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 > > >
