Thanks for the comments John and Guozhang, I'll address each one of your comments in turn.
John, > I'm wondering about a missing quadrant from the truth table involving > whether a Materialized is stored or not and querying is > enabled/disabled... What should be the behavior if there is no store > configured (e.g., if Materialized with only serdes) and querying is enabled? > It seems we have two choices: > 1. we can force creation of a state store in this case, so the store > can be used to serve the queries > 2. we can provide just a queriable view, basically letting IQ query > into the "KTableValueGetter", which would transparently construct the > query response by applying the operator logic to the upstream state if > the operator state isn't already stored. I agree with your assertion about a missing quadrant from the truth table. Additionally, I too like the concept of a queriable view. But I think that goes a bit beyond the scope of this KIP and would like to pursue that feature as follow-on work. Also thinking about this KIP some more, I'm thinking of the changes to Materialized might be a reach as well. Separating the naming from a store and its queryable state seems like a complex issue in and of itself and should be treated accordingly. So here's what I'm thinking now. We add Materialzied to Join, but for now, we internally disable querying. I know this breaks our current semantic approach, but I think it's crucial that we do two things in this KIP 1. Break the naming of the state stores from Joined to Materialized, so the naming of state stores follows our current pattern and enables upgrades from 2.3 to 2.4 2. Offer the ability to configure the state stores of the join, even providing a different implementation (i.e. in-memory) if desired. With that in mind I'm considering changing the KIP to remove the changes to Materialized, and we document very clearly that by providing a Materialized object with a name is only for naming the state store, hence the changelog topics and any possible configurations of the store, but this store *will not be available for IQ.* WDYT? Guozhang, > 1. About not breaking compatibility of stream-stream join materialized > stores: I think this is a valid issue to tackle, but after thinking about > it once more I'm not sure if exposing Materialized would be a good solution > here. My rationles: > > 1.a For stream-stream join, our current usage of window-store is not ideal, > and we want to modify it in the near future to be more efficient. Not > allowing users to override such state store backend gives us such freedom > (which was also considered in the original DSL design), whereas getting a > Materialized<WindowStore> basically kicks out that freedom out of the > window. > 1.b For strema-stream join, in our original design we intend to "never" > want users to query the state, since it is just for buffering the upcoming > records from the stream. Now I know that some users may indeed want to > query it from the debugging perspective, but still I concerned about > whether leveraging IQ for debugging purposes would be the right solution > here. And adding Materialized object opens the door to let users query > about it (unless we did something intentionally to still forbids it), which > also restricts us in the future. > > 2. About the coupling between Materialized.name() and queryable: again I > think this is a valid issue. But I'm not sure if the current > "withQuerryingDisabled / Enabled" at Materialized is the best approach. > Here I think I agree with John, that generally speaking it's better be a > control function on the `KTable` itself, rather than on `Materialized`, so > fixing it via adding functions through `Materialized` seems not a natural approach either. I understand your thoughts here, and up to a point, I agree with you. But concerning not providing Materialized as it may restrict us in the future for delivering different implementations, I'm wondering if we are doing some premature optimization here. My rationale for saying so 1. I think the cost of not allowing the naming of state stores for joins is too big of a gap to leave. IMHO for joins to follow the current pattern of using Materialized for naming state stores would be what most users would expect to use. As I said in my comments above, I think we should *not include* the changes to Materialized and enforce named stores for joins as unavailable for IQ. 2. We'll still have the join methods available without a Materialized allowing us to do something different internally if a Materialized is not provided. > Overall, I'm thinking maybe we should still use two stones rather than one > to kill these two birds, and probably for this KIP we just focus on 1) > above. And for that I'd like to not expose the Materialized either for > rationales that I've listed above. Instead, we just restrict KIP-307 to NOT > use the Joined.name for state store names and always use internal names as > well, which admittedly indeed leaves a hole of not being able to cover all > internal names here, but now I feel this `hole` may better be filled by, > e.g. not creating changelog topics but just use the upstream to > re-bootstrap the materialized store, more concretely: when materializing > the store, try to piggy-back the changelog topic on an existing topic, e.g. > a) if the stream is coming directly from some source topic (including > repartition topic), make that as changelog topic and if it is repartition > topic change the retention / data purging policy necessarily as well; b) if > the stream is coming from some stateless operators, delegate that stateless > operator to the parent stream similar as a); if the stream is coming from a > stream-stream join which is the only stateful operator that can result in a > stream, consider merging the join into multi-way joins (yes, this is a very > hand-wavy thought, but the point here is that we do not try to tackle it > now but leave it for a better solution :). I really like this idea! I agree with you in that this approach to too much for adding in this KIP, but we could pick it up later and leverage the Optimization framework to accomplish this re-use. Again, while I agree we should break the naming of join state stores from KIP-307, IMHO it's something we should fix now as it will be the last piece we can provide to give users the ability to completely make their topologies "upgrade proof" when adding additional operations. Thanks again to both of you for comments and I look forward to hearing back from you. Regards, Bill On Thu, Jun 20, 2019 at 2:33 PM Guozhang Wang <wangg...@gmail.com> wrote: > Hello Bill, > > Thanks for the KIP. Glad to see that we can likely shooting two birds with > one stone. I have some concerns though about those "two birds" themselves: > > 1. About not breaking compatibility of stream-stream join materialized > stores: I think this is a valid issue to tackle, but after thinking about > it once more I'm not sure if exposing Materialized would be a good solution > here. My rationles: > > 1.a For stream-stream join, our current usage of window-store is not ideal, > and we want to modify it in the near future to be more efficient. Not > allowing users to override such state store backend gives us such freedom > (which was also considered in the original DSL design), whereas getting a > Materialized<WindowStore> basically kicks out that freedom out of the > window. > 1.b For strema-stream join, in our original design we intend to "never" > want users to query the state, since it is just for buffering the upcoming > records from the stream. Now I know that some users may indeed want to > query it from the debugging perspective, but still I concerned about > whether leveraging IQ for debugging purposes would be the right solution > here. And adding Materialized object opens the door to let users query > about it (unless we did something intentionally to still forbids it), which > also restricts us in the future. > > 2. About the coupling between Materialized.name() and queryable: again I > think this is a valid issue. But I'm not sure if the current > "withQuerryingDisabled / Enabled" at Materialized is the best approach. > Here I think I agree with John, that generally speaking it's better be a > control function on the `KTable` itself, rather than on `Materialized`, so > fixing it via adding functions through `Materialized` seems not a natural > approach either. > > > Overall, I'm thinking maybe we should still use two stones rather than one > to kill these two birds, and probably for this KIP we just focus on 1) > above. And for that I'd like to not expose the Materialized either for > rationales that I've listed above. Instead, we just restrict KIP-307 to NOT > use the Joined.name for state store names and always use internal names as > well, which admittedly indeed leaves a hole of not being able to cover all > internal names here, but now I feel this `hole` may better be filled by, > e.g. not creating changelog topics but just use the upstream to > re-bootstrap the materialized store, more concretely: when materializing > the store, try to piggy-back the changelog topic on an existing topic, e.g. > a) if the stream is coming directly from some source topic (including > repartition topic), make that as changelog topic and if it is repartition > topic change the retention / data purging policy necessarily as well; b) if > the stream is coming from some stateless operators, delegate that stateless > operator to the parent stream similar as a); if the stream is coming from a > stream-stream join which is the only stateful operator that can result in a > stream, consider merging the join into multi-way joins (yes, this is a very > hand-wavy thought, but the point here is that we do not try to tackle it > now but leave it for a better solution :). > > > Guozhang > > > > On Wed, Jun 19, 2019 at 11:41 AM John Roesler <j...@confluent.io> wrote: > > > Hi Bill, > > > > Thanks for the KIP! Awesome job catching this unexpected consequence > > of the prior KIPs before it was released. > > > > The proposal looks good to me. On top of just fixing the problem, it > > seems to address two other pain points: > > * that naming a state store automatically causes it to become queriable. > > * that there's currently no way to configure the bytes store for join > > windows. > > > > It's awesome that we can fix this issue and two others with one feature. > > > > I'm wondering about a missing quadrant from the truth table involving > > whether a Materialized is stored or not and querying is > > enabled/disabled... What should be the behavior if there is no store > > configured (e.g., if Materialized with only serdes) and querying is > > enabled? > > > > It seems we have two choices: > > 1. we can force creation of a state store in this case, so the store > > can be used to serve the queries > > 2. we can provide just a queriable view, basically letting IQ query > > into the "KTableValueGetter", which would transparently construct the > > query response by applying the operator logic to the upstream state if > > the operator state isn't already stored. > > > > Offhand, it seems like the second is actually a pretty awesome > > capability. But it might have an awkward interaction with the current > > semantics. Presently, if I provide a Materialized.withName, it implies > > that querying should be enabled AND that the view should actually be > > stored in a state store. Under option 2 above, this behavior would > > change to NOT provision a state store and instead just consult the > > ValueGetter. To get back to the current behavior, users would have to > > add a "bytes store supplier" to the Materialized to indicate that, > > yes, they really want a state store there. > > > > Behavior changes are always kind of scary, but I think in this case, > > it might actually be preferable. In the event where only the name is > > provided, it means that people just wanted to make the operation > > result queriable. If we automatically convert this to a non-stored > > view, then simply upgrading results in the same observable behavior > > and semantics, but a linear reduction in local storage requirements > > and disk i/o, as well as a corresponding linear reduction in memory > > usage both on and off heap. > > > > What do you think? > > -John > > > > On Tue, Jun 18, 2019 at 9:21 PM Bill Bejeck <bbej...@gmail.com> wrote: > > > > > > All, > > > > > > I'd like to start a discussion for adding a Materialized configuration > > > object to KStream.join for naming state stores involved in joins. > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-479%3A+Add+Materialized+to+Join > > > > > > Your comments and suggestions are welcome. > > > > > > Thanks, > > > Bill > > > > > -- > -- Guozhang >
