If no additional comments, I will proceed to start the a vote thread. Thanks a lot for your feedback!
On Fri, May 22, 2020 at 9:25 AM Jorge Esteban Quilcate Otoya < quilcate.jo...@gmail.com> wrote: > Thanks Sophie. I like the `reverseAll()` idea. > > I updated the KIP with your feedback. > > > > On Fri, May 22, 2020 at 4:22 AM Sophie Blee-Goldman <sop...@confluent.io> > wrote: > >> Hm, the case of `all()` does seem to present a dilemma in the case of >> variable-length keys. >> >> In the case of fixed-length keys, you can just compute the keys that >> correspond >> to the maximum and minimum serialized bytes, then perform a `range()` >> query >> instead of an `all()`. If your keys don't have a well-defined ordering >> such >> that >> you can't determine the MAX_KEY, then you probably don't care about the >> iterator order anyway. >> >> But with variable-length keys, there is no MAX_KEY. If all your keys were >> just >> of the form 'a', 'aa', 'aaaaa', 'aaaaaaa' then in fact the only way to >> figure out the >> maximum key in the store is by using `all()` -- and without a reverse >> iterator, you're >> doomed to iterate through every single key just to answer that simple >> question. >> >> That said, I still think determining the iterator order based on the >> to/from bytes >> makes a lot of intuitive sense and gives the API a nice symmetry. What if >> we >> solved the `all()` problem by just giving `all()` a reverse form to >> complement it? >> Ie we would have `all()` and `reverseAll()`, or something to that effect. >> >> On Thu, May 21, 2020 at 3:41 PM Jorge Esteban Quilcate Otoya < >> quilcate.jo...@gmail.com> wrote: >> >> > Thanks John. >> > >> > Agree. I like the first approach as well, with StreamsConfig flag >> passing >> > by via ProcessorContext. >> > >> > Another positive effect with "reverse parameters" is that in the case of >> > `fetch(keyFrom, keyTo, timeFrom, timeTo)` users can decide _which_ pair >> to >> > flip, whether with `ReadDirection` enum it apply to both. >> > >> > The only issue I've found while reviewing the KIP is that `all()` won't >> fit >> > within this approach. >> > >> > We could remove it from the KIP and argue that for WindowStore, >> > `fetchAll(0, Long.MAX_VALUE)` can be used to get all in reverse order, >> and >> > for KeyValueStore, no ordering guarantees are provided. >> > >> > If there is consensus with this changes, I will go and update the KIP. >> > >> > On Thu, May 21, 2020 at 3:33 PM John Roesler <vvcep...@apache.org> >> wrote: >> > >> > > Hi Jorge, >> > > >> > > Thanks for that idea. I agree, a feature flag would protect anyone >> > > who may be depending on the current behavior. >> > > >> > > It seems better to locate the feature flag in the initialization >> logic of >> > > the store, rather than have a method on the "live" store that changes >> > > its behavior on the fly. >> > > >> > > It seems like there are two options here, one is to add a new config: >> > > >> > > StreamsConfig.ENABLE_BACKWARDS_ITERATION = >> > > "enable.backwards.iteration >> > > >> > > Or we can add a feature flag in Materialized, like >> > > >> > > Materialized.enableBackwardsIteration() >> > > >> > > I think I'd personally lean toward the config, for the following >> reason. >> > > The concern that Sophie raised is that someone's program may depend >> > > on the existing contract of getting an empty iterator. We don't want >> to >> > > switch behavior when they aren't expecting it, so we provide them a >> > > config to assert that they _are_ expecting the new behavior, which >> > > means they take responsibility for updating their code to expect the >> new >> > > behavior. >> > > >> > > There doesn't seem to be a reason to offer a choice of behaviors on a >> > > per-query, or per-store basis. We just want people to be not surprised >> > > by this change in general. >> > > >> > > What do you think? >> > > Thanks, >> > > -John >> > > >> > > On Wed, May 20, 2020, at 17:37, Jorge Quilcate wrote: >> > > > Thank you both for the great feedback. >> > > > >> > > > I like the "fancy" proposal :), and how it removes the need for >> > > > additional API methods. And with a feature flag on `StateStore`, >> > > > disabled by default, should no break current users. >> > > > >> > > > The only side-effect I can think of is that: by moving the flag >> > upwards, >> > > > all later operations become affected; which might be ok for most >> (all?) >> > > > cases. I can't think of an scenario where this would be an issue, >> just >> > > > want to point this out. >> > > > >> > > > If moving to this approach, I'd like to check if I got this right >> > before >> > > > updating the KIP: >> > > > >> > > > - only `StateStore` will change by having a new method: >> > > > `backwardIteration()`, `false` by default to keep things compatible. >> > > > - then all `*Stores` will have to update their implementation based >> on >> > > > this flag. >> > > > >> > > > >> > > > On 20/05/2020 21:02, Sophie Blee-Goldman wrote: >> > > > >> There's no possibility that someone could be relying >> > > > >> on iterating over that range in increasing order, because that's >> not >> > > what >> > > > >> happens. However, they could indeed be relying on getting an >> empty >> > > > > iterator >> > > > > >> > > > > I just meant that they might be relying on the assumption that the >> > > range >> > > > > query >> > > > > will never return results with decreasing keys. The empty iterator >> > > wouldn't >> > > > > break that contract, but of course a surprise reverse iterator >> would. >> > > > > >> > > > > FWIW I actually am in favor of automatically converting to a >> reverse >> > > > > iterator, >> > > > > I just thought we should consider whether this should be off by >> > > default or >> > > > > even possible to disable at all. >> > > > > >> > > > > On Tue, May 19, 2020 at 7:42 PM John Roesler <vvcep...@apache.org >> > >> > > wrote: >> > > > > >> > > > >> Thanks for the response, Sophie, >> > > > >> >> > > > >> I wholeheartedly agree we should take as much into account as >> > possible >> > > > >> up front, rather than regretting our decisions later. I actually >> do >> > > share >> > > > >> your vague sense of worry, which was what led me to say initially >> > > that I >> > > > >> thought my counterproposal might be "too fancy". Sometimes, it's >> > > better >> > > > >> to be explicit instead of "elegant", if we think more people >> will be >> > > > >> confused >> > > > >> than not. >> > > > >> >> > > > >> I really don't think that there's any danger of "relying on a >> bug" >> > > here, >> > > > >> although >> > > > >> people certainly could be relying on current behavior. One thing >> to >> > be >> > > > >> clear >> > > > >> about (which I just left a more detailed comment in KAFKA-8159 >> > about) >> > > is >> > > > >> that >> > > > >> when we say something like key1 > key2, this ordering is defined >> by >> > > the >> > > > >> serde's output and nothing else. >> > > > >> >> > > > >> Currently, thanks to your fix in >> > > https://github.com/apache/kafka/pull/6521 >> > > > >> , >> > > > >> the store contract is that for range scans, if from > to, then >> the >> > > store >> > > > >> must >> > > > >> return an empty iterator. There's no possibility that someone >> could >> > be >> > > > >> relying >> > > > >> on iterating over that range in increasing order, because that's >> not >> > > what >> > > > >> happens. However, they could indeed be relying on getting an >> empty >> > > > >> iterator. >> > > > >> >> > > > >> My counterproposal was to actually change this contract to say >> that >> > > the >> > > > >> store >> > > > >> must return an iterator over the keys in that range, but in the >> > > reverse >> > > > >> order. >> > > > >> So, in addition to considering whether this idea is "too fancy" >> (aka >> > > > >> confusing), >> > > > >> we should also consider the likelihood of breaking an existing >> > > program with >> > > > >> this behavior/contract change. >> > > > >> >> > > > >> To echo your clarification, I'm also not advocating strongly in >> > favor >> > > of my >> > > > >> proposal. I just wanted to present it for consideration alongside >> > > Jorge's >> > > > >> original one. >> > > > >> >> > > > >> Thanks for raising these very good points, >> > > > >> -John >> > > > >> >> > > > >> On Tue, May 19, 2020, at 20:49, Sophie Blee-Goldman wrote: >> > > > >>>> Rather than working around it, I think we should just fix it >> > > > >>> Now *that's* a "fancy" idea :P >> > > > >>> >> > > > >>> That was my primary concern, although I do have a vague sense of >> > > worry >> > > > >>> that we might be allowing users to get into trouble without >> > > realizing it. >> > > > >>> For example if their custom serdes suffer a similar bug as the >> > above, >> > > > >>> and/or >> > > > >>> they rely on getting results in increasing order (of the keys) >> even >> > > when >> > > > >>> to < from. Maybe they're relying on the fact that the range >> query >> > > returns >> > > > >>> nothing in that case. >> > > > >>> >> > > > >>> Not sure if that qualifies as relying on a bug or not, but in >> that >> > > past >> > > > >>> we've >> > > > >>> taken the stance that we should not break compatibility even if >> the >> > > user >> > > > >>> was relying on bugs or unintentional behavior. >> > > > >>> >> > > > >>> Just to clarify I'm not advocating strongly against this >> proposal, >> > > just >> > > > >>> laying >> > > > >>> out some considerations we should take into account. At the end >> of >> > > the >> > > > >> day >> > > > >>> we should do what's right rather than maintain compatibility >> with >> > > > >> existing >> > > > >>> bugs, but sometimes there's a reasonable middle ground. >> > > > >>> >> > > > >>> On Tue, May 19, 2020 at 6:15 PM John Roesler < >> vvcep...@apache.org> >> > > > >> wrote: >> > > > >>>> Thanks Sophie, >> > > > >>>> >> > > > >>>> Woah, that’s a nasty bug. Rather than working around it, I >> think >> > we >> > > > >> should >> > > > >>>> just fix it. I’ll leave some comments on the Jira. >> > > > >>>> >> > > > >>>> It doesn’t seem like it should be this KIP’s concern that some >> > > serdes >> > > > >>>> might be incorrectly written. >> > > > >>>> >> > > > >>>> Were there other practical concerns that you had in mind? >> > > > >>>> >> > > > >>>> Thanks, >> > > > >>>> John >> > > > >>>> >> > > > >>>> On Tue, May 19, 2020, at 19:10, Sophie Blee-Goldman wrote: >> > > > >>>>> I like this "fancy idea" to just flip the to/from bytes but I >> > think >> > > > >> there >> > > > >>>>> are some practical limitations to implementing this. In >> > particular >> > > > >>>>> I'm thinking about this issue >> > > > >>>>> <https://issues.apache.org/jira/browse/KAFKA-8159> with the >> > > built-in >> > > > >>>> signed >> > > > >>>>> number serdes. >> > > > >>>>> >> > > > >>>>> This trick would actually fix the problem for >> negative-negative >> > > > >> queries >> > > > >>>>> (ie where to & from are negative) but would cause undetectable >> > > > >>>>> incorrect results for negative-positive queries. For example, >> say >> > > you >> > > > >>>>> call #range with from = -1 and to = 1, using the Short serdes. >> > The >> > > > >>>>> serialized bytes for that are >> > > > >>>>> >> > > > >>>>> from = 1111111111111111 >> > > > >>>>> to = 0000000000000001 >> > > > >>>>> >> > > > >>>>> so we would end up flipping those and iterating over all keys >> > from >> > > > >>>>> 0000000000000001 to 1111111111111111. Iterating in >> > lexicographical >> > > > >>>>> order means we would iterate over every key in the space >> *except* >> > > for >> > > > >>>>> 0, but 0 is actually the *only* other key we meant to be >> included >> > > in >> > > > >> the >> > > > >>>>> range query. >> > > > >>>>> >> > > > >>>>> Currently we just log a warning and return an empty iterator >> when >> > > > >>>>> to < from, which is obviously also incorrect but feels >> slightly >> > > more >> > > > >>>>> palatable. If we start automatically converting to reverse >> > queries >> > > we >> > > > >>>>> can't even log a warning in this case unless we wanted to log >> a >> > > > >> warning >> > > > >>>>> every time, which would be weird to do for a valid usage of a >> new >> > > > >>>>> feature. >> > > > >>>>> >> > > > >>>>> All that said, I still like the idea overall. Off the top of >> my >> > > head >> > > > >> I >> > > > >>>> guess >> > > > >>>>> we could add a store config to enable/disable automatic >> reverse >> > > > >>>> iteration, >> > > > >>>>> which is off by default? >> > > > >>>>> >> > > > >>>>> Thanks for the KIP! This will be a nice addition >> > > > >>>>> >> > > > >>>>> Sophie >> > > > >>>>> >> > > > >>>>> >> > > > >>>>> On Tue, May 19, 2020 at 3:21 PM John Roesler < >> > vvcep...@apache.org> >> > > > >>>> wrote: >> > > > >>>>>> Hi there Jorge, >> > > > >>>>>> >> > > > >>>>>> Thanks for the KIP! >> > > > >>>>>> >> > > > >>>>>> I think this feature sounds very reasonable. >> > > > >>>>>> >> > > > >>>>>> I'm not 100% sure if this is "too fancy", but what do you >> think >> > > > >>>>>> about avoiding the enum by instead allowing people to flip >> > > > >>>>>> the "from" and "to" endpoints? I.e., reading from "A" to "Z" >> > would >> > > > >>>>>> be a forward scan, and from "Z" to "A" would be a backward >> one? >> > > > >>>>>> >> > > > >>>>>> Thanks, >> > > > >>>>>> -John >> > > > >>>>>> >> > > > >>>>>> On Tue, May 19, 2020, at 16:20, Jorge Quilcate wrote: >> > > > >>>>>>> Hi everyone, >> > > > >>>>>>> >> > > > >>>>>>> I would like to start the discussion for KIP-617: >> > > > >>>>>>> >> > > > >> >> > > >> > >> https://cwiki.apache.org/confluence/display/KAFKA/KIP-617%3A+Allow+Kafka+Streams+State+Stores+to+be+iterated+backwards >> > > > >>>>>>> Looking forward to your feedback. >> > > > >>>>>>> >> > > > >>>>>>> Thanks! >> > > > >>>>>>> Jorge. >> > > > >>>>>>> >> > > > >>>>>>> >> > > > >>>>>>> Attachments: >> > > > >>>>>>> * 0x5F2C6E22064982DF.asc >> > > > >> > > > >> > > > Attachments: >> > > > * 0x5F2C6E22064982DF.asc >> > > >> > >> >
