Hello Colt,
Thanks for writing the KIP! I have read through the updated KIP and
overall it looks great. I only have minor naming comments (well,
aren't naming the least boring stuff to discuss and that takes the
most of the time for KIPs :P):
1. I tend to agree with Sophie regarding whether or not to include
"Standby" in the functions of "onStandbyUpdateStart/Suspended", since
it is also more consistent with the functions of
"StateRestoreListener" where we do not name it as
"onStateRestoreState" etc.
2. I know in community discussions we sometimes say "a standby is
promoted to active", but in the official code / java docs we did not
have a term of "promotion", since what the code does is really recycle
the task (while keeping its state stores open), and create a new
active task that takes in the recycled state stores and just changing
the other fields like task type etc. After thinking about this for a
bit, I tend to feel that "promoted" is indeed a better name for user
facing purposes while "recycle" is more of a technical detail inside
the code and could be abstracted away from users. So I feel keeping
the name "PROMOTED" is fine.
3. Regarding "earliestOffset", it does feel like we cannot always
avoid another call to the Kafka API. And on the other hand, I also
tend to think that such bookkeeping may be better done at the app
level than from the Streams' public API level. I.e. the app could keep
a "first ever starting offset" per "topic-partition-store" key, and a
when we have rolling restart and hence some standby task keeps
"jumping" from one client to another via task assignment, the app
would update this value just one when it finds the
""topic-partition-store" was never triggered before. What do you
think?
4. I do not have a strong opinion either, but what about "onBatchUpdated" ?
Guozhang
On Wed, Oct 11, 2023 at 9:31 PM Colt McNealy <c...@littlehorse.io> wrote:
>
> Sohpie—
>
> Thank you very much for such a detailed review of the KIP. It might
> actually be longer than the original KIP in the first place!
>
> 1. Ack'ed and fixed.
>
> 2. Correct, this is a confusing passage and requires context:
>
> One thing on our list of TODO's regarding reliability is to determine how
> to configure `session.timeout.ms`. In our Kubernetes Environment, an
> instance of our Streams App can be terminated, restarted, and get back into
> the "RUNNING" Streams state in about 20 seconds. We have two options here:
> a) set session.timeout.ms to 30 seconds or so, and deal with 20 seconds of
> unavailability for affected partitions, but avoid shuffling Tasks; or b)
> set session.timeout.ms to a low value, such as 6 seconds (
> heartbeat.interval.ms of 2000), and reduce the unavailability window during
> a rolling bounce but incur an "extra" rebalance. There are several
> different costs to a rebalance, including the shuffling of standby tasks.
> JMX metrics are not fine-grained enough to give us an accurate picture of
> what's going on with the whole Standby Task Shuffle Dance. I hypothesize
> that the Standby Update Listener might help us clarify just how the
> shuffling actually (not theoretically) works, which will help us make a
> more informed decision about the session timeout config.
>
> If you think this is worth putting in the KIP, I'll polish it and do so;
> else, I'll remove the current half-baked explanation.
>
> 3. Overall, I agree with this. In our app, each Task has only one Store to
> reduce the number of changelog partitions, so I sometimes forget the
> distinction between the two concepts, as reflected in the KIP (:
>
> 3a. I don't like the word "Restore" here, since Restoration refers to an
> Active Task getting caught up in preparation to resume processing.
> `StandbyUpdateListener` is fine by me; I have updated the KIP. I am a
> native Python speaker so I do prefer shorter names anyways (:
>
> 3b1. +1 to removing the word 'Task'.
>
> 3b2. I like `onUpdateStart()`, but with your permission I'd prefer
> `onStandbyUpdateStart()` which matches the name of the Interface
> "StandbyUpdateListener". (the python part of me hates this, however)
>
> 3b3. Going back to question 2), `earliestOffset` was intended to allow us
> to more easily calculate the amount of state _already loaded_ in the store
> by subtracting (startingOffset - earliestOffset). This would help us see
> how much inefficiency is introduced in a rolling restart—if we end up going
> from a situation with an up-to-date standby before the restart, and then
> after the whole restart, the Task is shuffled onto an instance where there
> is no previous state, then that is expensive. However, if the final
> shuffling results in the Task back on an instance with a lot of pre-built
> state, it's not expensive.
>
> If a call over the network is required to determine the earliestOffset,
> then this is a "hard no-go" for me, and we will remove it (I'll have to
> check with Eduwer as he is close to having a working implementation). I
> think we can probably determine what we wanted to see in a different
> way, but it will take more thinking.. If `earliestOffset` is confusing,
> perhaps rename it to `earliestChangelogOffset`?
>
> `startingOffset` is easy to remove as it can be determined from the first
> call to `onBatch{Restored/Updated/Processed/Loaded}()`.
>
> Anyways, I've updated the JavaDoc in the interface; hopefully it's more
> clear. Awaiting further instructions here.
>
> 3c. Good point; after thinking, my preference is `onBatchLoaded()` ->
> `onBatchUpdated()` -> `onBatchProcessed()` -> `onBatchRestored()`. I am
> less fond of "processed" because when I was first learning Streams I
> mistakenly thought that standby tasks actually processed the input topic
> rather than loaded from the changelog. I'll defer to you here.
>
> 3d. +1 to `onUpdateSuspended()`, or better yet
> `onStandbyUpdateSuspended()`. Will check about the implementation of
> keeping track of the number of records loaded.
>
> 4a. I think this might be best in a separate KIP, especially given that
> this is my and Eduwer's first time contributing to Kafka (so we want to
> minimize the blast radius).
>
> 4b. I might respectfully (and timidly) push back here, RECYCLED for an
> Active Task is a bit confusing to me. DEMOTED and MIGRATED make sense from
> the standpoint of an Active Task, recycling to me sounds like throwing
> stuff away, such that the resources (i.e. disk space) can be used by a
> separate Task. As an alternative rather than trying to reuse the same enum,
> maybe rename it to `StandbySuspendReason` to avoid naming conflicts with
> `ActiveSuspendReason`? However, I could be convinced to rename PROMOTED ->
> RECYCLED, especially if Eduwer agrees.
>
> TLDR:
>
> T1. Agreed, will remove the word "Task" as it's incorrect.
> T2. Will update to `onStandbyUpdateStart()`
> T3. Awaiting further instructions on earliestOffset and startingOffset.
> T4. I don't like `onBatchProcessed()` too much, perhaps `onBatchLoaded()`?
> T5. Will update to `onStandbyUpdateSuspended()`
> T6. Thoughts on renaming SuspendReason to StandbySuspendReason, rather than
> renaming PROMOTED to RECYCLED? @Eduwer?
>
> Long Live the Otter,
> Colt McNealy
>
> *Founder, LittleHorse.dev*
>
>
> On Wed, Oct 11, 2023 at 9:32 AM Sophie Blee-Goldman <sop...@responsive.dev>
> wrote:
>
> > Hey Colt! Thanks for the KIP -- this will be a great addition to Streams, I
> > can't believe we've gone so long without this.
> >
> > Overall the proposal makes sense, but I had a handful of fairly minor
> > questions and suggestions/requests
> >
> > 1. Seems like the last sentence in the 2nd paragraph of the Motivation
> > section is cut off and incomplete -- "want to be able to know " what
> > exactly?
> >
> > 2. This isn't that important since the motivation as a whole is clear to me
> > and convincing enough, but I'm not quite sure I understand the example at
> > the end of the Motivation section. How are standby tasks (and the ability
> > to hook into and monitor their status) related to the session.timeout.ms
> > config?
> >
> > 3. To help both old and new users of Kafka Streams understand this new
> > restore listener and its purpose/semantics, can we try to name the class
> > and
> > callbacks in a way that's more consistent with the active task restore
> > listener?
> >
> > 3a. StandbyTaskUpdateListener:
> > The existing restore listener is called StateRestoreListener, so the new
> > one could be called something like StandbyStateRestoreListener. Although
> > we typically refer to standby tasks as "processing" rather than "restoring"
> > records -- ie restoration is a term for active task state specifically. I
> > actually
> > like the original suggestion if we just drop the "Task" part of the name,
> > ie StandbyUpdateListener. I think either that or StandbyRestoreListener
> > would be fine and probably the two best options.
> > Also, this probably goes without saying but any change to the name of this
> > class should of course be reflected in the KafkaStreams#setXXX API as well
> >
> > 3b. #onTaskCreated
> > I know the "start" callback feels a bit different for the standby task
> > updater vs an active task beginning restoration, but I think we should try
> > to
> > keep the various callbacks aligned to their active restore listener
> > counterpart. We can/should just replace the term "restore" with "update"
> > for the
> > callback method names the same way we do for the class name, which in this
> > case would give us #onUpdateStart. Personally I like this better,
> > but it's ultimately up to you. However, I would push back against anything
> > that includes the word "Task" (eg #onTaskCreated) as the listener
> > is actually not scoped to the task itself but instead to the individual
> > state store(s). This is the main reason I would prefer calling it something
> > like #onUpdateStart, which keeps the focus on the store being updated
> > rather than the task that just happens to own this store
> > One last thing on this callback -- do we really need both the
> > `earliestOffset` and `startingOffset`? I feel like this might be more
> > confusing than it
> > is helpful (tbh even I'm not completely sure I know what the earliestOffset
> > is supposed to represent) More importantly, is this all information
> > that is already available and able to be passed in to the callback by
> > Streams? I haven't checked on this but it feels like the earliestOffset is
> > likely to require a remote call, either by the embedded consumer or via the
> > admin client. If so, the ROI on including this parameter seems
> > quite low (if not outright negative)
> >
> > 3c. #onBatchRestored
> > If we opt to use the term "update" in place of "restore" elsewhere, then we
> > should consider doing so here as well. What do you think about
> > #onBatchUpdated, or even #onBatchProcessed?
> > I'm actually not super concerned about this particular API, and honestly I
> > think we can use restore or update interchangeably here, so if you
> > don't like any of the suggested names (and no one can think of anything
> > better), I would just stick with #onBatchRestored. In this case,
> > it kind of makes the most sense.
> >
> > 3d. #onTaskSuspended
> > Along the same lines as 3b above, #onUpdateSuspended or just
> > #onRestoreSuspended probably makes more sense for this callback. Also,
> > I notice the StateRestoreListener passes in the total number of records
> > restored to its #onRestoreSuspended. Assuming we already track
> > that information in Streams and have it readily available to pass in at
> > whatever point we would be invoking this callback, that might be a
> > useful parameter for the standby listener to have as well
> >
> > 4. I totally love the SuspendReason thing, just two notes/requests:
> >
> > 4a. Feel free to push back against adding onto the scope of this KIP, but
> > it would be great to expand the active state restore listener with this
> > SuspendReason enum as well. It would be really useful for both variants of
> > restore listener
> >
> > 4b. Assuming we do 4a, let's rename PROMOTED to RECYCLED -- for standby
> > tasks it means basically the same thing, the point is that active
> > tasks can also be recycled into standbys through the same mechanism. This
> > way they can share the SuspendReason enum -- not that it's
> > necessary for them to share, I just think it would be a good idea to keep
> > the two restore listeners aligned to the highest degree possible for as
> > we can.
> > I was actually considering proposing a short KIP with a new
> > RecyclingListener (or something) specifically for this exact kind of thing,
> > since we
> > currently have literally zero insight into the recycling process. It's
> > practically impossible to tell when a store has been converted from active
> > to
> > standby, or vice versa. So having access to the SuspendReason, and more
> > importantly having a callback guaranteed to notify you when a
> > state store is recycled whether active or standby, would be amazing.
> >
> > Thanks for the KIP!
> >
> > -Sophie "otterStandbyTaskUpdateListener :P" Blee-Goldman
> >
> >
> > ---------- Forwarded message ---------
> > > From: Colt McNealy <c...@littlehorse.io>
> > > Date: Tue, Oct 3, 2023 at 12:48 PM
> > > Subject: [DISCUSS] KIP-988 Streams Standby Task Update Listener
> > > To: <dev@kafka.apache.org>
> > >
> > >
> > > Hi all,
> > >
> > > We would like to propose a small KIP to improve the ability of Streams
> > apps
> > > to monitor the progress of their standby tasks through a callback
> > > interface.
> > >
> > > We have a nearly-working implementation on our fork and are curious for
> > > feedback.
> > >
> > >
> > >
> > https://cwiki.apache.org/confluence/display/KAFKA/KIP-988%3A+Streams+Standby+Task+Update+Listener
> > >
> > > Thank you,
> > > Colt McNealy
> > >
> > > *Founder, LittleHorse.dev*
> > >
> >
