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* >
