Thanks everyone for the great discussions so far! I first saw the JIRA and left some quick thoughts without being aware of the already-written KIP (kudos to Almog, very great one) and the DISCUSS thread here. And I happily find some of my initial thoughts align with the KIP already :)
Would like to add a bit more of my 2c after reading through the KIP and the thread here: 1. On the high level, I'm in favor of pushing this KIP through without waiting on the other gaps to be closed. In my back pocket's "dependency graph" of Kafka Streams roadmap of large changes or feature gaps, the edges of dependencies are defined based on my understanding of whether doing one first would largely complicate / negate the effort of the other but not vice versa, in which case we should consider getting the other done first. In this case, I feel such a dependency is not strong enough, so encouraging the KIP contributor to finish what he/she would love to do to close some gaps early would be higher priorities. I did not see by just doing this we could end up in a worse intermediate stage yet, but I could be corrected. 2. Regarding the store types --- gain here I'd like to just clarify the terms a bit since in the past it has some confusions: we used "impl types" (in hindsight it may not be a good name) for rocksdb / memory / custom, and we used "store types" for kv / windowed / sessioned etc, as I said in the JIRA I think the current proposal also have a good side effect as quality bar to really enforce us think twice when trying to add more store types in the future as it will impact API instantiations. In the ideal world, I would consider: * We have (timestamped) kv store, versioned kv store, window store, session store as first-class DSL store types. Some DSL operators could accept multiple store types (e.g. versioned and non versioned kv-store) for semantics / efficiency trade-offs. But I think we would remove un-timestamped kv stores eventually since that efficiency trade-off is so minimal compared to its usage limitations. * As for list-value store (for stream-stream Join), memory-lru-cache (for PAPI use only), memory-time-ordered-buffer (for suppression), they would not be exposed as DSL first-class store types in the future. Instead, they would be treated as internal used stores (e.g. list-value store is built on key-value store with specialized serde and putInternal), or continue to be just convenient OOTB PAPI used stores only. * As we move on, we will continue to be very, very strict on what would be added as DSL store types (and hence requires changes to the proposed APIs), what to be added as convenient OOTB PAPI store impls only, what to be added as internal used store types that should not be exposed to users nor customizable at all. 3. Some more detailed thoughts below: 3.a) I originally also think that we can extend the existing config, rather than replacing it. The difference was that I was thinking that order-wise, the runtime would look at the API first, and then the config, whereas in your rejected alternative it was looking at the config first, and then the API --- that I think is a minor thing and either is fine. I'm in agreement that having two configs would be more confusing to users to learn about their precedence rather than helpful, but if we keep both a config and a public API, then the precedence ordering would not be so confusing as long as we state them clearly. For example: * We have DefaultStoreTypeSpec OOTB, in that impl we look at the config only, and would only expect either ROCKS or MEMORY, and return corresponding OOTB store impls; if any other values configured, we error out. * Users extend that by having MyStoreTypeSpec, in which they could do arbituray things without respecting the config at all, but our recommended pattern in docs would still say "look into the config, if it is ROCKS or MEMORY just return fall back to DefaultStoreTypeSepc; otherwise if it's some String you recognize, then return your customized store based on the string value, otherwise error out". 3.b) About the struct-like Params classes, I like the idea a lot and wished we would pursue this in the first place, but if we only do this in Spec it would leave some inconsistencies with the StoreBuilders though arguably the latter is only for PAPI. I'm wondering if we should consider including the changes in StoreBuilders (e.g. WindowStoreBuilder(WindowSupplierParams)), and if yes, maybe we should also consider renaming that e.g. `WindowSupplierParams` to `WindowStoreSpecParams` too? For this one I only have a "weak feeling" so I can be convinced otherwise :P Thanks, Guozhang On Sun, Jul 23, 2023 at 9:52 AM Matthias J. Sax <mj...@apache.org> wrote: > > Thanks for all the input. My intention was not to block the KIP, but > just to take a step back and try get a holistic picture and discussion, > to explore if there are good/viable alternative designs. As said > originally, I really like to close this gap, and was always aware that > the current config is not flexible enough. > > > I guess, my "concern" is that the KIP does increase the API surface area > significantly, as we need all kind of `StoreTypeSpec` implementations, > and it might also imply that we need follow up KIPs for new feature > (like in-memory versioned store) that might not need a KIP otherwise. > > The second question is if it might make the already "patchy" situation > with regard to customization worse. > > We did de-scope the original KIP-591 for this reason, and given the new > situation of the DSL, it seems that it actually got worse compared to > back in the days. > > Lastly, I hope to make the new versioned stores the default in the DSL > and we did not do it in the previous KIP due to backward compatibility > issues. Thus, from a DSL point of view, I believe there should be only > "RocksDB", "InMemory", and "Custom" in an ideal world. Introducing (I am > exaggerating to highlight my point) "KvRocksDbSpec", > "TimestampeKvRocksDbSpec", "VersionedRocksDbSpec", plus the > corresponding in-memory specs seems to head into the opposite direction. > -- My goal is to give users a handle of the _physical_ store (RocksDB vs > InMemory vs Custom) but not the _logical_ stores (plain kv, ts-kv, > versioned) which is "dictated" by the DSL itself and should not be > customizable (we are just in a weird intermediate situation that we need > to clean up, but not "lean into" IMHO). > > Thus, I am also not sure if adding "VersionedRocksDbSpec" would be ideal > (also, given that it only changes a single store, but not the two > windowed stores)? > > Furthermore, I actually hope that we could use the new versioned store > to replace the window- and sessions- stores, and thus to decrease the > number of required store types. > > > Admittedly, I am talking a lot about a potential future, but the goal is > only to explore opportunities to not get into "worse" intermediate > state, that will require a huge deprecation surface area later on. Of > course, if there is no better way, and my concerns are not shared, I am > ok to move forward with the KIP. > > > Bottom line: I would personally prefer to keep the current config and > add a `Custom` option to it, plus adding one new config that allows > people to set their custom `StoreTypeSpec` class. -- I would not add a > built-in spec for versioned stores at this point (or any other built-in > `StorytypeSpec` implementations). I guess, users could write a custom > spec if they want to enable versioned store across the board for now > (until we make them the default anyway)? > > > Hope my train of thoughts is halfway reasonable and not totally off track? > > > -Matthias > > On 7/21/23 15:27, Sophie Blee-Goldman wrote: > > I agree with everything Almog said above, and will just add on to two > > points: > > > > 1. Regarding whether to block this KIP on the completion of any or all > > future implementations of in-memory version stores (or persist suppression > > buffers), I feel that would be unfair to this feature which is completely > > unrelated to the semantic improvements offered by versioned state stores. > > It seems like the responsibility of those driving the versioned state > > stores feature, not Almog/this KIP, to make sure that those bases are > > covered. Further, if anything, this KIP will help with the massive > > proliferation of StoreSuppliers on the Stores factory class, and provide > > users with a much easier way to leverage the versioned stores without > > having to muck around directly with the StoreSuppliers. > > > > I also thought about it a bit, and really like Almog's suggestion to > > introduce an additional StoreSpec for the Versioned state stores. Obviously > > we can add the RocksDB one to this KIP now, and then as he mentioned, > > there's an easy way to get users onto the IMVersionedStateStore types once > > they are completed. > > > > Lastly, on this note, I want to point out how smoothly this solved the > > issue of timestamped stores, which are intended to be the DSL default but > > are only a special case for RocksDB. Right now it can be confusing for a > > user scrolling through the endless Stores class and seeing a timestamped > > version of the persistent but not in-memory stores. One could easily assume > > there was no timestamped option for IM stores and that this feature was > > incomplete, if they weren't acutely aware of the internal implementation > > details (namely that it's only required for RocksDB for compatibility > > reasons). However, with this KIP, all that is handled completely > > transparently to the user, and we the devs, who *are* aware of the internal > > implementation details, are now appropriately the ones responsible for > > handing the correct store type to a particular operator. While versioned > > state stores may not be completely comparable, depending on whether we want > > users to remain able to easily choose between using them or not (vs > > timestamped which should be used by all), I still feel this KIP is a great > > step in the right direction that not only should not be blocked on the > > completion of the IM implementations, but in fact should specifically be > > done first as it enables an easier way to utilize those IM versioned > > stores. Just my 2 cents :) > > > > 2. The idea to expand the existing the config with a CUSTOM enum without > > introducing another config to specify the CUSTOM store spec does not seem > > appropriate, or even possible (for the reasons Almog mentioned above about > > config types, though perhaps there is a way I'm not seeing). I do not buy > > the argument that we should optimize the API to make it easy for users who > > just want to switch to all in-memory stores, as I truly believe this is a > > very small fraction of the potential userbase of this feature (anyone who's > > actually using this should please chime in!). It seems very likely that the > > majority of users of this feature are actually those with custom state > > stores, as to my knowledge, this has been the case any/every time this > > feature was requested by users. > > > > That said, while I don't see any way to get around introducing a new > > config, I don't personally have a preference w.r.t whether to keep around > > the old config or deprecate it. I simply don't get the impression it is > > very heavily used as it stands today, while it only works for those who > > want all in-memory stores. Again, input from actual users would be very > > valuable here. In the absence of that data, I will just point to the fact > > that this KIP was proposed by a Streams dev (you :P), abandoned, picked up > > by another Streams dev, and finally implemented without ever hearing from a > > user that they would find this useful. This is not to disparage the > > original KIP, but just to say again, as I stated back then, it seemed like > > a major opportunity loss to leave out custom state stores > > > > Cheers, > > Sophie > > > > On Fri, Jul 21, 2023 at 1:52 PM Almog Gavra <almog.ga...@gmail.com> wrote: > > > >> Thanks for all the feedback folk! Responses inline. > >> > >>> Basically, I'm suggesting two things: first, call out in some way > >> (perhaps the StoreTypeSpec javadocs) that each StoreTypeSpec is considered > >> a public contract in itself and should outline any semantic guarantees it > >> does, or does not, make. Second, we should add a note on ordering > >> guarantees in the two OOTB specs: for RocksDB we assert that range queries > >> will honor serialized byte ordering, whereas the InMemory flavor gives no > >> ordering guarantee whatsoever at this time. > >> > >> That makes sense to me Sophie! I'll make the changes to the KIP. And @Colt, > >> yes I believe that would be the new javadoc for the generic > >> ReadOnlyKeyValueStore. > >> > >>> However, I am wondering if we should close others gaps first? > >> > >> @Matthias, thanks for the review and thoughts! I think we should separate > >> closing other gaps in the product from providing this as useful > >> functionality to avoid feature creep so long as the API proposed here will > >> be suitable for when we want to close those implementation gaps! My general > >> proposal is that for things that are not customizable today by > >> default.dsl.store they remain not customizable after this KIP. The good > >> news is, however, that there's no reason why this cannot be extended to > >> cover those in the future if we want to - see specifics below. > >> > >> Comments on the specifics below > >> > >>> In particular, this holds for the new versioned-store ... Should > >> versioned stores also be covered by the KIP > >> > >> Is there a reason why we can't introduce a VersionedRocksDBStoreTypeSpec > >> and if we ever support an in-memory an equivalent > >> VersionedInMemoryRocksDBStoreTypeSpec? If so, then there would not need to > >> be any additional changes to the API proposed in this KIP. > >> > >>> For `suppress()` it's actually other way around we only have an in-memory > >> implementation. Do you aim to allow custom stores for `suppress()`, too? > >> > >> We have three options here: > >> 1) we can decide to maintain existing behavior and use the in-memory > >> implementation for all stores (not even going through the API at all) > >> 2a) we can introduce a new parameter to the KeyValueParams class (boolean > >> isTimeOrderedBuffer or something like that) and return an in-memory store > >> in the implementation of RocksDBStoreTypeSpec (this maintains the existing > >> behavior, and would allow us in the future to make the change to return a > >> RocksDB store if we ever provide one) > >> 2b) same as 2a but we throw an exception if the requested store type does > >> not support that (this is backwards incompatible, and since ROCKS_DB is the > >> default we probably shouldn't do this) > >> > >> My proposal for now is 1) because as of KIP-825 > >> EmitStrategy#ON_WINDOW_CLOSE is the preferred way of suppressing and that > >> is accounted for in this API already. > >> > >>> Last, I am not sure if the new parameter replacing the existing one is > >> the > >> best way to go? > >> > >> I'm happy either way, just let me know which you prefer - the discussion > >> around CUSTOM is in the rejected alternatives but I'm happy to differ to > >> whatever the project conventions are :) > >> > >>> If it's matches existing `ROCKS_DB` or `IN_MEMORY` we just process it as > >> we > >> do know, and if know we assume it's a fully qualified class name and try to > >> instantiate it? > >> > >> Note that there is no functionality for this kind of thing in > >> AbstractConfig (it's either a String validated enum or a class) so this > >> would be a departure from convention. Again, I'm happy to implement that if > >> it's preferred. > >> > >>> Also wondering how it would related to the existing `Stores` factory? > >> > >> StoreTypeSpec will depend on Stores factory - they're one layer removed. > >> You can imagine that StoreTypeSpec is just a grouping of methods from the > >> Stores factory into a convenient package for default configuration > >> purposes. > >> > >> Thanks again for all the detailed thoughts Matthias! > >> > >> On Fri, Jul 21, 2023 at 11:50 AM Matthias J. Sax <mj...@apache.org> wrote: > >> > >>> Thanks for the KIP. Overall I like the idea to close this gap. > >>> > >>> However, I am wondering if we should close others gaps first? In > >>> particular, IIRC, we have a few cases for which we only have a RocksDB > >>> implementation for a store, and thus, adding an in-memory version for > >>> these stores first, to make the current `IN_MEMORY` parameter work, > >>> might be the first step? > >>> > >>> In particular, this holds for the new versioned-store (but I actually > >>> believe the is some other internal store with no in-memory > >>> implementation). -- For `suppress()` it's actually other way around we > >>> we only have an in-memory implementation. Do you aim to allow custom > >>> stores for `suppress()`, too? > >>> > >>> Btw: Should versioned stores also be covered by the KIP (ie, > >>> `StoreTypeSpec`)? We did consider to add a new option `VERSIONED` to the > >>> existing `default.dsl.store` config, but opted out for various reasons. > >>> > >>> Last, I am not sure if the new parameter replacing the existing one is > >>> the best way to go? Did you put the idea to add `CUSTOM` to the existing > >>> config into rejected alternative. Personally, I would prefer to add > >>> `CUSTOM` as I would like to optimize to easy of use for the majority of > >>> users (which don't implement a custom store), but only switch to > >>> in-memory sometimes. -- As an alternative, you would also just extend > >>> `dsl.default.store` (it's just a String) and allow to pass in anything. > >>> If it's matches existing `ROCKS_DB` or `IN_MEMORY` we just process it as > >>> we do know, and if know we assume it's a fully qualified class name and > >>> try to instantiate it? -- Given that we plan to keep the store-enum, is > >>> seems cleaner to keep the existing config and keep both the config and > >>> enum aligned to each other? > >>> > >>> > >>> It's just preliminary thought. I will need to go back an take a more > >>> detailed look into the code to grok how the propose `StoreTypeSpec` > >>> falls into place. Also wondering how it would related to the existing > >>> `Stores` factory? > >>> > >>> -Matthias > >>> > >>> > >>> On 7/21/23 6:45 AM, Colt McNealy wrote: > >>>> Sophie— > >>>> > >>>> Thanks for chiming in here. +1 to the idea of specifying the ordering > >>>> guarantees that we make in the StorageTypeSpec javadocs. > >>>> > >>>> Quick question then. Is the javadoc that says: > >>>> > >>>>> Order is not guaranteed as bytes lexicographical ordering might not > >>>> represent key order. > >>>> > >>>> no longer correct, and should say: > >>>> > >>>>> Order guarantees depend on the underlying implementation of the > >>>> ReadOnlyKeyValueStore. For more information, please consult the > >>>> [StorageTypeSpec javadocs](....) > >>>> > >>>> Thanks, > >>>> Colt McNealy > >>>> > >>>> *Founder, LittleHorse.dev* > >>>> > >>>> > >>>> On Thu, Jul 20, 2023 at 9:28 PM Sophie Blee-Goldman < > >>> ableegold...@gmail.com> > >>>> wrote: > >>>> > >>>>> Hey Almog, first off, thanks for the KIP! I (and others) raised > >> concerns > >>>>> over how restrictive the default.dsl.store config would be if not > >>>>> extendable to custom store types, especially given that this seems to > >> be > >>>>> the primary userbase of such a feature. At the time we didn't really > >>> have > >>>>> any better ideas for a clean way to achieve that, but what you > >> proposed > >>>>> makes a lot of sense to me. Happy to see a good solution to this, and > >>>>> hopefully others will share my satisfaction :P > >>>>> > >>>>> I did have one quick piece of feedback which arose from an unrelated > >>>>> question posed to the dev mailing list w/ subject line > >>>>> "ReadOnlyKeyValueStore#range() > >>>>> Semantics" > >>>>> <https://lists.apache.org/thread/jbckmth8d3mcgg0rd670cpvsgwzqlwrm>. I > >>>>> recommend checking out the full thread for context, but it made me > >> think > >>>>> about how we can leverage the new StoreTypeSpec concept as an answer > >> to > >>> the > >>>>> long-standing question in Streams: where can we put guarantees of the > >>>>> public contract for RocksDB (or other store implementations) when all > >>> the > >>>>> RocksDB stuff is technically internal. > >>>>> > >>>>> Basically, I'm suggesting two things: first, call out in some way > >>> (perhaps > >>>>> the StoreTypeSpec javadocs) that each StoreTypeSpec is considered a > >>> public > >>>>> contract in itself and should outline any semantic guarantees it does, > >>> or > >>>>> does not, make. Second, we should add a note on ordering guarantees in > >>> the > >>>>> two OOTB specs: for RocksDB we assert that range queries will honor > >>>>> serialized byte ordering, whereas the InMemory flavor gives no > >> ordering > >>>>> guarantee whatsoever at this time. > >>>>> > >>>>> Thoughts? > >>>>> > >>>>> -Sophie > >>>>> > >>>>> On Thu, Jul 20, 2023 at 4:28 PM Almog Gavra <almog.ga...@gmail.com> > >>> wrote: > >>>>> > >>>>>> Hi All, > >>>>>> > >>>>>> I would like to propose a KIP to expand support for default store > >> types > >>>>>> (KIP-591) to encompass custom store implementations: > >>>>>> > >>>>>> > >>>>> > >>> > >> https://cwiki.apache.org/confluence/display/KAFKA/KIP-954%3A+expand+default+DSL+store+configuration+to+custom+types > >>>>>> > >>>>>> Looking forward to your feedback! > >>>>>> > >>>>>> Cheers, > >>>>>> Almog > >>>>>> > >>>>> > >>>> > >>> > >> > >
