Thanks for the discussion, Bruno, Sagar, and Matthias! It seems we've reached consensus on almost all of the discussion points. I've updated the KIP with the following: 1) renamed "timestampTo" in `get(key, timestampTo)` to "asOfTimestamp" to clarify that this timestamp bound is inclusive, per the SQL guideline that "AS OF <timestamp>" queries are inclusive. In the future, if we want to introduce a timestamp range query, we can use `get(key, timestampFrom, timestampTo)` and specify that timestampTo is exclusive in this method, while avoiding confusing with the inclusive asOfTimestamp parameter in the other method, given that the names are different. 2) added a description of "history retention" semantics into the VersionedKeyValueStore interface Javadoc, and updated the Javadoc for `get(key, asOfTimestamp)` to mention explicitly that a null result is returned if the provided timestamp bound is not within history retention. 3) added a `delete(key, timestamp)` method (with return type `ValueAndTimestamp<V>`) to the VersionedKeyValueStore interface. 4) updated the Javadoc for `segmentInterval` to clarify that the only reason a user might be interested in this parameter is performance.
Other points we discussed which did not result in updates include: 5) whether to automatically update the `min.compaction.lag.ms` config on changelog topics when history retention is changed -- there's support for this but let's not bundle it with this KIP. We can have a separate KIP to change this behavior for the existing windowed changelog topics, in addition to versioned changelog topics. 6) should we expose segmentInterval in this KIP -- let's go ahead and expose it now since we'll almost certainly expose it (in this same manner) in a follow-on KIP anyway, and so that poor performance for user workloads is less likely to be a barrier for users getting started with this feature. I updated the Javadoc for this parameter to clarify why the Javadoc mentions performance despite Javadocs typically not doing so. 7) `get(timestampFrom, timestampTo)` and other methods for IQ -- very important but deferred to a future KIP 8) `purge(key)`/`deleteAllVersions(key)` -- deferred to a future KIP That leaves only one unresolved discussion point: 9) whether to include validTo in the return types from `get(...)`. If we go with the current proposal of not including validTo in the return type, then it will not be easy to add it in the future (unless we want to add validTo to ValueAndTimestamp, which feels odd to me). If we think we might want to have validTo in the future, we can change the return type of `get(...)` and `delete(...)` in this proposal from `ValueAndTimestamp<V>` to a new type, e.g., `VersionedRecord<V>` or `RecordVersion<V>`, which today will look the same as `ValueAndTimestamp<V>` but in the future we can add validTo if we want. The cost is a new type which today looks the same as ValueAndTimestamp. Now that I think about it more, the cost to introducing a new type seems relatively low. I've added a proposal towards the bottom of the KIP here <https://cwiki.apache.org/confluence/display/KAFKA/KIP-889%3A+Versioned+State+Stores#KIP889:VersionedStateStores-Additionalreturntimestampsfromget(key,asOfTimestamp)>. If others also believe that the cost of introducing this new interface is low (particularly relative to the flexibility it provides us for being able to evolve the class in the future), I will incorporate this proposal into the KIP. I think the hardest part of this will be deciding on a name for the new class :) Pending objections, I'd like to make a call on item (9) and call a vote on this KIP at the end of this week. Thanks, Victoria On Thu, Dec 1, 2022 at 9:47 PM Matthias J. Sax <mj...@apache.org> wrote: > Thanks Victoria! > > (1) About `ReadOnlyVersionedKeyValueStore` -- I am not sure about IQv1 > vs IQv2. But you might be right that adding the interface later might > not be an issue -- so it does not matter. Just wanted to double check. > > > > (2) About `delete(key, ts)` -- as already discussed, I agree that it > should have same semantics as `put(key, null, ts)` (delete() needs a > timestamp). Not sure if `delete()` really needs to return anything? I > would be ok to make it `void` -- but I think it's also semantically > sound if it returns the "old" value at timestamps `ts` that the delete > actually deleted, as you mentioned -- in the end, an "delete" is a > physical append anyway (ie, "soft delete") as we want to track history. > > > > (3) > > Ah, great question. I think the question boils down to: do we want to > > require that all versioned stores (including custom user implementations) > > use "history retention" to determine when to expire old record versions? > > I personally think, yes. The main reason for this is, that I think we > need to have a clear contract so we can plug-in custom implementations > into the DSL later? -- I guess, having a stricter contract initially, > and relaxing it later if necessary, is the easier was forward, than the > other way around. > > For PAPI users, they are not bound to implement the interface anyway and > can just add any store they like by extending the top level `StateStore` > interface. > > > > (4) About `segmentInterval`: I am personally fine both ways. Seems it's > your call to expose it or not. It seems there is a slight preference to > expose it. > > > > (5) About `validTo`: based on my experience, it's usually simpler to > have it exclusive. It's also how it's defined in "system versioned > temporal tables" in the SQL standard, and how `AS OF <ts>` queries work. > > For a join, it of course implies that if a table record has [100,200) as > inclusive `validFrom=100` and exclusive `validTo=200` it would only join > with a stream-side record with 100 <= ts <= 199 (or 100 <= ts < 200 :)). > > I would strongly advocate to make the upper bound exclusive (it did > serve us well in the past to align to SQL semantics). It must be clearly > documented of course and we can also name variable accordingly if > necessary. > > > > (6) About including `validTo` in return types -- it's not easy to change > the return type, because the signature of a method is only determined by > it's name in input parameter types, ie, we cannot overload an existing > method to just change the return type, but would need to change its name > or parameter list... Not sure if we can or cannot add `validTo` to > `ValueAndTimestamp` though, but it's a tricky question. Would be good to > get some more input from other if we think that it would be important > enough to worry about it now or not. > > > > (7) About `get(k)` vs `get(k, ts)` vs `getAsOf(k, ts)`: I would prefer > to just keep `get()` with two overloads and not add `getAsOf()`; the > fact that we pass in a timestamp implies we have a point in time query. > (It's cleaner API design to leverage method overloads IMHO, and it's > what we did in the past). Of course, we can name the parameter `get(key, > asOfTimestamp)` if we think it's helpful. And in alignment to have > `validTo` exclusive, `validTo` would be `asOfTimestampe+1` (or larger), > in case we return it. > > > > (8) About updating topic config (ie, history retention and compaction > lag): It think it was actually some oversight to not update topic > configs if the code changes. There is actually a Jira ticket about it. I > would prefer to keep the behavior consistent though and not change it > just for the new versioned-store, but change it globally in one shot > independent of this KIP. > > > -Matthias > > > > On 12/1/22 10:15 AM, Sagar wrote: > > Thanks Victoria, > > > > I guess an advantage of exposing a method like delete(key, timestamp) > could > > be that from a user's standpoint, it is a single operation and not 2. The > > equivalent of this method i.e put followed by get is not atomic so > exposing > > it certainly sounds like a good idea. > > > > Thanks! > > Sagar. > > > > On Tue, Nov 29, 2022 at 1:15 AM Victoria Xia > > <victoria....@confluent.io.invalid> wrote: > > > >> Thanks, Sagar and Bruno, for your insights and comments! > >> > >>> Sagar: Can we name according to the semantics that you want to > >> support like `getAsOf` or something like that? I am not sure if we do > that > >> in our codebase though. Maybe the experts can chime in. > >> > >> Because it is a new method that will be added, we should be able to > name it > >> whatever we like. I agree `getAsOf` is more clear, albeit wordier. > >> Introducing `getAsOf(key, timestamp)` means we could leave open > `get(key, > >> timeFrom, timeTo)` to have an exclusive `timeTo` without introducing a > >> collision. (We could introduce `getBetween(key, timeFrom, timeTo)` > instead > >> to delineate even more clearly, though this is better left for a future > >> KIP.) > >> > >> I don't think there's any existing precedent in codebase to follow here > but > >> I'll leave that to the experts. Curious to hear what others prefer as > well. > >> > >>> Sagar: With delete, we would stlll keep the older versions of the key > >> right? > >> > >> We could certainly choose this for the semantics of delete(...) -- and > it > >> sounds like we should too, based on Bruno's confirmation below that this > >> feels more natural to him as well -- but as Bruno noted in his message > >> below I think we'll want the method signature to be `delete(key, > >> timestamp)` then, so that there is an explicit timestamp to associate > with > >> the deletion. In other words, `delete(key, timestamp)` has the same > effect > >> as `put(key, null, timestamp)`. The only difference is that the > `put(...)` > >> method has a `void` return type, while `delete(key, timestamp)` can have > >> `ValueAndTimestamp` as return type in order to return the record which > is > >> replaced (if any). In other words, `delete(key, timestamp)` is > equivalent > >> to `put(key, null, timestamp)` followed by `get(key, timestamp)`. > >> > >>> Bruno: I would also not change the semantics so that it deletes all > >> versions of > >> a key. I would rather add a new method purge(key) or > >> deleteAllVersions(key) or similar if we want to have such a method in > >> this first KIP. > >> > >> Makes sense; I'm convinced. Let's defer > >> `purge(key)`/`deleteAllVersions(key)` to a future KIP. If there's > agreement > >> that `delete(key, timestamp)` (as described above) is valuable, we can > keep > >> it in this first KIP even though it is syntactic sugar. If this turns > into > >> a larger discussion, we can defer this to a future KIP as well. > >> > >>> Bruno: I would treat the history retention as a strict limit. [...] You > >> could also add historyRetentionMs() to the VersionedKeyValueStore<K, V> > >> interface to make the concept of the history retention part of the > >> interface. > >> > >> OK. That's the second vote for rewording the javadoc for > >> `VersionedKeyValueStore#get(key, timestampTo)` to remove the > parenthetical > >> and clarify that history retention should be used to dictate this case, > so > >> I'll go ahead and do that. I'll leave out adding `historyRetentionMs()` > to > >> the interface for now, though, for the sake of consistency with other > >> stores (e.g., window stores) which don't expose similar types of > >> configurations from their interfaces. > >> > >>> Bruno: exclusive vs inclusive regarding validTo timestamp in get(). > >> Doesn't this decision depend on the semantics of the join for which this > >> state store should be used? > >> > >> Yes, you are correct. As a user I would expect that a stream-side record > >> with the same timestamp as a table-side record _would_ produce a join > >> result, which is consistent with the proposal for timestampTo to be > >> inclusive. (FWIW I tried this out with a Flink temporal join just now > and > >> observed this result as well. Not sure where to look for other > standards to > >> validate this expectation.) > >> > >>> Bruno: If Streams does not update min.compaction.lag.ms during > >> rebalances, > >> users have to do it each time they change history retention in the code, > >> right? That seems odd to me. What is the actual reason for not updating > >> the config? How does Streams handle updates to windowed stores? > >> > >> Yes, users will have to update min.compaction.lag.ms for the changelog > >> topic themselves if they update history retention in their code. This is > >> consistent with what happens for window stores today: e.g., if a user > >> updates grace period for a windowed aggregation, then they are > responsible > >> for updating retention.ms on their windowed changelog topic as well. > >> > >> I'm not familiar with the historical context around why this is the > case -- > >> Matthias, do you know? > >> > >> My best guess is that Streams does not want to interfere with any > potential > >> out-of-band changes by the user between application restarts, though I'm > >> not sure why a user would want to change this specific config to a value > >> which does not accord with the specified history retention. I notice > that > >> there is code for validating topic configs and collecting validation > errors > >> ( > >> > >> > https://github.com/apache/kafka/blob/be032735b39360df1a6de1a7feea8b4336e5bcc0/streams/src/main/java/org/apache/kafka/streams/processor/internals/InternalTopicManager.java#L318-L319 > >> ) > >> but this method is not called from anywhere, even though there are unit > >> tests for it. I was unable to find history of this validation after a > quick > >> search. Hopefully Matthias (or others) has context, otherwise I will > have a > >> closer look. > >> > >> - Victoria > >> > >> On Wed, Nov 23, 2022 at 8:52 AM Bruno Cadonna <cado...@apache.org> > wrote: > >> > >>> Hi all, > >>> > >>> Thanks for the KIP, Victoria! > >>> > >>> I have a couple of comments. > >>> > >>> 1. delete(key) > >>> I think delete(key) should not remove all versions of a key. We should > >>> use it to close the validity interval of the last version. > >>> Assuming we have records of different versions for key A: > >>> (A, e, 0, 2), > >>> (A, f, 2, 3), > >>> (A, g, 3, MAX) > >>> > >>> delete(A) would update them to > >>> > >>> (A, e, 0, 2), > >>> (A, f, 2, 3), > >>> (A, g, 3, 5) > >>> (A, null, 5, MAX) > >>> > >>> But then the question arises where does timestamp 5 that closes the > >>> interval in (A, g, 3, 5) and opens the interval in (A, null, 5, MAX) > >>> come from. We could use the timestamp at which delete(A) is called, but > >>> actually I do not like that because it seems to me it opens the doors > to > >>> non-determinism. If we use event time for put() we should also use it > >>> for delete(). Actually, put(A, null, 5) would have the same effect as > >>> delete(A) in the example above. As a syntactical sugar, we could add > >>> delete(key, validFrom). (I just realized now that I just repeated what > >>> Victoria said in her previous e-mail.) > >>> I agree with Victoria that delete(A) as defined for other state stores > >>> is hard to re-use in the versioned key-value store. > >>> I would also not change the semantics so that it deletes all versions > of > >>> a key. I would rather add a new method purge(key) or > >>> deleteAllVersions(key) or similar if we want to have such a method in > >>> this first KIP. > >>> > >>> > >>> 2. history retention > >>> I would remove "(up to store implementation discretion when this is the > >>> case)". I would treat the history retention as a strict limit. If users > >>> want to implement a less strict behavior, they can still do it. Maybe > >>> mention in the javadocs the implications of not adhering strictly to > the > >>> history retention. That is, the DSL might become non-deterministic. You > >>> could also add historyRetentionMs() to the VersionedKeyValueStore<K, V> > >>> interface to make the concept of the history retention part of the > >>> interface. > >>> > >>> 3. null vs. exception for out-of-bound queries > >>> I am in favor of null. The record version is not there anymore because > >>> it expired. This seems to me normal and nothing exceptional. That would > >>> also consistent with the behavior of other APIs as already mentioned. > >>> > >>> > >>> 4. Exposing segmentInterval > >>> Since we have evidence that the segment interval affects performance, I > >>> would expose it. But I find it also OK to expose it once we have a > >>> corresponding metric. > >>> > >>> 5. exclusive vs inclusive regarding validTo timestamp in get() > >>> Doesn't this decision depend on the semantics of the join for which > this > >>> state store should be used? Should a record on the table side that has > >>> the same timestamp as the record on the stream side join? Or should > only > >>> records in the table that are strictly before the record on the stream > >>> side join? > >>> > >>> > >>> 6. Not setting min.compaction.lag.ms during rebalances > >>> If Streams does not update min.compaction.lag.ms during rebalances, > >>> users have to do it each time they change history retention in the > code, > >>> right? That seems odd to me. What is the actual reason for not updating > >>> the config? How does Streams handle updates to windowed stores? That > >>> should be a similar situation for the retention time config of the > >>> changelog topic. > >>> > >>> > >>> Best, > >>> Bruno > >>> > >>> > >>> > >>> On 23.11.22 09:11, Sagar wrote: > >>>> Hi Vicky, > >>>> > >>>> Thanks for your response! > >>>> > >>>> I would just use numbers to refer to your comments. > >>>> > >>>> 1) Thanks for your response. Even I am not totally sure whether these > >>>> should be supported via IQv2 or via store interface. That said, I > >>> wouldn't > >>>> definitely qualify this as blocking the KIP for sure so we can live > >>>> without it :) > >>>> > >>>> 2) Yeah if the 2 APIs for get have different semantics for > timestampTo, > >>>> then it could be confusing. I went through the link for temporal > tables > >>>> (TFS!) and I now get why the AS OF semantics would have it inclusive. > I > >>>> think part of the problem is that the name get on it's own is not as > >>>> expressive as SQL. Can we name according to the semantics that you > want > >>> to > >>>> support like `getAsOf` or something like that? I am not sure if we do > >>> that > >>>> in our codebase though. Maybe the experts can chime in. > >>>> > >>>> 3) hmm I would have named it `validUpto` But again not very picky > about > >>> it. > >>>> After going through the link and your KIP, it's a lot clearer to me. > >>>> > >>>> 4) I think delete(key) should be sufficient. With delete, we would > >>>> stlll keep the older versions of the key right? > >>>> > >>>> Thanks! > >>>> Sagar. > >>>> > >>>> On Wed, Nov 23, 2022 at 12:17 AM Victoria Xia > >>>> <victoria....@confluent.io.invalid> wrote: > >>>> > >>>>> Thanks, Matthias and Sagar, for your comments! I've responded here > for > >>> now, > >>>>> and will update the KIP afterwards with the outcome of our > discussions > >>> as > >>>>> they resolve. > >>>>> > >>>>> ----------- Matthias's comments ----------- > >>>>> > >>>>>> (1) Why does the new store not extend KeyValueStore, but StateStore? > >>>>> In the end, it's a KeyValueStore? > >>>>> > >>>>> A `VersionedKeyValueStore<K, V>` is not a `KeyValueStore<K, V>` > >> because > >>>>> many of the KeyValueStore methods would not make sense for a > versioned > >>>>> store. For example, `put(K key, V value)` is not meaningful for a > >>> versioned > >>>>> store because the record needs a timestamp associated with it. > >>>>> > >>>>> A `VersionedKeyValueStore<K, V>` is more similar to a > >> `KeyValueStore<K, > >>>>> ValueAndTimestamp<V>>` (i.e., `TimestampedKeyValueStore<K, V>`), but > >>> some > >>>>> of the TimestampedKeyValueStore methods are still problematic. For > >>> example, > >>>>> what does it mean for `delete(K key)` to have return type > >>>>> `ValueAndTimestamp<V>`? Does this mean that `delete(K key)` only > >> deletes > >>>>> (and returns) the latest record version for the key? Probably we want > >> a > >>>>> versioned store to have `delete(K key)` delete all record versions > for > >>> the > >>>>> given key, in which case the return type is better suited as an > >>>>> iterator/collection of KeyValueTimestamp. `putIfAbsent(K key, > >>>>> ValueAndTimestamp value)` also has ambiguous semantics for versioned > >>> stores > >>>>> (i.e., what does it mean for the key/record to be "absent"). > >>>>> > >>>>> I agree that conceptually a versioned key-value store is just a > >>> key-value > >>>>> store, though. In the future if we redesign the store interfaces, > it'd > >>> be > >>>>> great to unify them by having a more generic KeyValueStore interface > >>> that > >>>>> allows for extra flexibility to support different types of key-value > >>>>> stores, including versioned stores. (Or, if you can think of a way to > >>>>> achieve this with the existing interfaces today, I'm all ears!) > >>>>> > >>>>>> (2) Should we have a ReadOnlyVersionedKeyValueStore? Even if we > don't > >>>>> want to support IQ in this KIP, it might be good to add this > interface > >>>>> right away to avoid complications for follow up KIPs? Or won't there > >> by > >>>>> any complications anyway? > >>>>> > >>>>> I don't think there will be complications for refactoring to add this > >>>>> interface in the future. Refactoring out > >> ReadOnlyVersionedKeyValueStore > >>>>> from VersionedKeyValueStore would leave VersionedKeyValueStore > >> unchanged > >>>>> from the outside. > >>>>> > >>>>> Also, is it true that the ReadOnlyKeyValueStore interface is only > used > >>> for > >>>>> IQv1 and not IQv2? I think it's an open question as to whether we > >> should > >>>>> support IQv1 for versioned stores or only IQv2. If the latter, then > >>> maybe > >>>>> we won't need the extra interface at all. > >>>>> > >>>>>> (3) Why do we not have a `delete(key)` method? I am ok with not > >>>>> supporting all methods from existing KV-store, but a `delete(key)` > >> seems > >>>>> to be fundamentally to have? > >>>>> > >>>>> What do you think the semantics of `delete(key)` should be for > >> versioned > >>>>> stores? Should `delete(key)` delete (and return) all record versions > >> for > >>>>> the key? Or should we have `delete(key, timestamp)` which is > >> equivalent > >>> to > >>>>> `put(key, null, timestamp)` except with a return type to return > >>>>> ValueAndTimestamp representing the record it replaced? > >>>>> > >>>>> If we have ready alignment on what the interface and semantics for > >>>>> `delete(key)` should be, then adding it in this KIP sounds good. I > >> just > >>>>> didn't want the rest of the KIP to be hung up over additional > >>> interfaces, > >>>>> given that we can always add extra interfaces in the future. > >>>>> > >>>>>> (4a) Do we need `get(key)`? It seems to be the same as `get(key, > >>>>> MAX_VALUE)`? Maybe is good to have as syntactic sugar though? Just > for > >>>>> my own clarification (should we add something to the JavaDocs?). > >>>>> > >>>>> Correct, it is just syntactic sugar. I will add a clarification into > >> the > >>>>> Javadocs as you've suggested. > >>>>> > >>>>>> (4b) Should we throw an exception if a user queries out-of-bound > >>>>> instead of returning `null` (in `get(key,ts)`)? > >>>>> -> You put it into "rejected alternatives", and I understand > your > >>>>> argument. Would love to get input from others about this question > >>>>> though. -- It seems we also return `null` for windowed stores, so > >> maybe > >>>>> the strongest argument is to align to existing behavior? Or do we > have > >>>>> case for which the current behavior is problematic? > >>>>> > >>>>> Sure; curious to hear what others think as well. > >>>>> > >>>>>> (4c) JavaDoc on `get(key,ts)` says: "(up to store implementation > >>>>> discretion when this is the case)" -> Should we make it a stricter > >>>>> contract such that the user can reason about it better (there is WIP > >> to > >>>>> make retention time a strict bound for windowed stores atm) > >>>>> -> JavaDocs on `persistentVersionedKeyValueStore` seems to > >> suggest a > >>>>> strict bound, too. > >>>>> > >>>>> Ah, great question. I think the question boils down to: do we want to > >>>>> require that all versioned stores (including custom user > >>> implementations) > >>>>> use "history retention" to determine when to expire old record > >> versions? > >>>>> > >>>>> Because the `persistentVersionedKeyValueStore(...)` method returns > >>>>> instances of the provided RocksDB-based versioned store > >> implementation, > >>>>> which does use history retention for this purpose, that's why we can > >>> very > >>>>> clearly say that for this store, `get(key, ts)` will return null if > >> the > >>>>> provided timestamp bound has fallen out of history retention. The > >>> reason I > >>>>> left the `VersionedKeyValueStore#get(key, ts)` Javadoc more generic > >>> (i.e., > >>>>> does not mention history retention) is because maybe a user > >> implementing > >>>>> their own custom store will choose a different expiry mechanism, > e.g., > >>> keep > >>>>> the three latest versions for each key regardless of how old the > >>> timestamps > >>>>> are. > >>>>> > >>>>> If we want to require that all versioned stores use history retention > >> in > >>>>> order to determine when to expire old records, then I will certainly > >>> update > >>>>> the Javadoc to clarify. This is already a requirement for DSL users > >>> because > >>>>> the VersionedBytesStoreSupplier interface requires history retention > >> to > >>> be > >>>>> provided (in order for changelog topic configs to be properly set), > so > >>> it's > >>>>> just a question of whether we also want to require PAPI users to use > >>>>> history retention too. I had a look at the existing window stores and > >>>>> didn't see precedent for requiring all window stores have a standard > >>>>> "retention time" concept for how long to keep windows, but if we want > >> to > >>>>> have a standard "history retention" concept for versioned stores we > >>>>> certainly can. WDYT? > >>>>> > >>>>>> (5a) Do we need to expose `segmentInterval`? For windowed-stores, we > >>>>> also use segments but hard-code it to two (it was exposed in earlier > >>>>> versions but it seems not useful, even if we would be open to expose > >> it > >>>>> again if there is user demand). > >>>>> > >>>>> If we want to leave it out of this first KIP (and potentially expose > >> it > >>> in > >>>>> the future), that works for me. The performance benchmarks I ran > >> suggest > >>>>> that this parameter greatly impacts store performance though and is > >> very > >>>>> workload dependent. If a user reported poor performance using > >> versioned > >>>>> stores for their workload, this is the first parameter I would want > to > >>>>> tune. That said, metrics/observability for versioned stores (which > >>> would be > >>>>> helpful for determining how this parameter should be adjusted) have > >> been > >>>>> deferred to a follow-up KIP, so perhaps that's reason to defer > >> exposing > >>>>> this parameter as well. > >>>>> > >>>>>> (5b) JavaDocs says: "Performance degrades as more record versions > for > >>>>> the same key are collected in a single segment. On the other hand, > >>>>> out-of-order writes and reads which access older segments may slow > >> down > >>>>> if there are too many segments." -- Wondering if JavaDocs should make > >>>>> any statements about expected performance? Seems to be an > >> implementation > >>>>> detail? > >>>>> > >>>>> I included this sentence to explain why a user might want to tune > this > >>>>> value / help guide how to think about the parameter, but if we want > to > >>>>> remove it entirely (per the discussion point above) then this Javadoc > >>> will > >>>>> be removed with it. > >>>>> > >>>>>> (6) validTo timestamp is "exclusive", right? Ie, if I query > >>>>> `get(key,ts[=validToV1])` I would get `null` or the "next" record v2 > >>>>> with validFromV2=ts? > >>>>> > >>>>> I actually intended for it to be inclusive (will update the KIP). Do > >> you > >>>>> think exclusive is more intuitive? The reason I had inclusive in my > >>> mind is > >>>>> because it's like a "AS OF <time>" query, which treats the time bound > >> as > >>>>> inclusive. > >>>>> > >>>>>> (7) The KIP says, that segments are stores in the same RocksDB -- > for > >>>>> this case, how are efficient deletes handled? For windowed-store, we > >> can > >>>>> just delete a full RocksDB. > >>>>> > >>>>> The way that multiple segments are represented in the same RocksDB is > >>> that > >>>>> the RocksDB keys are prefixed with segment ID. An entire segment is > >>> deleted > >>>>> with a single `deleteRange()` call to RocksDB. > >>>>> > >>>>>> (8) Rejected alternatives: you propose to not return the validTo > >>>>> timestamp -- if we find it useful in the future to return it, would > >>>>> there be a clean path to change it accordingly? > >>>>> > >>>>> With the current proposal, there's no clean path. If we think there's > >> a > >>>>> good chance we might want to do this in the future, then we should > >>> update > >>>>> the proposed interfaces. > >>>>> > >>>>> The current proposed return type from `VersionedKeyValueStore<K, > >>>>> V>#get(key, tsTo)` is `ValueAndTimestamp<V>`. There's no way to add a > >>>>> second timestamp into `ValueAndTimestamp<V>`, which is why there's no > >>> clean > >>>>> path to include validTo timestamp in the future under the existing > >>>>> proposal. > >>>>> > >>>>> If we wanted to allow for including validTo timestamp in the future, > >>> we'd > >>>>> instead update the return type to be a new `VersionedRecord<V>` > >> object. > >>>>> Today a `VersionedRecord<V>` could just include `value` and > >> `timestamp`, > >>>>> and in the future we could add `validTo` (names subject to change) > >> into > >>> the > >>>>> `VersionedRecord` as well. (It'd look a little strange for now since > >>>>> VersionedRecord is the same as ValueAndTimestamp, but that seems > >> fine.) > >>>>> > >>>>> If we choose to do this, I think we should also update the return > type > >>> of > >>>>> `VersionedKeyValueStore#get(key)` to be VersionedRecord as well, > >> rather > >>>>> than having one return TimestampAndValue while the other returns > >>>>> VersionedRecord. > >>>>> > >>>>> ----------- Sagar's comments ----------- > >>>>> > >>>>>> 1) Did you consider adding a method similar to : > >>>>> List<ValueAndTimeStamp<V>> get(K key, long from, long to)? > >>>>> I think this could be useful considering that this > >>>>> versioning scheme unlocks time travel at a key basis. WDYT? > >>>>> > >>>>> Yes, I do think this method is valuable. I think we will definitely > >>> want to > >>>>> support time-range based queries at some point (hopefully soon), and > >>> likely > >>>>> also key-range based queries (to achieve feature parity with existing > >>>>> key-value stores). > >>>>> > >>>>> It's not immediately clear to me whether these types of queries > should > >>> be > >>>>> supported as part of the store interface or if they should only be > >>>>> supported via the `query(...)` method for IQv2. (It's an open > question > >>> as > >>>>> to whether we should support IQv1 for versioned stores or only IQv2. > A > >>>>> benefit of IQv2 over IQv1 is that we won't need to add individual > >> store > >>>>> methods for each type of query, including for all wrapped store > >> layers.) > >>>>> > >>>>> If we have clear non-IQ use cases for these methods (e.g., use cases > >>> within > >>>>> processors), then they'll need to be added as part of the store > >>> interface > >>>>> for sure. I'm leaning towards adding them as part of the store > >> interface > >>>>> but given the ambiguity here, it may be preferrable to defer to a > >>> follow-up > >>>>> KIP. OTOH, if you think the versioned store interface as proposed in > >>> this > >>>>> KIP is too bare bones to be useful, I'm open to adding it in now as > >>> well. > >>>>> > >>>>>> 2) I have a similar question as Matthias, about the timestampTo > >>> argument > >>>>> when doing a get. Is it inclusive or exclusive? > >>>>> > >>>>> Same answer (and follow-up question) as above. Do you think it will > be > >>>>> confusing for `get(key, tsTo)` to use an inclusive time bound, while > >>>>> `get(key, tsFrom, tsTo)` would use an exclusive tsTo time bound? > Maybe > >>> we > >>>>> should rename `get(key, tsFrom, tsTo)` to `getVersions(...)` or > >>>>> `getRange(...)` in order to avoid confusion. > >>>>> > >>>>>> 3) validFrom sounds slightly confusing to me. It is essentially the > >>>>> timestamp at which the record was inserted. validFrom makes it sound > >>> like > >>>>> validTo which can keep changing based on new records while *from* is > >>> fixed. > >>>>> WDYT? > >>>>> > >>>>> "It is essentially the timestamp at which the record was inserted" > <-- > >>> Yes, > >>>>> that's correct. > >>>>> > >>>>> I borrowed the "validFrom/validTo" terminology from temporal tables, > >>> e.g., > >>>>> > >>>>> > >>> > >> > https://learn.microsoft.com/en-us/sql/relational-databases/tables/temporal-tables?view=sql-server-ver16 > >>>>> . > >>>>> I don't believe the terms "validFrom" or "validTo" are currently > >> exposed > >>>>> anywhere in any of the user-facing interfaces (or Javadocs); I just > >>> needed > >>>>> a way to refer to the concepts in the KIP. Hopefully this is a > >> non-issue > >>>>> (at least for now) as a result. Do you have a suggestion for > >> terminology > >>>>> that would've been less confusing? > >>>>> > >>>>>> 4) Even I think delete api should be supported. > >>>>> > >>>>> Makes sense. It'd be to get your input on the same follow-up > >> questions I > >>>>> asked Matthias above as well :) > >>>>> > >>>>> On Tue, Nov 22, 2022 at 4:25 AM Sagar <sagarmeansoc...@gmail.com> > >>> wrote: > >>>>> > >>>>>> Hi Victoria, > >>>>>> > >>>>>> Thanks for the KIP. Seems like a very interesting idea! > >>>>>> > >>>>>> I have a couple of questions: > >>>>>> > >>>>>> 1) Did you consider adding a method similar to : > >>>>>> List<ValueAndTimeStamp<V>> get(K key, long from, long to)? > >>>>>> > >>>>>> I think this could be useful considering that this > >>>>>> versioning scheme unlocks time travel at a key basis. WDYT? > >>>>>> > >>>>>> 2) I have a similar question as Matthias, about the timestampTo > >>> argument > >>>>>> when doing a get. Is it inclusive or exclusive? > >>>>>> > >>>>>> 3) validFrom sounds slightly confusing to me. It is essentially the > >>>>>> timestamp at which the record was inserted. validFrom makes it sound > >>> like > >>>>>> validTo which can keep changing based on new records while *from* is > >>>>> fixed. > >>>>>> WDYT? > >>>>>> > >>>>>> 4) Even I think delete api should be supported. > >>>>>> > >>>>>> Thanks! > >>>>>> Sagar. > >>>>>> > >>>>>> On Tue, Nov 22, 2022 at 8:02 AM Matthias J. Sax <mj...@apache.org> > >>>>> wrote: > >>>>>> > >>>>>>> Thanks for the KIP Victoria. Very well written! > >>>>>>> > >>>>>>> > >>>>>>> Couple of questions (many might just require to add some more > >> details > >>>>> to > >>>>>>> the KIP): > >>>>>>> > >>>>>>> (1) Why does the new store not extend KeyValueStore, but > >>> StateStore? > >>>>>>> In the end, it's a KeyValueStore? > >>>>>>> > >>>>>>> (2) Should we have a ReadOnlyVersionedKeyValueStore? Even if we > >>> don't > >>>>>>> want to support IQ in this KIP, it might be good to add this > >> interface > >>>>>>> right away to avoid complications for follow up KIPs? Or won't > there > >>> by > >>>>>>> any complications anyway? > >>>>>>> > >>>>>>> (3) Why do we not have a `delete(key)` method? I am ok with not > >>>>>>> supporting all methods from existing KV-store, but a `delete(key)` > >>>>> seems > >>>>>>> to be fundamentally to have? > >>>>>>> > >>>>>>> (4a) Do we need `get(key)`? It seems to be the same as > `get(key, > >>>>>>> MAX_VALUE)`? Maybe is good to have as syntactic sugar though? Just > >> for > >>>>>>> my own clarification (should we add something to the JavaDocs?). > >>>>>>> > >>>>>>> (4b) Should we throw an exception if a user queries > out-of-bound > >>>>>>> instead of returning `null` (in `get(key,ts)`)? > >>>>>>> -> You put it into "rejected alternatives", and I understand > >> your > >>>>>>> argument. Would love to get input from others about this question > >>>>>>> though. -- It seems we also return `null` for windowed stores, so > >>> maybe > >>>>>>> the strongest argument is to align to existing behavior? Or do we > >> have > >>>>>>> case for which the current behavior is problematic? > >>>>>>> > >>>>>>> (4c) JavaDoc on `get(key,ts)` says: "(up to store > implementation > >>>>>>> discretion when this is the case)" -> Should we make it a stricter > >>>>>>> contract such that the user can reason about it better (there is > WIP > >>> to > >>>>>>> make retention time a strict bound for windowed stores atm) > >>>>>>> -> JavaDocs on `persistentVersionedKeyValueStore` seems to > >>> suggest a > >>>>>>> strict bound, too. > >>>>>>> > >>>>>>> (5a) Do we need to expose `segmentInterval`? For > windowed-stores, > >>> we > >>>>>>> also use segments but hard-code it to two (it was exposed in > earlier > >>>>>>> versions but it seems not useful, even if we would be open to > expose > >>> it > >>>>>>> again if there is user demand). > >>>>>>> > >>>>>>> (5b) JavaDocs says: "Performance degrades as more record > versions > >>> for > >>>>>>> the same key are collected in a single segment. On the other hand, > >>>>>>> out-of-order writes and reads which access older segments may slow > >>> down > >>>>>>> if there are too many segments." -- Wondering if JavaDocs should > >> make > >>>>>>> any statements about expected performance? Seems to be an > >>>>> implementation > >>>>>>> detail? > >>>>>>> > >>>>>>> (6) validTo timestamp is "exclusive", right? Ie, if I query > >>>>>>> `get(key,ts[=validToV1])` I would get `null` or the "next" record > v2 > >>>>>>> with validFromV2=ts? > >>>>>>> > >>>>>>> (7) The KIP says, that segments are stores in the same RocksDB > -- > >>> for > >>>>>>> this case, how are efficient deletes handled? For windowed-store, > we > >>>>> can > >>>>>>> just delete a full RocksDB. > >>>>>>> > >>>>>>> (8) Rejected alternatives: you propose to not return the > validTo > >>>>>>> timestamp -- if we find it useful in the future to return it, would > >>>>>>> there be a clean path to change it accordingly? > >>>>>>> > >>>>>>> > >>>>>>> -Matthias > >>>>>>> > >>>>>>> > >>>>>>> On 11/16/22 9:57 PM, Victoria Xia wrote: > >>>>>>>> Hi everyone, > >>>>>>>> > >>>>>>>> I have a proposal for introducing versioned state stores in Kafka > >>>>>>> Streams. > >>>>>>>> Versioned state stores are similar to key-value stores except they > >>>>> can > >>>>>>>> store multiple record versions for a single key. This KIP focuses > >> on > >>>>>>>> interfaces only in order to limit the scope of the KIP. > >>>>>>>> > >>>>>>>> > >>>>>>> > >>>>>> > >>>>> > >>> > >> > https://cwiki.apache.org/confluence/display/KAFKA/KIP-889%3A+Versioned+State+Stores > >>>>>>>> > >>>>>>>> Thanks, > >>>>>>>> Victoria > >>>>>>>> > >>>>>>> > >>>>>> > >>>>> > >>>> > >>> > >> > > >
