Just to clarify, though "CombinedKey" will still be exposed as public APIs it would not be used in any of the returned key types, so users do not need to worry providing a serde for it at all; it will only be used in the Mapper parameter, and internally Streams library would know how to serde it if ever necessary.
Guozhang On Tue, Nov 21, 2017 at 3:14 PM, Matthias J. Sax <matth...@confluent.io> wrote: > Jan, > > Thanks for explaining the Serde issue! This makes a lot of sense. > > I discussed with Guozhang about this issue and came up with the > following idea that bridges both APIs: > > We still introduce CombinedKey as a public interface and exploit it to > manage the key in the store and the changelog topic. For this case we > can construct a suitable Serde internally based on the Serdes of both > keys that are combined. > > However, the type of the result table is user defined and can be > anything. To bridge between the CombinedKey and the user defined result > type, users need to hand in a `ValueMapper<CombinedKey, KO>` that > convert the CombinedKey into the desired result type. > > Thus, the method signature would be something like > > > <KO, VO, K1, V1> KTable<KO,VO> oneToManyJoin(> KTable<K1, V1> other, > > ValueMapper<V1, K> keyExtractor,> ValueJoiner<V, V1, VO> joiner, > > ValueMapper<CombinedKey<K,K1>, KO> resultKeyMapper); > > The interface parameters are still easy to understand and don't leak > implementation details IMHO. > > WDYT about this idea? > > > -Matthias > > > On 11/19/17 11:28 AM, Guozhang Wang wrote: > > Hello Jan, > > > > I think I get your point about the cumbersome that CombinedKey would > > introduce for serialization and tooling based on serdes. What I'm still > > wondering is the underlying of joinPrefixFakers mapper: from your latest > > comment it seems this mapper will be a one-time mapper: we use this to > map > > the original resulted KTable<combined<K1, K2>, V0> to KTable<K0, V0> and > > then that mapper can be thrown away and be forgotten. Is that true? My > > original thought is that you propose to carry this mapper all the way > along > > the rest of the topology to "abstract" the underlying combined keys. > > > > If it is the other way (i.e. the former approach), then the diagram of > > these two approaches would be different: for the less intrusive approach > we > > would add one more step in this diagram to always do a mapping after the > > "task perform join" block. > > > > Also another minor comment on the internal topic: I think many readers > may > > not get the schema of this topic, so it is better to indicate that what > > would be the key of this internal topic used for compaction, and what > would > > be used as the partition-key. > > > > Guozhang > > > > > > On Sat, Nov 18, 2017 at 2:30 PM, Jan Filipiak <jan.filip...@trivago.com> > > wrote: > > > >> -> it think the relationships between the different used types, K0,K1,KO > >> should be explains explicitly (all information is there implicitly, but > >> one need to think hard to figure it out) > >> > >> > >> I'm probably blind for this. can you help me here? how would you > formulate > >> this? > >> > >> Thanks, > >> > >> Jan > >> > >> > >> On 16.11.2017 23:18, Matthias J. Sax wrote: > >> > >>> Hi, > >>> > >>> I am just catching up on this discussion and did re-read the KIP and > >>> discussion thread. > >>> > >>> In contrast to you, I prefer the second approach with CombinedKey as > >>> return type for the following reasons: > >>> > >>> 1) the oneToManyJoin() method had less parameter > >>> 2) those parameters are easy to understand > >>> 3) we hide implementation details (joinPrefixFaker, leftKeyExtractor, > >>> and the return type KO leaks internal implementation details from my > >>> point of view) > >>> 4) user can get their own KO type by extending CombinedKey interface > >>> (this would also address the nesting issue Trevor pointed out) > >>> > >>> That's unclear to me is, why you care about JSON serdes? What is the > >>> problem with regard to prefix? It seems I am missing something here. > >>> > >>> I also don't understand the argument about "the user can stick with his > >>> default serde or his standard way of serializing"? If we have > >>> `CombinedKey` as output, the use just provide the serdes for both input > >>> combined-key types individually, and we can reuse both internally to do > >>> the rest. This seems to be a way simpler API. With the KO output type > >>> approach, users need to write an entirely new serde for KO in contrast. > >>> > >>> Finally, @Jan, there are still some open comments you did not address > >>> and the KIP wiki page needs some updates. Would be great if you could > do > >>> this. > >>> > >>> Can you also explicitly describe the data layout of the store that is > >>> used to do the range scans? > >>> > >>> Additionally: > >>> > >>> -> some arrows in the algorithm diagram are missing > >>> -> was are those XXX in the diagram > >>> -> can you finish the "Step by Step" example > >>> -> it think the relationships between the different used types, > K0,K1,KO > >>> should be explains explicitly (all information is there implicitly, but > >>> one need to think hard to figure it out) > >>> > >>> > >>> Last but not least: > >>> > >>> But noone is really interested. > >>>> > >>> Don't understand this statement... > >>> > >>> > >>> > >>> -Matthias > >>> > >>> > >>> On 11/16/17 9:05 AM, Jan Filipiak wrote: > >>> > >>>> We are running this perfectly fine. for us the smaller table changes > >>>> rather infrequent say. only a few times per day. The performance of > the > >>>> flush is way lower than the computing power you need to bring to the > >>>> table to account for all the records beeing emmited after the one > single > >>>> update. > >>>> > >>>> On 16.11.2017 18:02, Trevor Huey wrote: > >>>> > >>>>> Ah, I think I see the problem now. Thanks for the explanation. That > is > >>>>> tricky. As you said, it seems the easiest solution would just be to > >>>>> flush the cache. I wonder how big of a performance hit that'd be... > >>>>> > >>>>> On Thu, Nov 16, 2017 at 9:07 AM Jan Filipiak < > jan.filip...@trivago.com > >>>>> <mailto:jan.filip...@trivago.com>> wrote: > >>>>> > >>>>> Hi Trevor, > >>>>> > >>>>> I am leaning towards the less intrusive approach myself. Infact > >>>>> that is how we implemented our Internal API for this and how we > >>>>> run it in production. > >>>>> getting more voices towards this solution makes me really happy. > >>>>> The reason its a problem for Prefix and not for Range is the > >>>>> following. Imagine the intrusive approach. They key of the > RockDB > >>>>> would be CombinedKey<A,B> and the prefix scan would take an A, > and > >>>>> the range scan would take an CombinedKey<A,B> still. As you can > >>>>> see with the intrusive approach the keys are actually different > >>>>> types for different queries. With the less intrusive apporach we > >>>>> use the same type and rely on Serde Invariances. For us this > works > >>>>> nice (protobuf) might bite some JSON users. > >>>>> > >>>>> Hope it makes it clear > >>>>> > >>>>> Best Jan > >>>>> > >>>>> > >>>>> On 16.11.2017 16:39, Trevor Huey wrote: > >>>>> > >>>>>> 1. Going over KIP-213, I am leaning toward the "less intrusive" > >>>>>> approach. In my use case, I am planning on performing a > sequence > >>>>>> of several oneToMany joins, From my understanding, the more > >>>>>> intrusive approach would result in several nested levels of > >>>>>> CombinedKey's. For example, consider Tables A, B, C, D with > >>>>>> corresponding keys KA, KB, KC. Joining A and B would produce > >>>>>> CombinedKey<KA, KB>. Then joining that result on C would > produce > >>>>>> CombinedKey<KC, CombinedKey<KA, KB>>. My "keyOtherSerde" in > this > >>>>>> case would need to be capable of deserializing CombinedKey<KA, > >>>>>> KB>. This would just get worse the more tables I join. I > realize > >>>>>> that it's easier to shoot yourself in the foot with the less > >>>>>> intrusive approach, but as you said, " the user can stick with > >>>>>> his default serde or his standard way of serializing". In the > >>>>>> simplest case where the keys are just strings, they can do > simple > >>>>>> string concatenation and Serdes.String(). It also allows the > user > >>>>>> to create and use their own version of CombinedKey if they feel > >>>>>> so inclined. > >>>>>> > >>>>>> 2. Why is there a problem for prefix, but not for range? > >>>>>> https://github.com/apache/kafka/pull/3720/files#diff-8f863b7 > >>>>>> 4c3c5a0b989e89d00c149aef1L162 > >>>>>> > >>>>>> > >>>>>> > >>>>>> On Thu, Nov 16, 2017 at 2:57 AM Jan Filipiak > >>>>>> <jan.filip...@trivago.com <mailto:jan.filip...@trivago.com>> > >>>>>> wrote: > >>>>>> > >>>>>> Hi Trevor, > >>>>>> > >>>>>> thank you very much for your interested. Too keep > discussion > >>>>>> mailing list focused and not Jira or Confluence I decided > to > >>>>>> reply here. > >>>>>> > >>>>>> 1. its tricky activity is indeed very low. In the KIP-213 > >>>>>> there are 2 proposals about the return type of the join. I > >>>>>> would like to settle on one. > >>>>>> Unfortunatly its controversal and I don't want to have the > >>>>>> discussion after I settled on one way and implemented it. > But > >>>>>> noone is really interested. > >>>>>> So discussing with YOU, what your preferred return type > would > >>>>>> look would be very helpfull already. > >>>>>> > >>>>>> 2. > >>>>>> The most difficult part is implementing > >>>>>> this > >>>>>> https://github.com/apache/kafka/pull/3720/files#diff- > ac41b4d > >>>>>> fb9fc6bb707d966477317783cR68 > >>>>>> > >>>>>> here > >>>>>> https://github.com/apache/kafka/pull/3720/files#diff- > 8f863b7 > >>>>>> 4c3c5a0b989e89d00c149aef1R244 > >>>>>> > >>>>>> and here > >>>>>> https://github.com/apache/kafka/pull/3720/files#diff- > b1a1281 > >>>>>> dce5219fd0cb5afad380d9438R207 > >>>>>> > >>>>>> One can get an easy shot by just flushing the underlying > >>>>>> rocks and using Rocks for range scan. > >>>>>> But as you can see the implementation depends on the API. > For > >>>>>> wich way the API discussion goes > >>>>>> I would implement this differently. > >>>>>> > >>>>>> 3. > >>>>>> I only have so and so much time to work on this. I filed > the > >>>>>> KIP because I want to pull it through and I am pretty > >>>>>> confident that I can do it. > >>>>>> But I am still waiting for the full discussion to happen on > >>>>>> this. To get the discussion forward it seems to be that I > >>>>>> need to fill out the table in > >>>>>> the KIP entirly (the one describing the events, change > >>>>>> modifications and output). Feel free to continue the > >>>>>> discussion w/o the table. I want > >>>>>> to finish the table during next week. > >>>>>> > >>>>>> Best Jan thank you for your interest! > >>>>>> > >>>>>> _____ Jira Quote ______ > >>>>>> > >>>>>> Jan Filipiak > >>>>>> <https://issues.apache.org/jira/secure/ViewProfile.jspa? > name > >>>>>> =jfilipiak> > >>>>>> Please bear with me while I try to get caught up. I'm not > yet > >>>>>> familiar with the Kafka code base. I have a few questions > to > >>>>>> try to figure out how I can get involved: > >>>>>> 1. It seems like we need to get buy-in on your KIP-213? It > >>>>>> doesn't seem like there's been much activity on it besides > >>>>>> yourself in a while. What's your current plan of attack for > >>>>>> getting that approved? > >>>>>> 2. I know you said that the most difficult part is yet to > be > >>>>>> done. Is there some code you can point me toward so I can > >>>>>> start digging in and better understand why this is so > >>>>>> difficult? > >>>>>> 3. This issue has been open since May '16. How far out do > you > >>>>>> think we are from getting this implemented? > >>>>>> > >>>>>> > >>>> > >> > > > > > > -- -- Guozhang
