Hi Richard,
Sorry for the slow reply. I actually think we should avoid checking
equals() for now. Your reasoning is good, but the truth is that
depending on the implementation of equals() is non-trivial,
semantically, and (though I proposed it before), I'm not convinced
it's worth the risk. Much better to start with exactly one kind of
"idempotence detection".
Even if someone does update their serdes, we know that the new
serde would still be able to _de_serialize the old format, or the whole
app would break. The situation is that the new result gets encoded
in the new binary format, which means we don't detect an idempotent
update for what it is. In this case, we'd write the new binary format to
disk and the changelog, and forward it downstream. However, we only
do this once. Now that the binary format for that record has been updated,
we would correctly detect idempotence of any subsequent updates.
Plus, we would still be able to filter out idempotent updates in repartition
sinks, since for those, we use the new serde to serialize both the "old" and
"new" result.
It's certainly a good observation, but I think we can just make a note of it
in "rejected alternatives" for now, and plan to refine it later, if it does
pose a big performance problem.
Thanks!
-John
On Sat, Feb 22, 2020, at 18:14, Richard Yu wrote:
> Hi all,
>
> Updated the KIP.
>
> Just a question: do you think it would be a good idea if we check for both
> Object#equals() and binary equality?
> Because there might be some subtle changes in the serialization (for
> example, if the user decides to upgrade their serialization procedure to a
> new one), but the underlying values of the result might be the same.
> (therefore equals() might return true)
>
> Do you think this would be plausible?
>
> Cheers,
> Richard
>
> On Fri, Feb 21, 2020 at 2:37 PM Richard Yu <yohan.richard...@gmail.com>
> wrote:
>
> > Hello,
> >
> > Just to make some updates. I changed the name of the metric so that it was
> > more in line with usual Kafka naming conventions for metrics / sensors.
> > Below is the updated description of the metric:
> >
> > dropped-idempotent-updates : (Level 2 - Per Task) DEBUG (rate | total)
> >
> > Description: This metric will record the number of updates that have been
> > dropped since they are essentially re-performing an earlier operation.
> >
> > Note:
> >
> > - The rate option indicates the ratio of records dropped to actual
> > volume of records passing through the task.
> > - The total option will just give a raw count of the number of records
> > dropped.
> >
> >
> > I hope that this is more on point.
> >
> > Best,
> > Richard
> >
> > On Fri, Feb 21, 2020 at 2:20 PM Richard Yu <yohan.richard...@gmail.com>
> > wrote:
> >
> >> Hi all,
> >>
> >> Thanks for the clarification. I was just confused a little on what was
> >> going on.
> >>
> >> So I guess then that for the actual proposal. We got the following:
> >>
> >> 1. We check for binary equality, and perform no extra look ups.
> >> 2. Emphasize that this applies only to materialized tables.
> >> 3. We drop aggregation updates if key, value and timestamp is the same.
> >>
> >> Then that settles the behavior changes. So it looks like the Metric that
> >> is the only thing that is left. In this case, I think the metric would be
> >> named the following: IdempotentUpdateMetric. This is mostly off the top of
> >> my head. So if you think that we change it, feel free to say so.
> >> The metric will report the number of dropped operations inherently.
> >>
> >> It will probably be added as a Sensor, similar to the dropped records
> >> sensor we already have.
> >>
> >> If there isn't anything else, I will probably start the voting process
> >> next week!
> >>
> >> Cheers,
> >> Richard
> >>
> >>
> >> On Fri, Feb 21, 2020 at 11:23 AM John Roesler <vvcep...@apache.org>
> >> wrote:
> >>
> >>> Hi Bruno,
> >>>
> >>> Thanks for the clarification. Indeed, I was thinking two things:
> >>> 1. For the initial implementation, we can just avoid adding any extra
> >>> lookups, but only do the comparison when we already happen to have
> >>> the prior value.
> >>> 2. I think, as a result of the timestamp semantics, we actually _do_ look
> >>> up the prior value approximately all the time, so the idempotence check
> >>> should be quite effective.
> >>>
> >>> I think that second point is the same thing you're referring to
> >>> potentially
> >>> being unnecessary. It does mean that we do fetch the whole value in a
> >>> lot of cases where we really only need the timestamp, so it could
> >>> certainly
> >>> be optimized in the future. In that future, we would need to weigh that
> >>> optimization against losing the idempotence check. But, that's a problem
> >>> for tomorrow :)
> >>>
> >>> I'm 100% on board with scrutinizing the performance as we implement
> >>> this feature.
> >>>
> >>> Thanks again,
> >>> -John
> >>>
> >>> On Thu, Feb 20, 2020, at 03:25, Bruno Cadonna wrote:
> >>> > Hi John,
> >>> >
> >>> > I am glad to help you with your imagination. With overhead, I mainly
> >>> > meant the additional lookup into the state store to get the current
> >>> > value, but I see now in the code that we do that lookup anyways
> >>> > (although I think we could avoid that in the cases where we do not
> >>> > need the old value). With or without config, we need to evaluate the
> >>> > performance benefits of this change, in any case.
> >>> >
> >>> > Best,
> >>> > Bruno
> >>> >
> >>> > On Wed, Feb 19, 2020 at 7:48 PM John Roesler <vvcep...@apache.org>
> >>> wrote:
> >>> > >
> >>> > > Thanks for your remarks, Bruno!
> >>> > >
> >>> > > I'm in favor of standardizing on terminology like "not forwarding
> >>> > > idempotent updates" or "dropping idempotent updates". Maybe we
> >>> > > should make a pass on the KIP and just convert everything to this
> >>> > > phrasing. In retrospect, even the term "emit-on-change" has too much
> >>> > > semantic baggage, since it implies the semantics from the SECRET
> >>> > > paper, which we don't really want to imply here.
> >>> > >
> >>> > > I'm also in favor of the metric as you propose.
> >>> > >
> >>> > > Likewise with stream aggregations, I was also under the impression
> >>> > > that we agreed on dropping idempotent updates to the aggregation
> >>> > > result, any time we find that our "new" (key, value, timestamp)
> >>> result
> >>> > > is identical to the prior one.
> >>> > >
> >>> > > Also, I'm +1 on all your recommendations for updating the KIP
> >>> document
> >>> > > for clarity.
> >>> > >
> >>> > > Regarding the opt-out config. Perhaps I'm suffering from a failure of
> >>> > > imagination, but I don't see how the current proposal could really
> >>> have
> >>> > > a measurable impact on latency. If all we do is make a single extra
> >>> pass
> >>> > > to compare two byte arrays for equality, only in the cases where we
> >>> already
> >>> > > have the byte arrays available, it seems unlikely to measurably
> >>> affect the
> >>> > > processing of non-idempotent updates. It seems guaranteed to
> >>> _decrease_
> >>> > > the latency of processing idempotent updates, since we get to skip a
> >>> > > store#put, at least one producer#send, and also all downstream
> >>> processing,
> >>> > > including all the disk and network operations associated with
> >>> downstream
> >>> > > operations.
> >>> > >
> >>> > > It seems like if we're pretty sure this change would only _help_, we
> >>> shouldn't
> >>> > > introduce the operational burden of an extra configuration. If we
> >>> want to
> >>> > > be more aggressive about dropping idempotent operations in the
> >>> future,
> >>> > > such as depending on equals() or adding a ChangeDetector interface,
> >>> then
> >>> > > we should consider adding a configuration as part of that future
> >>> work. In
> >>> > > fact, if we add a simple "opt-in/opt-out" switch right now, we might
> >>> find
> >>> > > that it's actually insufficient for whatever future feature we might
> >>> propose,
> >>> > > then we have a mess of deprecating the opt-out config and replacing
> >>> it.
> >>> > >
> >>> > > What do you think?
> >>> > > -John
> >>> > >
> >>> > > On Wed, Feb 19, 2020, at 09:50, Bruno Cadonna wrote:
> >>> > > > Hi all,
> >>> > > >
> >>> > > > Sorry for the late reply!
> >>> > > >
> >>> > > > I am also in favour of baby steps.
> >>> > > >
> >>> > > > I am undecided whether the KIP should contain a opt-out config or
> >>> not.
> >>> > > > The overhead of emit-on-change might affect latency. For
> >>> applications
> >>> > > > where low latency is crucial and there are not too many idempotent
> >>> > > > updates, it would be better to fall back to emit-on-update.
> >>> However,
> >>> > > > we do not know how much emit-on-change impacts latency. We would
> >>> first
> >>> > > > need to benchmark that before we can decide about the
> >>> opt-out-config.
> >>> > > >
> >>> > > > A metric of dropped idempotent updates seems useful to me to be
> >>> > > > informed about potential upstream applications or upstream
> >>> operators
> >>> > > > that produce too many idempotent updates. The KIP should state the
> >>> > > > name of the metric, its group, its tags, and its recording level
> >>> (see
> >>> > > > KIP-444 or KIP-471 for examples). I propose DEBUG as reporting
> >>> level.
> >>> > > >
> >>> > > > Richard, what competing proposals for emit-on-change for
> >>> aggregations
> >>> > > > do you mean? I have the feeling that we agreed to get rid of
> >>> > > > idempotent updates if the aggregate is updated with the same key,
> >>> > > > value, AND timestamp. I am also fine if we do not include this into
> >>> > > > this KIP (remember: baby steps).
> >>> > > >
> >>> > > > You write that "emit-on-change is more correct". Since we agreed
> >>> that
> >>> > > > this is an optimization, IMO you cannot argue this way.
> >>> > > >
> >>> > > > Please put "Alternative Approaches" under "Rejected Alternatives",
> >>> so
> >>> > > > that it becomes clear that we are not going to implement them. In
> >>> > > > general, I think the KIP needs a bit of clean-up (probably, you
> >>> > > > already planned for it). "Design Reasoning" is a bit of behavior
> >>> > > > changes, rejected alternatives and duplicates a bit the content in
> >>> > > > those sections.
> >>> > > >
> >>> > > > I do not like the name "no-op operations" or "no-ops", because they
> >>> > > > are rather generic. I like more "idempotent updates".
> >>> > > >
> >>> > > > Best,
> >>> > > > Bruno
> >>> > > >
> >>> > > >
> >>> > > > On Tue, Feb 18, 2020 at 7:25 PM Richard Yu <
> >>> yohan.richard...@gmail.com> wrote:
> >>> > > > >
> >>> > > > > Hi all,
> >>> > > > >
> >>> > > > > We are definitely making progress!
> >>> > > > >
> >>> > > > > @John should I emphasize in the proposed behavior changes that
> >>> we are only
> >>> > > > > doing binary equality checks for stateful operators?
> >>> > > > > It looks like we have come close to finalizing this part of the
> >>> KIP. (I
> >>> > > > > will note in the KIP that this proposal is intended for
> >>> optimization, not
> >>> > > > > semantics correctness)
> >>> > > > >
> >>> > > > > I do think maybe we still have one other detail we need to
> >>> discuss. So far,
> >>> > > > > there has been quite a bit of back and forth about what the
> >>> behavior of
> >>> > > > > aggregations should look like in emit on change. I have seen
> >>> > > > > multiple competing proposals, so I am not completely certain
> >>> which one we
> >>> > > > > should go with, or how we will be able to compromise in between
> >>> them.
> >>> > > > >
> >>> > > > > Let me know what your thoughts are on this matter, since we are
> >>> probably
> >>> > > > > close to wrapping up most other stuff.
> >>> > > > > @Matthias J. Sax <matth...@confluent.io> and @Bruno, see what
> >>> you think
> >>> > > > > about this.
> >>> > > > >
> >>> > > > > Best,
> >>> > > > > Richard
> >>> > > > >
> >>> > > > >
> >>> > > > >
> >>> > > > > On Tue, Feb 18, 2020 at 9:06 AM John Roesler <
> >>> vvcep...@apache.org> wrote:
> >>> > > > >
> >>> > > > > > Thanks, Matthias!
> >>> > > > > >
> >>> > > > > > Regarding numbers, it would be hard to know how many
> >>> applications
> >>> > > > > > would benefit, since we don't know how many applications there
> >>> are,
> >>> > > > > > or anything about their data sets or topologies. We could do a
> >>> survey,
> >>> > > > > > but it seems overkill if we take the conservative approach.
> >>> > > > > >
> >>> > > > > > I have my own practical stream processing experience that
> >>> tells me this
> >>> > > > > > is absolutely critical for any moderate-to-large relational
> >>> stream
> >>> > > > > > processing use cases. I'll leave it to you to decide if you
> >>> find that
> >>> > > > > > convincing, but it's definitely not an _assumption_. I've also
> >>> heard from
> >>> > > > > > a few Streams users who have already had to implement their own
> >>> > > > > > noop-suppression transformers in order to get to production
> >>> scale.
> >>> > > > > >
> >>> > > > > > Regardless, it sounds like we can agree on taking an
> >>> opportunistic approach
> >>> > > > > > and targeting the optimization just to use a binary-equality
> >>> check at
> >>> > > > > > stateful operators. (I'd also suggest in sink nodes, when we
> >>> are about to
> >>> > > > > > send old and new values, since they are also already present
> >>> and serialized
> >>> > > > > > at that point.) We could make the KIP even more vague, and
> >>> just say that
> >>> > > > > > we'll drop no-op updates "when possible".
> >>> > > > > >
> >>> > > > > > I'm curious what Bruno and the others think about this. If it
> >>> seems like
> >>> > > > > > a good starting point, perhaps we could move to a vote soon
> >>> and get to
> >>> > > > > > work on the implementation!
> >>> > > > > >
> >>> > > > > > Thanks,
> >>> > > > > > -John
> >>> > > > > >
> >>> > > > > > On Mon, Feb 17, 2020, at 20:54, Matthias J. Sax wrote:
> >>> > > > > > > Talking about optimizations and reducing downstream load:
> >>> > > > > > >
> >>> > > > > > > Do we actually have any numbers? I have the impression that
> >>> this KIP is
> >>> > > > > > > more or less build on the _assumption_ that there is a
> >>> problem. Yes,
> >>> > > > > > > there are some use cases that would benefit from this; But
> >>> how many
> >>> > > > > > > applications would actually benefit? And how much load
> >>> reduction would
> >>> > > > > > > they get?
> >>> > > > > > >
> >>> > > > > > > The simplest approach (following John idea to make baby
> >>> steps) would be
> >>> > > > > > > to apply the emit-on-change pattern only if there is a
> >>> store. For this
> >>> > > > > > > case we need to serialize old and new result anyway and thus
> >>> a simple
> >>> > > > > > > byte-array comparison is no overhead.
> >>> > > > > > >
> >>> > > > > > > Sending `oldValues` by default would become expensive
> >>> because we would
> >>> > > > > > > need to serialize the recomputed old result, as well as the
> >>> new result,
> >>> > > > > > > to make the comparison (and we now the serialization is not
> >>> cheap). We
> >>> > > > > > > are facing a trade-off between CPU overhead and downstream
> >>> load and I am
> >>> > > > > > > not sure if we should hard code this. My original argument
> >>> for sending
> >>> > > > > > > `oldValues` was about semantics; but for an optimization, I
> >>> am not sure
> >>> > > > > > > if this would be the right choice.
> >>> > > > > > >
> >>> > > > > > > For now, users who want to opt-in can force a
> >>> materialization. A
> >>> > > > > > > materialization may be expensive and if we see future
> >>> demand, we could
> >>> > > > > > > still add an option to send `oldValues` instead of
> >>> materialization (this
> >>> > > > > > > would at least save the store overhead). As we consider the
> >>> KIP an
> >>> > > > > > > optimization, a "config" seems to make sense.
> >>> > > > > > >
> >>> > > > > > >
> >>> > > > > > > -Matthias
> >>> > > > > > >
> >>> > > > > > >
> >>> > > > > > > On 2/17/20 5:21 PM, Richard Yu wrote:
> >>> > > > > > > > Hi John!
> >>> > > > > > > >
> >>> > > > > > > > Thanks for the reply.
> >>> > > > > > > >
> >>> > > > > > > > About the changes we have discussed so far. I think upon
> >>> further
> >>> > > > > > > > consideration, we have been mostly talking about this from
> >>> the
> >>> > > > > > perspective
> >>> > > > > > > > that no stop-gap effort is acceptable. However, in recent
> >>> discussion,
> >>> > > > > > > if we
> >>> > > > > > > > consider optimization, then it appears that the
> >>> perspective I
> >>> > > > > > mentioned no
> >>> > > > > > > > longer applies. After all, we are no longer concerned so
> >>> much about
> >>> > > > > > > > semantics correctness, then reducing traffic as much as
> >>> possible
> >>> > > > > > without
> >>> > > > > > > > performance tradeoffs.
> >>> > > > > > > >
> >>> > > > > > > > In this case, I think a cache would be a good idea for
> >>> stateless
> >>> > > > > > > > operations. This cache will not be backed by a store
> >>> obviously. We can
> >>> > > > > > > > probably use Kafka's ThreadCache. We should be able to
> >>> catch a large
> >>> > > > > > > > portion of the no-ops if we at least store some results in
> >>> the cache.
> >>> > > > > > Not
> >>> > > > > > > > all will be caught, but I think the impact will be
> >>> significant.
> >>> > > > > > > >
> >>> > > > > > > > On another note, I think that we should implement
> >>> competing proposals
> >>> > > > > > i.e.
> >>> > > > > > > > one where we forward both old and new values with a
> >>> reasonable
> >>> > > > > > proportion
> >>> > > > > > > > of artificial no-ops (we do not necessarily have to rely
> >>> on equals so
> >>> > > > > > much
> >>> > > > > > > > as comparing the serialized binary data after the
> >>> operation), and in
> >>> > > > > > > > another scenario, the cache for stateless ops. It would be
> >>> > > > > > unreasonable if
> >>> > > > > > > > we completely disregard either approach, since they both
> >>> have merit.
> >>> > > > > > The
> >>> > > > > > > > reason for implementing both is to perform benchmark tests
> >>> on them, and
> >>> > > > > > > > compare them with the original. This way, we can more
> >>> clearly see what
> >>> > > > > > is
> >>> > > > > > > > the drawbacks and the gains. So far, we have been
> >>> discussing only
> >>> > > > > > > > hypotheticals, and if we continue to do so, I think it is
> >>> likely no
> >>> > > > > > ground
> >>> > > > > > > > will be gained.
> >>> > > > > > > >
> >>> > > > > > > > After all, what we seek is optimization, and performance
> >>> benchmarks
> >>> > > > > > > will be
> >>> > > > > > > > mandatory for a KIP of this nature.
> >>> > > > > > > >
> >>> > > > > > > > Hope this helps,
> >>> > > > > > > > Richard
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > > On Mon, Feb 17, 2020 at 2:12 PM John Roesler <
> >>> vvcep...@apache.org>
> >>> > > > > > wrote:
> >>> > > > > > > >
> >>> > > > > > > >> Hi again, all,
> >>> > > > > > > >>
> >>> > > > > > > >> Sorry on my part for my silence.
> >>> > > > > > > >>
> >>> > > > > > > >> I've just taken another look over the recent history of
> >>> this
> >>> > > > > > > discussion. It
> >>> > > > > > > >> seems like the #1 point to clarify (because it affect
> >>> everything
> >>> > > > > > else) is
> >>> > > > > > > >> that,
> >>> > > > > > > >> yes, I was 100% envisioning this as an _optimization_.
> >>> > > > > > > >>
> >>> > > > > > > >> As a consequence, I don't think it's critical to make any
> >>> hard
> >>> > > > > > guarantees
> >>> > > > > > > >> about
> >>> > > > > > > >> what results get forwarded and what (no-op updates) get
> >>> dropped. I'd
> >>> > > > > > > >> initially
> >>> > > > > > > >> just been thinking about doing this opportunistically in
> >>> cases where
> >>> > > > > > we
> >>> > > > > > > >> already
> >>> > > > > > > >> had the "old" and "new" result in memory, thanks to a
> >>> request to
> >>> > > > > > > "emit old
> >>> > > > > > > >> values", or to the implementation of timestamp semantics.
> >>> > > > > > > >>
> >>> > > > > > > >> However, whether or not it's semantically critical, I do
> >>> think that
> >>> > > > > > > >> Matthias's
> >>> > > > > > > >> idea to use the change-forwarding mechanism to check for
> >>> no-ops even
> >>> > > > > > on
> >>> > > > > > > >> stateless operations is pretty interesting. Specifically,
> >>> this would
> >>> > > > > > > >> _really_
> >>> > > > > > > >> let you pare down useless updates by using mapValues to
> >>> strip down
> >>> > > > > > > records
> >>> > > > > > > >> only
> >>> > > > > > > >> to what you really need. However, the dependence on the
> >>> > > > > > implementation of
> >>> > > > > > > >> equals() is troubling.
> >>> > > > > > > >>
> >>> > > > > > > >> It might make sense to table this idea, as well as my
> >>> complicated
> >>> > > > > > no-op
> >>> > > > > > > >> detection algorithm, and initially propose just a
> >>> nonconfigurable
> >>> > > > > > feature
> >>> > > > > > > >> to
> >>> > > > > > > >> check "old" and "new" results for binary equality before
> >>> forwarding.
> >>> > > > > > > I.e.,
> >>> > > > > > > >> if
> >>> > > > > > > >> any operation determines that the old and new results are
> >>> > > > > > > >> binary-identical, we
> >>> > > > > > > >> would not forward.
> >>> > > > > > > >>
> >>> > > > > > > >> I'll admit that this doesn't serve Tommy's use case very
> >>> well, but it
> >>> > > > > > > >> might be
> >>> > > > > > > >> better to take baby steps with an optimization like this
> >>> and not risk
> >>> > > > > > > >> over-reaching in a way that actually harms performance or
> >>> > > > > > correctness. We
> >>> > > > > > > >> could
> >>> > > > > > > >> always expand the feature to use equals() or some kind of
> >>> > > > > > ChangeDetector
> >>> > > > > > > >> later
> >>> > > > > > > >> on, in a more focused discussion.
> >>> > > > > > > >>
> >>> > > > > > > >> Regarding metrics or debug logs, I guess I don't feel
> >>> strongly, but it
> >>> > > > > > > >> feels
> >>> > > > > > > >> like two things will happen that make it nicer to add
> >>> them:
> >>> > > > > > > >>
> >>> > > > > > > >> 1. This feature is going to surprise/annoy _somebody_,
> >>> and it would be
> >>> > > > > > > >> nice to
> >>> > > > > > > >> be able to definitively say the reason that updates are
> >>> dropped is
> >>> > > > > > that
> >>> > > > > > > >> they
> >>> > > > > > > >> were no-ops. The easiest smoking gun is if there are
> >>> debug-logs that
> >>> > > > > > > can be
> >>> > > > > > > >> enabled. This person might just be looking at the
> >>> dashboards,
> >>> > > > > > > wondering why
> >>> > > > > > > >> there are 100K updates per second going into their app,
> >>> but only 1K
> >>> > > > > > > >> results per
> >>> > > > > > > >> second coming out. Having the metric there makes the
> >>> accounting
> >>> > > > > > easier.
> >>> > > > > > > >>
> >>> > > > > > > >> 2. Somebody is going to struggle with high-volume
> >>> updates, and it
> >>> > > > > > > would be
> >>> > > > > > > >> nice
> >>> > > > > > > >> for them to know that this feature is saving them
> >>> X-thousand updates
> >>> > > > > > per
> >>> > > > > > > >> second,
> >>> > > > > > > >> etc.
> >>> > > > > > > >>
> >>> > > > > > > >> What does everyone think about this? Note, as I read it,
> >>> what I've
> >>> > > > > > said
> >>> > > > > > > >> above is
> >>> > > > > > > >> already reflected in the text of the KIP.
> >>> > > > > > > >>
> >>> > > > > > > >> Thanks,
> >>> > > > > > > >> -John
> >>> > > > > > > >>
> >>> > > > > > > >>
> >>> > > > > > > >> On Tue, Feb 11, 2020, at 18:27, Richard Yu wrote:
> >>> > > > > > > >>> Hi all,
> >>> > > > > > > >>>
> >>> > > > > > > >>> Bumping this. If you feel that this KIP is not too
> >>> urgent. Then let
> >>> > > > > > me
> >>> > > > > > > >>> know. :)
> >>> > > > > > > >>>
> >>> > > > > > > >>> Cheers,
> >>> > > > > > > >>> Richard
> >>> > > > > > > >>>
> >>> > > > > > > >>> On Thu, Feb 6, 2020 at 4:55 PM Richard Yu <
> >>> > > > > > yohan.richard...@gmail.com>
> >>> > > > > > > >>> wrote:
> >>> > > > > > > >>>
> >>> > > > > > > >>>> Hi all,
> >>> > > > > > > >>>>
> >>> > > > > > > >>>> I've had just a few thoughts regarding the forwarding
> >>> of <key,
> >>> > > > > > > >>>> change<old_value, new_value>>. As Matthias already
> >>> mentioned, there
> >>> > > > > > > >> are two
> >>> > > > > > > >>>> separate priorities by which we can judge this KIP:
> >>> > > > > > > >>>>
> >>> > > > > > > >>>> 1. A optimization perspective: In this case, the user
> >>> would prefer
> >>> > > > > > the
> >>> > > > > > > >>>> impact of this KIP to be as minimal as possible. By
> >>> such logic, if
> >>> > > > > > > >>>> stateless operations are performed twice, that could
> >>> prove
> >>> > > > > > > >> unacceptable for
> >>> > > > > > > >>>> them. (since operations can prove expensive)
> >>> > > > > > > >>>>
> >>> > > > > > > >>>> 2. Semantics correctness perspective: Unlike the
> >>> optimization
> >>> > > > > > > >> approach, we
> >>> > > > > > > >>>> are more concerned with all KTable operations obeying
> >>> the same
> >>> > > > > > emission
> >>> > > > > > > >>>> policy. i.e. emit on change. In this case, a
> >>> discrepancy would not
> >>> > > > > > be
> >>> > > > > > > >>>> tolerated, even though an extra performance cost will
> >>> be incurred.
> >>> > > > > > > >>>> Therefore, we will follow Matthias's approach, and then
> >>> perform the
> >>> > > > > > > >>>> operation once on the old value, and once on the new.
> >>> > > > > > > >>>>
> >>> > > > > > > >>>> The issue here I think is more black and white than in
> >>> between. The
> >>> > > > > > > >> second
> >>> > > > > > > >>>> option in particular would be favorable for users with
> >>> inexpensive
> >>> > > > > > > >>>> stateless operations, while for the former option, we
> >>> are probably
> >>> > > > > > > >> dealing
> >>> > > > > > > >>>> with more expensive ones. So the simplest solution is
> >>> probably to
> >>> > > > > > > >> allow the
> >>> > > > > > > >>>> user to choose one of the behaviors, and have a config
> >>> which can
> >>> > > > > > > >> switch in
> >>> > > > > > > >>>> between them.
> >>> > > > > > > >>>>
> >>> > > > > > > >>>> Its the simplest compromise I can come up with at the
> >>> moment, but if
> >>> > > > > > > >> you
> >>> > > > > > > >>>> think you have a better plan which could better balance
> >>> tradeoffs.
> >>> > > > > > Then
> >>> > > > > > > >>>> please let us know. :)
> >>> > > > > > > >>>>
> >>> > > > > > > >>>> Best,
> >>> > > > > > > >>>> Richard
> >>> > > > > > > >>>>
> >>> > > > > > > >>>> On Wed, Feb 5, 2020 at 5:12 PM John Roesler <
> >>> vvcep...@apache.org>
> >>> > > > > > > >> wrote:
> >>> > > > > > > >>>>
> >>> > > > > > > >>>>> Hi all,
> >>> > > > > > > >>>>>
> >>> > > > > > > >>>>> Thanks for the thoughtful comments!
> >>> > > > > > > >>>>>
> >>> > > > > > > >>>>> I need more time to reflect on your thoughts, but just
> >>> wanted to
> >>> > > > > > offer
> >>> > > > > > > >>>>> a quick clarification about equals().
> >>> > > > > > > >>>>>
> >>> > > > > > > >>>>> I only meant that we can't be sure if a class's
> >>> equals()
> >>> > > > > > > >> implementation
> >>> > > > > > > >>>>> returns true for two semantically identical instances.
> >>> I.e., if a
> >>> > > > > > > >> class
> >>> > > > > > > >>>>> doesn't
> >>> > > > > > > >>>>> override the default equals() implementation, then we
> >>> would see
> >>> > > > > > > >> behavior
> >>> > > > > > > >>>>> like:
> >>> > > > > > > >>>>>
> >>> > > > > > > >>>>> new MyPair("A", 1).equals(new MyPair("A", 1)) returns
> >>> false
> >>> > > > > > > >>>>>
> >>> > > > > > > >>>>> In that case, I would still like to catch no-op
> >>> updates by
> >>> > > > > > comparing
> >>> > > > > > > >> the
> >>> > > > > > > >>>>> serialized form of the records when we happen to have
> >>> it serialized
> >>> > > > > > > >> anyway
> >>> > > > > > > >>>>> (such as when the operation is stateful, or when we're
> >>> sending to a
> >>> > > > > > > >>>>> repartition topic and we have both the "new" and "old"
> >>> value from
> >>> > > > > > > >>>>> upstream).
> >>> > > > > > > >>>>>
> >>> > > > > > > >>>>> I didn't mean to suggest we'd try to use reflection to
> >>> detect
> >>> > > > > > whether
> >>> > > > > > > >>>>> equals
> >>> > > > > > > >>>>> is implemented, although that is a neat trick. I was
> >>> thinking more
> >>> > > > > > of
> >>> > > > > > > >> a
> >>> > > > > > > >>>>> belt-and-suspenders algorithm where we do the check
> >>> for no-ops
> >>> > > > > > based
> >>> > > > > > > >> on
> >>> > > > > > > >>>>> equals() and then _also_ check the serialized bytes
> >>> for equality.
> >>> > > > > > > >>>>>
> >>> > > > > > > >>>>> Thanks,
> >>> > > > > > > >>>>> -John
> >>> > > > > > > >>>>>
> >>> > > > > > > >>>>> On Wed, Feb 5, 2020, at 15:31, Ted Yu wrote:
> >>> > > > > > > >>>>>> Thanks for the comments, Matthias.
> >>> > > > > > > >>>>>>
> >>> > > > > > > >>>>>> w.r.t. requirement of an `equals()` implementation,
> >>> each template
> >>> > > > > > > >> type
> >>> > > > > > > >>>>>> would have an equals() method. We can use the
> >>> following code to
> >>> > > > > > know
> >>> > > > > > > >>>>>> whether it is provided by JVM or provided by user.
> >>> > > > > > > >>>>>>
> >>> > > > > > > >>>>>> boolean customEquals = false;
> >>> > > > > > > >>>>>> try {
> >>> > > > > > > >>>>>> Class cls = value.getClass().getMethod("equals",
> >>> > > > > > > >>>>>> Object.class).getDeclaringClass();
> >>> > > > > > > >>>>>> if (!Object.class.equals(cls)) {
> >>> > > > > > > >>>>>> customEquals = true;
> >>> > > > > > > >>>>>> }
> >>> > > > > > > >>>>>> } catch (NoSuchMethodException nsme) {
> >>> > > > > > > >>>>>> // equals is always defined, this wouldn't hit
> >>> > > > > > > >>>>>> }
> >>> > > > > > > >>>>>>
> >>> > > > > > > >>>>>> The next question is: what if the user doesn't
> >>> provide equals()
> >>> > > > > > > >> method ?
> >>> > > > > > > >>>>>> Would we automatically fall back to emit-on-update ?
> >>> > > > > > > >>>>>>
> >>> > > > > > > >>>>>> Cheers
> >>> > > > > > > >>>>>>
> >>> > > > > > > >>>>>> On Tue, Feb 4, 2020 at 1:37 PM Matthias J. Sax <
> >>> mj...@apache.org>
> >>> > > > > > > >>>>> wrote:
> >>> > > > > > > >>>>>>
> >>> > > > > > > > First a high level comment:
> >>> > > > > > > >
> >>> > > > > > > > Overall, I would like to make one step back, and make sure
> >>> we are
> >>> > > > > > > > discussion on the same level. Originally, I understood
> >>> this KIP
> >>> > > > > > > >>> as a
> >>> > > > > > > > proposed change of _semantics_, however, given the latest
> >>> > > > > > > >>> discussion
> >>> > > > > > > > it seems it's actually not -- it's more an _optimization_
> >>> > > > > > > >>> proposal.
> >>> > > > > > > > Hence, we only need to make sure that this optimization
> >>> does not
> >>> > > > > > > >>> break
> >>> > > > > > > > existing semantics. It this the right way to think about
> >>> it?
> >>> > > > > > > >
> >>> > > > > > > > If yes, than it might actually be ok to have different
> >>> behavior
> >>> > > > > > > > depending if there is a materialized KTable or not. So
> >>> far, we
> >>> > > > > > > >>> never
> >>> > > > > > > > defined a public contract about our emit strategy and it
> >>> seems
> >>> > > > > > > >>> this
> >>> > > > > > > > KIP does not define one either.
> >>> > > > > > > >
> >>> > > > > > > > Hence, I don't have as strong of an opinion about sending
> >>> > > > > > > >>> oldValues
> >>> > > > > > > > for example any longer. I guess the question is really,
> >>> what can
> >>> > > > > > > >>> we
> >>> > > > > > > > implement in a reasonable way.
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > > Other comments:
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > > @Richard:
> >>> > > > > > > >
> >>> > > > > > > > Can you please add the KIP to the KIP overview table: It's
> >>> missing
> >>> > > > > > > > (
> >>> > > > > > > >>>>>>
> >>> > > > > > > >>>
> >>> > > > > >
> >>> https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Improvement+Pro
> >>> > > > > > > > posals).
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > > @Bruno:
> >>> > > > > > > >
> >>> > > > > > > > You mentioned caching. I think it's irrelevant
> >>> (orthogonal) and
> >>> > > > > > > >>> we can
> >>> > > > > > > > discuss this KIP without considering it.
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > > @John:
> >>> > > > > > > >
> >>> > > > > > > >>>>>>>>> Even in the source table, we forward the updated
> >>> record with
> >>> > > > > > the
> >>> > > > > > > >>>>>>>>> higher of the two timestamps. So the example is
> >>> more like:
> >>> > > > > > > >
> >>> > > > > > > > That is not correct. Currently, we forward with the smaller
> >>> > > > > > > > out-of-order timestamp (changing the timestamp would
> >>> corrupt the
> >>> > > > > > > >>> data
> >>> > > > > > > > -- we don't know, because we don't check, if the value is
> >>> the
> >>> > > > > > > >>> same
> >>> > > > > > > >>>>>> or
> >>> > > > > > > > a different one, hence, we must emit the out-of-order
> >>> record
> >>> > > > > > > >>> as-is).
> >>> > > > > > > >
> >>> > > > > > > > If we start to do emit-on-change, we also need to emit a
> >>> new
> >>> > > > > > > >>> record if
> >>> > > > > > > > the timestamp changes due to out-of-order data, hence, we
> >>> would
> >>> > > > > > > >>> still
> >>> > > > > > > > need to emit <K,V,T1> because that give us correct
> >>> semantics:
> >>> > > > > > > >>> assume
> >>> > > > > > > > you have a filter() and afterward use the filter KTable in
> >>> a
> >>> > > > > > > > stream-table join -- the lower T1 timestamp must be
> >>> propagated to
> >>> > > > > > > >>> the
> >>> > > > > > > > filtered KTable to ensure that that the stream-table join
> >>> compute
> >>> > > > > > > >>> the
> >>> > > > > > > > correct result.
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > > Your point about requiring an `equals()` implementation is
> >>> > > > > > > >>> actually a
> >>> > > > > > > > quite interesting one and boils down to my statement from
> >>> above
> >>> > > > > > > >>> about
> >>> > > > > > > > "what can we actually implement". What I don't understand
> >>> is:
> >>> > > > > > > >
> >>> > > > > > > >>>>>>>>> This way, we still don't have to rely on the
> >>> existence of an
> >>> > > > > > > >>>>>>>>> equals() method, but if it is there, we can
> >>> benefit from it.
> >>> > > > > > > >
> >>> > > > > > > > Your bullet point (2) says it uses `equals()` -- hence, it
> >>> seems
> >>> > > > > > > >>> we
> >>> > > > > > > > actually to rely on it? Also, how can we detect if there
> >>> is an
> >>> > > > > > > > `equals()` method to do the comparison? Would be fail if
> >>> we don't
> >>> > > > > > > >>> have
> >>> > > > > > > > `equals()` nor corresponding serializes to do the
> >>> comparison?
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > >>>>>>>>> Wow, really good catch! Yes, we absolutely need
> >>> metrics and
> >>> > > > > > > >>> logs if
> >>> > > > > > > >>>>>>>>> we're going to drop any records. And, yes, we
> >>> should propose
> >>> > > > > > > >>>>>>>>> metrics and logs that are similar to the existing
> >>> ones when we
> >>> > > > > > > >>> drop
> >>> > > > > > > >>>>>>>>> records for other reasons.
> >>> > > > > > > >
> >>> > > > > > > > I am not sure about this point. In fact, we have already
> >>> some
> >>> > > > > > > >>> no-ops
> >>> > > > > > > > in Kafka Streams in our join-operators and don't report
> >>> any of
> >>> > > > > > > >>> those
> >>> > > > > > > > either. Emit-on-change is operator semantics and I don't
> >>> see why
> >>> > > > > > > >>> we
> >>> > > > > > > > would need to have a metric for it? It seems to be quite
> >>> different
> >>> > > > > > > > compared to dropping late or malformed records.
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > > -Matthias
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > >
> >>> > > > > > > > On 2/4/20 7:13 AM, Thomas Becker wrote:
> >>> > > > > > > >>>>>>>>> Thanks John for your thoughtful reply. Some
> >>> comments inline.
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> On Mon, 2020-02-03 at 11:51 -0600, John Roesler
> >>> wrote:
> >>> > > > > > > >>>>>>>>>> [EXTERNAL EMAIL] Attention: This email was sent
> >>> from outside
> >>> > > > > > > >>>>>>>>>> TiVo. DO NOT CLICK any links or attachments
> >>> unless you
> >>> > > > > > expected
> >>> > > > > > > >>>>>>>>>> them. ________________________________
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> Hi Tommy,
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> Thanks for the context. I can see the attraction
> >>> of
> >>> > > > > > considering
> >>> > > > > > > >>>>>>>>>> these use cases together.
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> To answer your question, if a part of the record
> >>> is not
> >>> > > > > > > >>> relevant
> >>> > > > > > > >>>>>>>>>> to downstream consumers, I was thinking you could
> >>> just use a
> >>> > > > > > > >>>>>>>>>> mapValue to remove it.
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> E.g., suppose you wanted to do a join between two
> >>> tables.
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> employeeInfo.join( employeePayroll, (info,
> >>> payroll) -> new
> >>> > > > > > > >>>>>>>>>> Result(info.name(), payroll.salary()) )
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> We only care about one attribute from the Info
> >>> table (name),
> >>> > > > > > > >>> and
> >>> > > > > > > >>>>>>>>>> one from the Payroll table (salary), and these
> >>> attributes
> >>> > > > > > > >>> change
> >>> > > > > > > >>>>>>>>>> rarely. On the other hand, there might be many
> >>> other
> >>> > > > > > attributes
> >>> > > > > > > >>>>>>>>>> that change frequently of these tables. We can
> >>> avoid
> >>> > > > > > triggering
> >>> > > > > > > >>>>>>>>>> the join unnecessarily by mapping the input
> >>> tables to drop the
> >>> > > > > > > >>>>>>>>>> unnecessary information before the join:
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> names = employeeInfo.mapValues(info -> info.name())
> >>> salaries
> >>> > > > > > =
> >>> > > > > > > >>>>>>>>>> employeePayroll.mapValues(payroll ->
> >>> payroll.salary())
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> names.join( salaries, (name, salary) -> new
> >>> Result(name,
> >>> > > > > > > >>> salary)
> >>> > > > > > > >>>>>>>>>> )
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> Ahh yes I see. This works, but in the case where
> >>> you're using
> >>> > > > > > > >>>>>>>>> schemas as we are (e.g. Avro), it seems like this
> >>> approach
> >>> > > > > > could
> >>> > > > > > > >>>>>>>>> lead to a proliferation of "skinny" record types
> >>> that just drop
> >>> > > > > > > >>>>>>>>> various fields.
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> Especially if we take Matthias's idea to drop
> >>> non-changes even
> >>> > > > > > > >>>>>>>>>> for stateless operations, this would be quite
> >>> efficient and is
> >>> > > > > > > >>>>>>>>>> also a very straightforward optimization to
> >>> understand once
> >>> > > > > > you
> >>> > > > > > > >>>>>>>>>> know that Streams provides emit-on-change.
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> From the context that you provided, it seems like
> >>> a slightly
> >>> > > > > > > >>>>>>>>>> different situation, though. Reading between the
> >>> lines a
> >>> > > > > > > >>> little,
> >>> > > > > > > >>>>>>>>>> it sounds like: in contrast to the example above,
> >>> in which we
> >>> > > > > > > >>> are
> >>> > > > > > > >>>>>>>>>> filtering out extra _data_, you have some extra
> >>> _metadata_
> >>> > > > > > that
> >>> > > > > > > >>>>>>>>>> you still wish to pass down with the data when
> >>> there is a
> >>> > > > > > > >>> "real"
> >>> > > > > > > >>>>>>>>>> update, but you don't want the metadata itself to
> >>> cause an
> >>> > > > > > > >>>>>>>>>> update.
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> Despite my lack of clarity, yes you've got it
> >>> right ;) This
> >>> > > > > > > >>>>>>>>> particular processor is the first stop for this
> >>> data after
> >>> > > > > > > >>> coming
> >>> > > > > > > >>>>>>>>> in from external users, who often simply post the
> >>> same content
> >>> > > > > > > >>> each
> >>> > > > > > > >>>>>>>>> time and we're trying to shield downstream
> >>> consumers from
> >>> > > > > > > >>>>>>>>> unnecessary churn.
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> It does seem handy to be able to plug in a custom
> >>> > > > > > > >>> ChangeDetector
> >>> > > > > > > >>>>>>>>>> for this purpose, but I worry about the API
> >>> complexity. Maybe
> >>> > > > > > > >>> you
> >>> > > > > > > >>>>>>>>>> can help think though how to provide the same
> >>> benefit while
> >>> > > > > > > >>>>>>>>>> limiting user-facing complexity.
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> Here's some extra context to consider:
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> We currently don't make any extra requirements
> >>> about the
> >>> > > > > > nature
> >>> > > > > > > >>>>>>>>>> of data that you can use in Streams. For example,
> >>> you don't
> >>> > > > > > > >>> have
> >>> > > > > > > >>>>>>>>>> to implement hashCode and equals, or compareTo,
> >>> etc. With the
> >>> > > > > > > >>>>>>>>>> current proposal, we can do an airtight
> >>> comparison based only
> >>> > > > > > > >>> on
> >>> > > > > > > >>>>>>>>>> the serialized form of the values, and we
> >>> actually don't have
> >>> > > > > > > >>> to
> >>> > > > > > > >>>>>>>>>> deserialize the "prior" value at all for a large
> >>> number of
> >>> > > > > > > >>>>>>>>>> operations. Admitedly, if we extend the proposal
> >>> to include
> >>> > > > > > > >>> no-op
> >>> > > > > > > >>>>>>>>>> detection for stateless operations, we'd probably
> >>> need to rely
> >>> > > > > > > >>> on
> >>> > > > > > > >>>>>>>>>> equals() for no-op checking, otherwise we'd wind
> >>> up requiring
> >>> > > > > > > >>>>>>>>>> serdes for stateless operations as well.
> >>> Actually, I'd
> >>> > > > > > probably
> >>> > > > > > > >>>>>>>>>> argue for doing exactly that:
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> 1. In stateful operations, drop if the serialized
> >>> byte[]s are
> >>> > > > > > > >>> the
> >>> > > > > > > >>>>>>>>>> same. After deserializing, also drop if the
> >>> objects are equal
> >>> > > > > > > >>>>>>>>>> according to Object#equals().
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> 2. In stateless operations, compare the "new" and
> >>> "old" values
> >>> > > > > > > >>>>>>>>>> (if "old" is available) based on Object#equals().
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> 3. As a final optimization, after serializing and
> >>> before
> >>> > > > > > > >>> sending
> >>> > > > > > > >>>>>>>>>> repartition records, compare the serialized data
> >>> and drop
> >>> > > > > > > >>>>>>>>>> no-ops.
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> This way, we still don't have to rely on the
> >>> existence of an
> >>> > > > > > > >>>>>>>>>> equals() method, but if it is there, we can
> >>> benefit from it.
> >>> > > > > > > >>>>>>>>>> Also, we don't require a serde in any new
> >>> situations, but we
> >>> > > > > > > >>> can
> >>> > > > > > > >>>>>>>>>> still leverage it when it is available.
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> For clarity, in my example above, even if the
> >>> employeeInfo and
> >>> > > > > > > >>>>>>>>>> employeePayroll and Result records all have
> >>> serdes, we need
> >>> > > > > > the
> >>> > > > > > > >>>>>>>>>> "name" field (presumably String) and the "salary"
> >>> field
> >>> > > > > > > >>>>>>>>>> (presumable a Double) to have serdes as well in
> >>> the naive
> >>> > > > > > > >>>>>>>>>> implementation. But if we can leverage equals(),
> >>> then the
> >>> > > > > > > >>> "right
> >>> > > > > > > >>>>>>>>>> thing" happens automatically.
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> I still don't totally follow why the individual
> >>> components
> >>> > > > > > > >>> (name,
> >>> > > > > > > >>>>>>>>> salary) would have to have serdes here. If Result
> >>> has one, we
> >>> > > > > > > >>>>>>>>> compare bytes, and if Result additionally has an
> >>> equals()
> >>> > > > > > method
> >>> > > > > > > >>>>>>>>> (which presumably includes equals comparisons on
> >>> the
> >>> > > > > > constituent
> >>> > > > > > > >>>>>>>>> fields), have we not covered our bases?
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> This dovetails in with my primary UX concern;
> >>> where would the
> >>> > > > > > > >>>>>>>>>> ChangeDetector actually be registered? None of
> >>> the operators
> >>> > > > > > in
> >>> > > > > > > >>>>>>>>>> my example have names or topics or any other
> >>> identifiable
> >>> > > > > > > >>>>>>>>>> characteristic that could be passed to a
> >>> ChangeDetector class
> >>> > > > > > > >>>>>>>>>> registered via config. You could say that we make
> >>> > > > > > > >>> ChangeDetector
> >>> > > > > > > >>>>>>>>>> an optional parameter to every operation in
> >>> Streams, but this
> >>> > > > > > > >>>>>>>>>> seems to carry quite a bit of mental burden with
> >>> it. People
> >>> > > > > > > >>> will
> >>> > > > > > > >>>>>>>>>> wonder what it's for and whether or not they
> >>> should be using
> >>> > > > > > > >>> it.
> >>> > > > > > > >>>>>>>>>> There would almost certainly be a misconception
> >>> that it's
> >>> > > > > > > >>>>>>>>>> preferable to implement it always, which would be
> >>> unfortunate.
> >>> > > > > > > >>>>>>>>>> Plus, to actually implment metadata flowing
> >>> through the
> >>> > > > > > > >>> topology
> >>> > > > > > > >>>>>>>>>> as in your use case, you'd have to do two things:
> >>> 1. make sure
> >>> > > > > > > >>>>>>>>>> that all operations actually preserve the
> >>> metadata alongside
> >>> > > > > > > >>> the
> >>> > > > > > > >>>>>>>>>> data (e.g., don't accidentally add a mapValues
> >>> like I did, or
> >>> > > > > > > >>> you
> >>> > > > > > > >>>>>>>>>> drop the metadata). 2. implement a ChangeDetector
> >>> for every
> >>> > > > > > > >>>>>>>>>> single operation in the topology, or you don't
> >>> get the benefit
> >>> > > > > > > >>> of
> >>> > > > > > > >>>>>>>>>> dropping non-changes internally 2b.
> >>> Alternatively, you could
> >>> > > > > > > >>> just
> >>> > > > > > > >>>>>>>>>> add the ChangeDetector to one operation toward
> >>> the end of the
> >>> > > > > > > >>>>>>>>>> topology. This would not drop redundant
> >>> computation
> >>> > > > > > internally,
> >>> > > > > > > >>>>>>>>>> but only drop redundant _outputs_. But this is
> >>> just about the
> >>> > > > > > > >>>>>>>>>> same as your current solution.
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> I definitely see your point regarding
> >>> configuration. I was
> >>> > > > > > > >>>>>>>>> originally thinking about this when the
> >>> deduplication was going
> >>> > > > > > > >>> to
> >>> > > > > > > >>>>>>>>> be opt-in, and it seemed very natural to say
> >>> something like:
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> employeeInfo.join(employeePayroll, (info, payroll)
> >>> -> new
> >>> > > > > > > >>>>>>>>> Result(info.name(), payroll.salary()))
> >>> > > > > > > >>>>>>>>>
> >>> .suppress(duplicatesAccordingTo(someChangeDetector))
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> Alternatively you can imagine a similar method
> >>> being on
> >>> > > > > > > >>>>>>>>> Materialized, though obviously this makes less
> >>> sense if we
> >>> > > > > > don't
> >>> > > > > > > >>>>>>>>> want to require materialization. If we're now
> >>> talking about
> >>> > > > > > > >>>>>>>>> changing the default behavior and not having any
> >>> configuration
> >>> > > > > > > >>>>>>>>> options, it's harder to find a place for this.
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>>> A final thought; if it really is a metadata
> >>> question, can we
> >>> > > > > > > >>> just
> >>> > > > > > > >>>>>>>>>> plan to finish up the support for headers in
> >>> Streams? I.e.,
> >>> > > > > > > >>> give
> >>> > > > > > > >>>>>>>>>> you a way to control the way that headers flow
> >>> through the
> >>> > > > > > > >>>>>>>>>> topology? Then, we could treat headers the same
> >>> way we treat
> >>> > > > > > > >>>>>>>>>> timestamps in the no-op checking... We completely
> >>> ignore them
> >>> > > > > > > >>>>>>>>>> for the sake of comparison. Thus, neither the
> >>> timestamp nor
> >>> > > > > > the
> >>> > > > > > > >>>>>>>>>> headers would get updated in internal state or in
> >>> downstream
> >>> > > > > > > >>>>>>>>>> views as long as the value itself doesn't change.
> >>> This seems
> >>> > > > > > to
> >>> > > > > > > >>>>>>>>>> give us a way to support your use case without
> >>> adding to the
> >>> > > > > > > >>>>>>>>>> mental overhead of using Streams for simple
> >>> things.
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> Agree headers could be a decent fit for this
> >>> particular case
> >>> > > > > > > >>>>>>>>> because it's mostly metadata, though to be honest
> >>> we haven't
> >>> > > > > > > >>> looked
> >>> > > > > > > >>>>>>>>> at headers much (mostly because, and to your
> >>> point, support
> >>> > > > > > > >>> seems
> >>> > > > > > > >>>>>>>>> to be lacking). I feel like there would be other
> >>> cases where
> >>> > > > > > > >>> this
> >>> > > > > > > >>>>>>>>> feature could be valuable, but I admit I can't
> >>> come up with
> >>> > > > > > > >>>>>>>>> anything right this second. Perhaps yuzhihong had
> >>> an example in
> >>> > > > > > > >>>>>>>>> mind?
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> I.e., simple things should be easy, and complex
> >>> things should
> >>> > > > > > > >>> be
> >>> > > > > > > >>>>>>>>>> possible.
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> What are your thoughts? Thanks, -John
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> On Mon, Feb 3, 2020, at 07:19, Thomas Becker
> >>> wrote:
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> Hi John, Can you describe how you'd use
> >>> filtering/mapping to
> >>> > > > > > > >>>>>>>>>> deduplicate records? To give some background on
> >>> my suggestion
> >>> > > > > > > >>> we
> >>> > > > > > > >>>>>>>>>> currently have a small stream processor that
> >>> exists solely to
> >>> > > > > > > >>>>>>>>>> deduplicate, which we do using a process that I
> >>> assume would
> >>> > > > > > be
> >>> > > > > > > >>>>>>>>>> similar to what would be done here (with a store
> >>> of keys and
> >>> > > > > > > >>> hash
> >>> > > > > > > >>>>>>>>>> values). But the records we are deduplicating
> >>> have some
> >>> > > > > > > >>> metadata
> >>> > > > > > > >>>>>>>>>> fields (such as timestamps of when the record was
> >>> posted) that
> >>> > > > > > > >>> we
> >>> > > > > > > >>>>>>>>>> don't consider semantically meaningful for
> >>> downstream
> >>> > > > > > > >>> consumers,
> >>> > > > > > > >>>>>>>>>> and therefore we also suppress updates that only
> >>> touch those
> >>> > > > > > > >>>>>>>>>> fields.
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> -Tommy
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> On Fri, 2020-01-31 at 19:30 -0600, John Roesler
> >>> wrote:
> >>> > > > > > > >>> [EXTERNAL
> >>> > > > > > > >>>>>>>>>> EMAIL] Attention: This email was sent from
> >>> outside TiVo. DO
> >>> > > > > > NOT
> >>> > > > > > > >>>>>>>>>> CLICK any links or attachments unless you
> >>> expected them.
> >>> > > > > > > >>>>>>>>>> ________________________________
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> Hi Thomas and yuzhihong, That’s an interesting
> >>> idea. Can you
> >>> > > > > > > >>> help
> >>> > > > > > > >>>>>>>>>> think of a use case that isn’t also served by
> >>> filtering or
> >>> > > > > > > >>>>>>>>>> mapping beforehand? Thanks for helping to design
> >>> this feature!
> >>> > > > > > > >>>>>>>>>> -John On Fri, Jan 31, 2020, at 18:56,
> >>> yuzhih...@gmail.com
> >>> > > > > > > >>>>>>>>>> <mailto:yuzhih...@gmail.com> wrote: I think this
> >>> is good
> >>> > > > > > > >>> idea. On
> >>> > > > > > > >>>>>>>>>> Jan 31, 2020, at 4:49 PM, Thomas Becker <
> >>> > > > > > > >>> thomas.bec...@tivo.com
> >>> > > > > > > >>>>>>>>>> <mailto:thomas.bec...@tivo.com>> wrote: How do
> >>> folks feel
> >>> > > > > > > >>> about
> >>> > > > > > > >>>>>>>>>> allowing the mechanism by which no-ops are
> >>> detected to be
> >>> > > > > > > >>>>>>>>>> pluggable? Meaning use something like a hash by
> >>> default, but
> >>> > > > > > > >>> you
> >>> > > > > > > >>>>>>>>>> could optionally provide an implementation of
> >>> something to use
> >>> > > > > > > >>>>>>>>>> instead, like a ChangeDetector. This could be
> >>> useful for
> >>> > > > > > > >>> example
> >>> > > > > > > >>>>>>>>>> to ignore changes to certain fields, which may
> >>> not be relevant
> >>> > > > > > > >>> to
> >>> > > > > > > >>>>>>>>>> the operation being performed.
> >>> > > > > > ________________________________
> >>> > > > > > > >>>>>>>>>> From: John Roesler <vvcep...@apache.org
> >>> > > > > > > >>>>>>>>>> <mailto:vvcep...@apache.org>> Sent: Friday,
> >>> January 31, 2020
> >>> > > > > > > >>> 4:51
> >>> > > > > > > >>>>>>>>>> PM To: dev@kafka.apache.org <mailto:
> >>> dev@kafka.apache.org>
> >>> > > > > > > >>>>>>>>>> <dev@kafka.apache.org <mailto:
> >>> dev@kafka.apache.org>> Subject:
> >>> > > > > > > >>> Re:
> >>> > > > > > > >>>>>>>>>> [KAFKA-557] Add emit on change support for Kafka
> >>> Streams
> >>> > > > > > > >>>>>>>>>> [EXTERNAL EMAIL] Attention: This email was sent
> >>> from outside
> >>> > > > > > > >>>>>>>>>> TiVo. DO NOT CLICK any links or attachments
> >>> unless you
> >>> > > > > > expected
> >>> > > > > > > >>>>>>>>>> them. ________________________________
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> Hello all, Sorry for my silence. It seems like we
> >>> are getting
> >>> > > > > > > >>>>>>>>>> close to consensus. Hopefully, we could move to a
> >>> vote soon!
> >>> > > > > > > >>> All
> >>> > > > > > > >>>>>>>>>> of the reasoning from Matthias and Bruno around
> >>> timestamp is
> >>> > > > > > > >>>>>>>>>> compelling. I would be strongly in favor of
> >>> stating a few
> >>> > > > > > > >>> things
> >>> > > > > > > >>>>>>>>>> very clearly in the KIP: 1. Streams will drop
> >>> no-op updates
> >>> > > > > > > >>> only
> >>> > > > > > > >>>>>>>>>> for KTable operations. That is, we won't make any
> >>> changes to
> >>> > > > > > > >>>>>>>>>> KStream aggregations at the moment. It does seem
> >>> like we can
> >>> > > > > > > >>>>>>>>>> potentially revisit the time semantics of that
> >>> operation in
> >>> > > > > > the
> >>> > > > > > > >>>>>>>>>> future, but we don't need to do it now. On the
> >>> other hand, the
> >>> > > > > > > >>>>>>>>>> proposed semantics for KTable timestamps (marking
> >>> the
> >>> > > > > > beginning
> >>> > > > > > > >>>>>>>>>> of the validity of that record) makes sense to
> >>> me. 2. Streams
> >>> > > > > > > >>>>>>>>>> will only drop no-op updates for _stateful_
> >>> KTable operations.
> >>> > > > > > > >>> We
> >>> > > > > > > >>>>>>>>>> don't want to add a hard guarantee that Streams
> >>> will _never_
> >>> > > > > > > >>>>>>>>>> emit a no-op table update because it would
> >>> require adding
> >>> > > > > > state
> >>> > > > > > > >>>>>>>>>> to otherwise stateless operations. If someone is
> >>> really
> >>> > > > > > > >>> concerned
> >>> > > > > > > >>>>>>>>>> about a particular stateless operation producing
> >>> a lot of
> >>> > > > > > no-op
> >>> > > > > > > >>>>>>>>>> results, all they have to do is materialize it,
> >>> and Streams
> >>> > > > > > > >>> would
> >>> > > > > > > >>>>>>>>>> automatically drop the no-ops. Additionally, I'm
> >>> +1 on not
> >>> > > > > > > >>> adding
> >>> > > > > > > >>>>>>>>>> an opt-out at this time. Regarding the KIP
> >>> itself, I would
> >>> > > > > > > >>> clean
> >>> > > > > > > >>>>>>>>>> it up a bit before calling for a vote. There is a
> >>> lot of
> >>> > > > > > > >>>>>>>>>> "discussion"-type language there, which is very
> >>> natural to
> >>> > > > > > > >>> read,
> >>> > > > > > > >>>>>>>>>> but makes it a bit hard to see what _exactly_ the
> >>> kip is
> >>> > > > > > > >>>>>>>>>> proposing. Richard, would you mind just making
> >>> the "proposed
> >>> > > > > > > >>>>>>>>>> behavior change" a simple and succinct list of
> >>> bullet points?
> >>> > > > > > > >>>>>>>>>> I.e., please drop glue phrases like "there has
> >>> been some
> >>> > > > > > > >>>>>>>>>> discussion" or "possibly we could do X". For the
> >>> final version
> >>> > > > > > > >>> of
> >>> > > > > > > >>>>>>>>>> the KIP, it should just say, "Streams will do X,
> >>> Streams will
> >>> > > > > > > >>> do
> >>> > > > > > > >>>>>>>>>> Y". Feel free to add an elaboration section to
> >>> explain more
> >>> > > > > > > >>> about
> >>> > > > > > > >>>>>>>>>> what X and Y mean, but we don't need to talk about
> >>> > > > > > > >>> possibilities
> >>> > > > > > > >>>>>>>>>> or alternatives except in the "rejected
> >>> alternatives" section.
> >>> > > > > > > >>>>>>>>>> Accordingly, can you also move the options you
> >>> presented in
> >>> > > > > > the
> >>> > > > > > > >>>>>>>>>> intro to the "rejected alternatives" section and
> >>> only mention
> >>> > > > > > > >>> the
> >>> > > > > > > >>>>>>>>>> final proposal itself? This just really helps
> >>> reviewers to
> >>> > > > > > know
> >>> > > > > > > >>>>>>>>>> what they are voting for, and it helps everyone
> >>> after the fact
> >>> > > > > > > >>>>>>>>>> when they are trying to get clarity on what
> >>> exactly the
> >>> > > > > > > >>> proposal
> >>> > > > > > > >>>>>>>>>> is, versus all the things it could have been.
> >>> Thanks, -John
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> On Mon, Jan 27, 2020, at 18:14, Richard Yu wrote:
> >>> Hello to
> >>> > > > > > all,
> >>> > > > > > > >>>>>>>>>> I've finished making some initial modifications
> >>> to the KIP. I
> >>> > > > > > > >>>>>>>>>> have decided to keep the implementation section
> >>> in the KIP for
> >>> > > > > > > >>>>>>>>>> record-keeping purposes. For now, we should focus
> >>> on only the
> >>> > > > > > > >>>>>>>>>> proposed behavior changes instead. See if you
> >>> have any
> >>> > > > > > > >>> comments!
> >>> > > > > > > >>>>>>>>>> Cheers, Richard On Sat, Jan 25, 2020 at 11:12 AM
> >>> Richard Yu
> >>> > > > > > > >>>>>>>>>> <yohan.richard...@gmail.com <mailto:
> >>> > > > > > yohan.richard...@gmail.com
> >>> > > > > > > >>>>>
> >>> > > > > > > >>>>>>>>>> wrote: Hi all, Thanks for all the discussion!
> >>> @John and @Bruno
> >>> > > > > > > >>> I
> >>> > > > > > > >>>>>>>>>> will survey other possible systems and see what I
> >>> can do. Just
> >>> > > > > > > >>> a
> >>> > > > > > > >>>>>>>>>> question, by systems, I suppose you would mean
> >>> the pros and
> >>> > > > > > > >>> cons
> >>> > > > > > > >>>>>>>>>> of different reporting strategies? I'm not
> >>> completely certain
> >>> > > > > > > >>> on
> >>> > > > > > > >>>>>>>>>> this point, so it would be great if you can
> >>> clarify on that.
> >>> > > > > > So
> >>> > > > > > > >>>>>>>>>> here's what I got from all the discussion so far:
> >>> - Since both
> >>> > > > > > > >>>>>>>>>> Matthias and John seems to have come to a
> >>> consensus on this,
> >>> > > > > > > >>> then
> >>> > > > > > > >>>>>>>>>> we will go for an all-round behavorial change for
> >>> KTables.
> >>> > > > > > > >>> After
> >>> > > > > > > >>>>>>>>>> some thought, I decided that for now, an opt-out
> >>> config will
> >>> > > > > > > >>> not
> >>> > > > > > > >>>>>>>>>> be added. As John have pointed out, no-op changes
> >>> tend to
> >>> > > > > > > >>>>>>>>>> explode further down the topology as they are
> >>> forwarded to
> >>> > > > > > more
> >>> > > > > > > >>>>>>>>>> and more processor nodes downstream. - About
> >>> using hash codes,
> >>> > > > > > > >>>>>>>>>> after some explanation from John, it looks like
> >>> hash codes
> >>> > > > > > > >>> might
> >>> > > > > > > >>>>>>>>>> not be as ideal (for implementation). For now, we
> >>> will omit
> >>> > > > > > > >>> that
> >>> > > > > > > >>>>>>>>>> detail, and save it for the PR. - @Bruno You do
> >>> have valid
> >>> > > > > > > >>>>>>>>>> concerns. Though, I am not completely certain if
> >>> we want to do
> >>> > > > > > > >>>>>>>>>> emit-on-change only for materialized KTables. I
> >>> will put it
> >>> > > > > > > >>> down
> >>> > > > > > > >>>>>>>>>> in the KIP regardless. I will do my best to
> >>> address all points
> >>> > > > > > > >>>>>>>>>> raised so far on the discussion. Hope we could
> >>> keep this
> >>> > > > > > going!
> >>> > > > > > > >>>>>>>>>> Best, Richard On Fri, Jan 24, 2020 at 6:07 PM
> >>> Bruno Cadonna
> >>> > > > > > > >>>>>>>>>> <br...@confluent.io <mailto:br...@confluent.io>>
> >>> wrote: Thank
> >>> > > > > > > >>> you
> >>> > > > > > > >>>>>>>>>> Matthias for the use cases! Looking at both use
> >>> cases, I think
> >>> > > > > > > >>>>>>>>>> you need to elaborate on them in the KIP,
> >>> Richard. Emit from
> >>> > > > > > > >>>>>>>>>> plain KTable: I agree with Matthias that the
> >>> lower timestamp
> >>> > > > > > > >>>>>>>>>> makes sense because it marks the start of the
> >>> validity of the
> >>> > > > > > > >>>>>>>>>> record. Idempotent records with a higher
> >>> timestamp can be
> >>> > > > > > > >>> safely
> >>> > > > > > > >>>>>>>>>> ignored. A corner case that I discussed with
> >>> Matthias offline
> >>> > > > > > > >>> is
> >>> > > > > > > >>>>>>>>>> when we do not materialize a KTable due to
> >>> optimization. Then
> >>> > > > > > > >>> we
> >>> > > > > > > >>>>>>>>>> cannot avoid the idempotent records because we do
> >>> not keep the
> >>> > > > > > > >>>>>>>>>> first record with the lower timestamp to compare
> >>> to. Emit from
> >>> > > > > > > >>>>>>>>>> KTable with aggregations: If we specify that an
> >>> aggregation
> >>> > > > > > > >>>>>>>>>> result should have the highest timestamp of the
> >>> records that
> >>> > > > > > > >>>>>>>>>> participated in the aggregation, we cannot ignore
> >>> any
> >>> > > > > > > >>> idempotent
> >>> > > > > > > >>>>>>>>>> records. Admittedly, the result of an aggregation
> >>> usually
> >>> > > > > > > >>>>>>>>>> changes, but there are aggregations where the
> >>> result may not
> >>> > > > > > > >>>>>>>>>> change like min and max, or sum when the incoming
> >>> records have
> >>> > > > > > > >>> a
> >>> > > > > > > >>>>>>>>>> value of zero. In those cases, we could benefit
> >>> of the emit on
> >>> > > > > > > >>>>>>>>>> change, but only if we define the semantics of the
> >>> > > > > > aggregations
> >>> > > > > > > >>>>>>>>>> to not use the highest timestamp of the
> >>> participating records
> >>> > > > > > > >>> for
> >>> > > > > > > >>>>>>>>>> the result. In Kafka Streams, we do not have min,
> >>> max, and sum
> >>> > > > > > > >>> as
> >>> > > > > > > >>>>>>>>>> explicit aggregations, but we need to provide an
> >>> API to define
> >>> > > > > > > >>>>>>>>>> what timestamp should be used for the result of
> >>> an aggregation
> >>> > > > > > > >>> if
> >>> > > > > > > >>>>>>>>>> we want to go down this path. All of this does
> >>> not block this
> >>> > > > > > > >>> KIP
> >>> > > > > > > >>>>>>>>>> and I just wanted to put this aspects up for
> >>> discussion. The
> >>> > > > > > > >>> KIP
> >>> > > > > > > >>>>>>>>>> can limit itself to emit from materialized
> >>> KTables. However,
> >>> > > > > > > >>> the
> >>> > > > > > > >>>>>>>>>> limits should be explicitly stated in the KIP.
> >>> Best, Bruno
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> On Fri, Jan 24, 2020 at 10:58 AM Matthias J. Sax
> >>> > > > > > > >>>>>>>>>> <matth...@confluent.io <mailto:
> >>> matth...@confluent.io>> wrote:
> >>> > > > > > > >>>>>>>>>> IMHO, the question about semantics depends on the
> >>> use case, in
> >>> > > > > > > >>>>>>>>>> particular on the origin of a KTable. If there is
> >>> a changlog
> >>> > > > > > > >>>>>>>>>> topic that one reads directly into a KTable,
> >>> emit-on-change
> >>> > > > > > > >>> does
> >>> > > > > > > >>>>>>>>>> actually make sense, because the timestamp
> >>> indicates _when_
> >>> > > > > > the
> >>> > > > > > > >>>>>>>>>> update was _effective_. For this case, it is
> >>> semantically
> >>> > > > > > sound
> >>> > > > > > > >>>>>>>>>> to _not_ update the timestamp in the store,
> >>> because the second
> >>> > > > > > > >>>>>>>>>> update is actually idempotent and advancing the
> >>> timestamp is
> >>> > > > > > > >>> not
> >>> > > > > > > >>>>>>>>>> ideal (one could even consider it to be wrong to
> >>> advance the
> >>> > > > > > > >>>>>>>>>> timestamp) because the "valid time" of the record
> >>> pair did not
> >>> > > > > > > >>>>>>>>>> change. This reasoning also applies to
> >>> KTable-KTable joins.
> >>> > > > > > > >>>>>>>>>> However, if the KTable is the result of an
> >>> aggregation, I
> >>> > > > > > think
> >>> > > > > > > >>>>>>>>>> emit-on-update is more natural, because the
> >>> timestamp reflects
> >>> > > > > > > >>>>>>>>>> the _last_ time (ie, highest timestamp) of all
> >>> input records
> >>> > > > > > > >>> the
> >>> > > > > > > >>>>>>>>>> contributed to the result. Hence, updating the
> >>> timestamp and
> >>> > > > > > > >>>>>>>>>> emitting a new record actually sounds correct to
> >>> me. This
> >>> > > > > > > >>> applies
> >>> > > > > > > >>>>>>>>>> to windowed and non-windowed aggregations IMHO.
> >>> However,
> >>> > > > > > > >>>>>>>>>> considering the argument that the timestamp
> >>> should not be
> >>> > > > > > > >>> update
> >>> > > > > > > >>>>>>>>>> in the first case in the store to begin with,
> >>> both cases are
> >>> > > > > > > >>>>>>>>>> actually the same, and both can be modeled as
> >>> emit-on-change:
> >>> > > > > > > >>> if
> >>> > > > > > > >>>>>>>>>> a `table()` operator does not update the
> >>> timestamp if the
> >>> > > > > > value
> >>> > > > > > > >>>>>>>>>> does not change, there is _no_ change and thus
> >>> nothing is
> >>> > > > > > > >>>>>>>>>> emitted. At the same time, if an aggregation
> >>> operator does
> >>> > > > > > > >>> update
> >>> > > > > > > >>>>>>>>>> the timestamp (even if the value does not change)
> >>> there _is_ a
> >>> > > > > > > >>>>>>>>>> change and we emit. Note that handling
> >>> out-of-order data for
> >>> > > > > > > >>>>>>>>>> aggregations would also work seamlessly with this
> >>> approach --
> >>> > > > > > > >>> for
> >>> > > > > > > >>>>>>>>>> out-of-order records, the timestamp does never
> >>> change, and
> >>> > > > > > > >>> thus,
> >>> > > > > > > >>>>>>>>>> we only emit if the result itself changes.
> >>> Therefore, I would
> >>> > > > > > > >>>>>>>>>> argue that we might not even need any config,
> >>> because the
> >>> > > > > > > >>>>>>>>>> emit-on-change behavior is just correct and
> >>> reduced the
> >>> > > > > > > >>>>>>>>>> downstream load, while our current behavior is
> >>> not ideal (even
> >>> > > > > > > >>> if
> >>> > > > > > > >>>>>>>>>> it's also correct). Thoughts? -Matthias On
> >>> 1/24/20 9:37 AM,
> >>> > > > > > > >>> John
> >>> > > > > > > >>>>>>>>>> Roesler wrote: Hi Bruno, Thanks for that idea. I
> >>> hadn't
> >>> > > > > > > >>>>>>>>>> considered that option before, and it does seem
> >>> like that
> >>> > > > > > would
> >>> > > > > > > >>>>>>>>>> be the right place to put it if we think it might
> >>> be
> >>> > > > > > > >>> semantically
> >>> > > > > > > >>>>>>>>>> important to control on a table-by-table basis. I
> >>> had been
> >>> > > > > > > >>>>>>>>>> thinking of it less semantically and more
> >>> practically. In the
> >>> > > > > > > >>>>>>>>>> context of a large topology, or more generally, a
> >>> large
> >>> > > > > > > >>> software
> >>> > > > > > > >>>>>>>>>> system that contains many topologies and other
> >>> event-driven
> >>> > > > > > > >>>>>>>>>> systems, each no-op result becomes an input that
> >>> is destined
> >>> > > > > > to
> >>> > > > > > > >>>>>>>>>> itself become a no-op result, and so on, all the
> >>> way through
> >>> > > > > > > >>> the
> >>> > > > > > > >>>>>>>>>> system. Thus, a single pointless processing
> >>> result becomes
> >>> > > > > > > >>>>>>>>>> amplified into a large number of pointless
> >>> computations, cache
> >>> > > > > > > >>>>>>>>>> perturbations, and network and disk I/O
> >>> operations. If you
> >>> > > > > > also
> >>> > > > > > > >>>>>>>>>> consider operations with fan-out implications,
> >>> like branching
> >>> > > > > > > >>> or
> >>> > > > > > > >>>>>>>>>> foreign-key joins, the wasted resources are
> >>> amplified not just
> >>> > > > > > > >>> in
> >>> > > > > > > >>>>>>>>>> proportion to the size of the system, but the
> >>> size of the
> >>> > > > > > > >>> system
> >>> > > > > > > >>>>>>>>>> times the average fan-out (to the power of the
> >>> number of
> >>> > > > > > > >>> fan-out
> >>> > > > > > > >>>>>>>>>> operations on the path(s) through the system). In
> >>> my time
> >>> > > > > > > >>>>>>>>>> operating such systems, I've observed these
> >>> effects to be very
> >>> > > > > > > >>>>>>>>>> real, and actually, the system and use case
> >>> doesn't have to be
> >>> > > > > > > >>>>>>>>>> very large before the amplification poses an
> >>> existential
> >>> > > > > > threat
> >>> > > > > > > >>>>>>>>>> to the system as a whole. This is the basis of my
> >>> advocating
> >>> > > > > > > >>> for
> >>> > > > > > > >>>>>>>>>> a simple behavior change, rather than an opt-in
> >>> config of any
> >>> > > > > > > >>>>>>>>>> kind. It seems like Streams should "do the right
> >>> thing" for
> >>> > > > > > the
> >>> > > > > > > >>>>>>>>>> majority use case. My theory (which may be wrong)
> >>> is that the
> >>> > > > > > > >>>>>>>>>> majority use case is more like "relational
> >>> queries" than "CEP
> >>> > > > > > > >>>>>>>>>> queries". Even if you were doing some
> >>> event-sensitive
> >>> > > > > > > >>>>>>>>>> computation, wouldn't you do them as Stream
> >>> operations (where
> >>> > > > > > > >>>>>>>>>> this feature is inapplicable anyway)? In keeping
> >>> with the
> >>> > > > > > > >>>>>>>>>> "practical" perspective, I suggested the opt-out
> >>> config only
> >>> > > > > > in
> >>> > > > > > > >>>>>>>>>> the (I think unlikely) event that filtering out
> >>> pointless
> >>> > > > > > > >>> updates
> >>> > > > > > > >>>>>>>>>> actually harms performance. I'd also be perfectly
> >>> fine without
> >>> > > > > > > >>>>>>>>>> the opt-out config. I really think that (because
> >>> of the
> >>> > > > > > > >>> timestamp
> >>> > > > > > > >>>>>>>>>> semantics work already underway), we're already
> >>> pre-fetching
> >>> > > > > > > >>> the
> >>> > > > > > > >>>>>>>>>> prior result most of the time, so there would
> >>> actually be very
> >>> > > > > > > >>>>>>>>>> little extra I/O involved in implementing
> >>> emit-on-change.
> >>> > > > > > > >>>>>>>>>> However, we should consider whether my experience
> >>> is likely to
> >>> > > > > > > >>>>>>>>>> be general. Do you have some use case in mind for
> >>> which you'd
> >>> > > > > > > >>>>>>>>>> actually want some KTable results to be
> >>> emit-on-update for
> >>> > > > > > > >>>>>>>>>> semantic reasons? Thanks, -John
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> On Fri, Jan 24, 2020, at 11:02, Bruno Cadonna
> >>> wrote: Hi
> >>> > > > > > > >>> Richard,
> >>> > > > > > > >>>>>>>>>> Thank you for the KIP. I agree with John that we
> >>> should focus
> >>> > > > > > > >>> on
> >>> > > > > > > >>>>>>>>>> the interface and behavior change in a KIP. We
> >>> can discuss the
> >>> > > > > > > >>>>>>>>>> implementation later. I am also +1 for the
> >>> survey. I had a
> >>> > > > > > > >>>>>>>>>> thought about this. Couldn't we consider
> >>> emit-on-change to be
> >>> > > > > > > >>> one
> >>> > > > > > > >>>>>>>>>> config of suppress (like `untilWindowCloses`)?
> >>> What you
> >>> > > > > > > >>>>>>>>>> basically propose is to suppress updates if they
> >>> do not change
> >>> > > > > > > >>>>>>>>>> the result. Considering emit on change as a
> >>> flavour of
> >>> > > > > > suppress
> >>> > > > > > > >>>>>>>>>> would be more flexible because it would specify
> >>> the behavior
> >>> > > > > > > >>>>>>>>>> locally for a KTable instead of globally for all
> >>> KTables.
> >>> > > > > > > >>>>>>>>>> Additionally, specifying the behavior in one
> >>> place instead of
> >>> > > > > > > >>>>>>>>>> multiple places feels more intuitive and
> >>> consistent to me.
> >>> > > > > > > >>> Best,
> >>> > > > > > > >>>>>>>>>> Bruno On Fri, Jan 24, 2020 at 7:49 AM John Roesler
> >>> > > > > > > >>>>>>>>>> <vvcep...@apache.org <mailto:vvcep...@apache.org>>
> >>> wrote: Hi
> >>> > > > > > > >>>>>>>>>> Richard, Thanks for picking this up! I know of at
> >>> least one
> >>> > > > > > > >>> large
> >>> > > > > > > >>>>>>>>>> community member for which this feature is
> >>> absolutely
> >>> > > > > > > >>> essential.
> >>> > > > > > > >>>>>>>>>> If I understand your two options, it seems like
> >>> the proposal
> >>> > > > > > is
> >>> > > > > > > >>>>>>>>>> to implement it as a behavior change regardless,
> >>> and the
> >>> > > > > > > >>> question
> >>> > > > > > > >>>>>>>>>> is whether to provide an opt-out config or not.
> >>> Given that any
> >>> > > > > > > >>>>>>>>>> implementation of this feature would have some
> >>> performance
> >>> > > > > > > >>> impact
> >>> > > > > > > >>>>>>>>>> under some workloads, and also that we don't know
> >>> if anyone
> >>> > > > > > > >>>>>>>>>> really depends on emit-on-update time semantics,
> >>> it seems like
> >>> > > > > > > >>> we
> >>> > > > > > > >>>>>>>>>> should propose to add an opt-out config. Can you
> >>> update the
> >>> > > > > > KIP
> >>> > > > > > > >>>>>>>>>> to mention the exact config key and value(s)
> >>> you'd propose?
> >>> > > > > > > >>> Just
> >>> > > > > > > >>>>>>>>>> to move the discussion forward, maybe something
> >>> like: emit.on
> >>> > > > > > > >>> :=
> >>> > > > > > > >>>>>>>>>> change|update with the new default being "change"
> >>> Thanks for
> >>> > > > > > > >>>>>>>>>> pointing out the timestamp issue in particular. I
> >>> agree that
> >>> > > > > > if
> >>> > > > > > > >>>>>>>>>> we discard the latter update as a no-op, then we
> >>> also have to
> >>> > > > > > > >>>>>>>>>> discard its timestamp (obviously, we don't
> >>> forward the
> >>> > > > > > > >>> timestamp
> >>> > > > > > > >>>>>>>>>> update, as that's the whole point, but we also
> >>> can't update
> >>> > > > > > the
> >>> > > > > > > >>>>>>>>>> timestamp in the store, as the store must remain
> >>> consistent
> >>> > > > > > > >>> with
> >>> > > > > > > >>>>>>>>>> what has been emitted). I have to confess that I
> >>> disagree with
> >>> > > > > > > >>>>>>>>>> your implementation proposal, but it's also not
> >>> necessary to
> >>> > > > > > > >>>>>>>>>> discuss implementation in the KIP. Maybe it would
> >>> be less
> >>> > > > > > > >>>>>>>>>> controversial if you just drop that section for
> >>> now, so that
> >>> > > > > > > >>> the
> >>> > > > > > > >>>>>>>>>> KIP discussion can focus on the behavior change
> >>> and config.
> >>> > > > > > > >>> Just
> >>> > > > > > > >>>>>>>>>> for reference, there is some research into this
> >>> domain. For
> >>> > > > > > > >>>>>>>>>> example, see the "Report" section (3.2.3) of the
> >>> SECRET paper:
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>
> >>> > > > > >
> >>> https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Fpeop
> >>> > > > > > > > le.csail.mit.edu
> >>> > > > > > > >>>>>>
> >>> %2Ftatbul%2Fpublications%2Fmaxstream_vldb10.pdf&data
> >>> > > > > > > > =02%7C01%7CThomas.Becker%40tivo.com
> >>> > > > > > > >>>>>> %7Ce0235483b1eb4f259c5c08d7a8d1c16b%7
> >>> > > > > > > >
> >>> > > > > > > >>>>>>
> >>> > > > > > > >>>
> >>> > > > > >
> >>> Cd05b7c6912014c0db45d7f1dcc227e4d%7C1%7C1%7C637163491160859282&sdata
> >>> > > > > > > >
> >>> =4dSGIS8jNPAPP7B48r9e%2BUgFh3WdmzVyXhyT63eP8dI%3D&reserved=0
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > > It might help to round out the proposal if you take a brief
> >>> > > > > > > >>> survey of
> >>> > > > > > > >>>>>>>>>> the behaviors of other systems, along with pros
> >>> and cons if
> >>> > > > > > any
> >>> > > > > > > >>>>>>>>>> are reported. Thanks, -John
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> On Fri, Jan 10, 2020, at 22:27, Richard Yu wrote:
> >>> Hi
> >>> > > > > > everybody!
> >>> > > > > > > >>>>>>>>>> I'd like to propose a change that we probably
> >>> should've added
> >>> > > > > > > >>> for
> >>> > > > > > > >>>>>>>>>> a long time now. The key benefit of this KIP
> >>> would be reduced
> >>> > > > > > > >>>>>>>>>> traffic in Kafka Streams since a lot of no-op
> >>> results would no
> >>> > > > > > > >>>>>>>>>> longer be sent downstream. Here is the KIP for
> >>> reference.
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>
> >>> > > > > >
> >>> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcwi
> >>> > > > > > > > ki.apache.org
> >>> > > > > > > >>>>>>
> >>> %2Fconfluence%2Fdisplay%2FKAFKA%2FKIP-557%253A%2BAdd%2Bemit
> >>> > > > > > > >
> >>> > > > > > > >>>>>>
> >>> > > > > > > >>>
> >>> > > > > >
> >>> %2Bon%2Bchange%2Bsupport%2Bfor%2BKafka%2BStreams&data=02%7C01%7CThom
> >>> > > > > > > > as.Becker%40tivo.com
> >>> > > > > > > >>>>>> %7Ce0235483b1eb4f259c5c08d7a8d1c16b%7Cd05b7c6912014c
> >>> > > > > > > >
> >>> > > > > > > >>>>>>
> >>> > > > > > > >>>
> >>> > > > > >
> >>> 0db45d7f1dcc227e4d%7C1%7C1%7C637163491160869277&sdata=zYpCSFOsyN4%2B
> >>> > > > > > > > 4rKRZBQ%2FZvcGQ4EINR9Qm6PLsB7EKrc%3D&reserved=0
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > > Currently, I seek to formalize our approach for this KIP
> >>> first
> >>> > > > > > > >>> before
> >>> > > > > > > >>>>>>>>>> we determine concrete API additions /
> >>> configurations. Some
> >>> > > > > > > >>>>>>>>>> configs might warrant adding, whiles others are
> >>> not necessary
> >>> > > > > > > >>>>>>>>>> since adding them would only increase complexity
> >>> of Kafka
> >>> > > > > > > >>>>>>>>>> Streams. Cheers, Richard
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> ________________________________ This email and
> >>> any
> >>> > > > > > attachments
> >>> > > > > > > >>>>>>>>>> may contain confidential and privileged material
> >>> for the sole
> >>> > > > > > > >>> use
> >>> > > > > > > >>>>>>>>>> of the intended recipient. Any review, copying, or
> >>> > > > > > distribution
> >>> > > > > > > >>>>>>>>>> of this email (or any attachments) by others is
> >>> prohibited. If
> >>> > > > > > > >>>>>>>>>> you are not the intended recipient, please
> >>> contact the sender
> >>> > > > > > > >>>>>>>>>> immediately and permanently delete this email and
> >>> any
> >>> > > > > > > >>>>>>>>>> attachments. No employee or agent of TiVo is
> >>> authorized to
> >>> > > > > > > >>>>>>>>>> conclude any binding agreement on behalf of TiVo
> >>> by email.
> >>> > > > > > > >>>>>>>>>> Binding agreements with TiVo may only be made by
> >>> a signed
> >>> > > > > > > >>> written
> >>> > > > > > > >>>>>>>>>> agreement. -- *Tommy Becker* *Principal Engineer *
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> *Personalized Content Discovery*
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> *O* +1 919.460.4747 *tivo.com* <
> >>> http://www.tivo.com/>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>>
> >>> > > > > > > >>>>>>>>>> This email and any attachments may contain
> >>> confidential and
> >>> > > > > > > >>>>>>>>>> privileged material for the sole use of the
> >>> intended
> >>> > > > > > recipient.
> >>> > > > > > > >>>>>>>>>> Any review, copying, or distribution of this
> >>> email (or any
> >>> > > > > > > >>>>>>>>>> attachments) by others is prohibited. If you are
> >>> not the
> >>> > > > > > > >>> intended
> >>> > > > > > > >>>>>>>>>> recipient, please contact the sender immediately
> >>> and
> >>> > > > > > > >>> permanently
> >>> > > > > > > >>>>>>>>>> delete this email and any attachments. No
> >>> employee or agent of
> >>> > > > > > > >>>>>>>>>> TiVo is authorized to conclude any binding
> >>> agreement on behalf
> >>> > > > > > > >>> of
> >>> > > > > > > >>>>>>>>>> TiVo by email. Binding agreements with TiVo may
> >>> only be made
> >>> > > > > > > >>> by a
> >>> > > > > > > >>>>>>>>>> signed written agreement.
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> --
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> *Tommy Becker* /Principal Engineer /
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> /Personalized Content Discovery/
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>> *O* +1 919.460.4747 *tivo.com* <
> >>> http://www.tivo.com/>
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>>>>
> >>> > > > > > > >>>>>>
> >>> > > > > >
> >>> ----------------------------------------------------------------------
> >>> > > > > > > >>>>>>>
> >>> > > > > > > >>>>>>
> >>> > > > > > > >>>>>
> >>> > > > > > > >>>>
> >>> > > > > > > >>>
> >>> > > > > > > >>
> >>> > > > > > > >
> >>> > > > > > >
> >>> > > > > > >
> >>> > > > > > > Attachments:
> >>> > > > > > > * signature.asc
> >>> > > > > >
> >>> > > >
> >>> >
> >>>
> >>
>
