >> > >>> > >> > make the
> > > > > >> > >>> > >> > > > > seemingly optimized assignment very
> > sub-optimal.
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > To be on the safer side we should consider
> > either
> > > > > prune
> > > > > >> > out
> > > > > >> > >>> > those
> > > > > >> > >>> > >> > tasks
> > > > > >> > >>> > >> > > > > that are "close to be cleaned up" in the
> > > > > subscription,
> > > > > >> or
> > > > > >> > we
> > > > > >> > >>> > >> should
> > > > > >> > >>> > >> > delay
> > > > > >> > >>> > >> > > > > the cleanup right after we've included them
> in
> > the
> > > > > >> > >>> subscription
> > > > > >> > >>> > in
> > > > > >> > >>> > >> > case
> > > > > >> > >>> > >> > > > > they are been selected as assigned tasks by
> the
> > > > > >> assignor.
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > 5. This is a meta comment: I think it would
> be
> > > > > helpful
> > > > > >> to
> > > > > >> > >>> add
> > > > > >> > >>> > some
> > > > > >> > >>> > >> > user
> > > > > >> > >>> > >> > > > > visibility on the standby tasks lagging as
> > well,
> > > > via
> > > > > >> > >>> metrics for
> > > > > >> > >>> > >> > example.
> > > > > >> > >>> > >> > > > > Today it is hard for us to observe how far
> are
> > our
> > > > > >> current
> > > > > >> > >>> > standby
> > > > > >> > >>> > >> > tasks
> > > > > >> > >>> > >> > > > > compared to the active tasks and whether that
> > lag
> > > > is
> > > > > >> being
> > > > > >> > >>> > >> > increasing or
> > > > > >> > >>> > >> > > > > decreasing. As a follow-up task, for example,
> > the
> > > > > >> > rebalance
> > > > > >> > >>> > should
> > > > > >> > >>> > >> > also
> > > > > >> > >>> > >> > > > be
> > > > > >> > >>> > >> > > > > triggered if we realize that some standby
> > task's
> > > > lag
> > > > > is
> > > > > >> > >>> > increasing
> > > > > >> > >>> > >> > > > > indefinitely means that it cannot keep up
> > (which is
> > > > > >> > another
> > > > > >> > >>> > >> indicator
> > > > > >> > >>> > >> > > > > either you need to add more resources with
> the
> > > > > >> > num.standbys
> > > > > >> > >>> or
> > > > > >> > >>> > >> your
> > > > > >> > >>> > >> > are
> > > > > >> > >>> > >> > > > > still not balanced enough).
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > On Tue, Aug 6, 2019 at 1:32 PM Sophie
> > Blee-Goldman
> > > > <
> > > > > >> > >>> > >> > sop...@confluent.io>
> > > > > >> > >>> > >> > > > > wrote:
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > > Hey all,
> > > > > >> > >>> > >> > > > > >
> > > > > >> > >>> > >> > > > > > I'd like to kick off discussion on KIP-441,
> > aimed
> > > > > at
> > > > > >> the
> > > > > >> > >>> long
> > > > > >> > >>> > >> > restore
> > > > > >> > >>> > >> > > > > times
> > > > > >> > >>> > >> > > > > > in Streams during which further active
> > processing
> > > > > >> and IQ
> > > > > >> > >>> are
> > > > > >> > >>> > >> > blocked.
> > > > > >> > >>> > >> > > > > > Please give it a read and let us know your
> > > > thoughts
> > > > > >> > >>> > >> > > > > >
> > > > > >> > >>> > >> > > > > >
> > > > > >> > >>> > >> > > > > >
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> >
> > > > > >> > >>> > >>
> > > > > >> > >>> >
> > > > > >> > >>>
> > > > > >> >
> > > > > >>
> > > > >
> > > >
> >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-441:+Smooth+Scaling+Out+for+Kafka+Streams
> > > > > >> > >>> > >> > > > > >
> > > > > >> > >>> > >> > > > > > Cheers,
> > > > > >> > >>> > >> > > > > > Sophie
> > > > > >> > >>> > >> > > > > >
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > > > --
> > > > > >> > >>> > >> > > > > -- Guozhang
> > > > > >> > >>> > >> > > > >
> > > > > >> > >>> > >> > > >
> > > > > >> > >>> > >> > >
> > > > > >> > >>> > >> > >
> > > > > >> > >>> > >> > > --
> > > > > >> > >>> > >> > > -- Guozhang
> > > > > >> > >>> > >> >
> > > > > >> > >>> > >>
> > > > > >> > >>> > >>
> > > > > >> > >>> > >> --
> > > > > >> > >>> > >> -- Guozhang
> > > > > >> > >>> > >>
> > > > > >> > >>> > >
> > > > > >> > >>> >
> > > > > >> > >>>
> > > > > >> > >>>
> > > > > >> > >>> --
> > > > > >> > >>> -- Guozhang
> > > > > >> > >>>
> > > > > >> > >>
> > > > > >> >
> > > > > >>
> > > > > >>
> > > > > >> --
> > > > > >> -- Guozhang
> > > > > >>
> > > > > >
> > > > >
> > > >
> > > >
> > > > --
> > > > -- Guozhang
> > > >
> >
> >
>
--
-- Guozhang
