On Fri, Nov 8, 2019 at 9:44 AM Steve Niemitz <sniem...@apache.org> wrote:
> Yeah that looks like what I had in mind too. I think the most useful > notification output would be a KV of (K, summary)? > Sounds about right. Some use cases may not care about the summary, but just the notification. But for most runners passing extra in-memory data to a subsequent projection which drops it is essentially free. Kenn > On Fri, Nov 8, 2019 at 12:38 PM Kenneth Knowles <k...@apache.org> wrote: > >> This sounds like a useful feature, if I understand it: a generic >> transform (build on a generic stateful DoFn) where the end-user provides a >> monotonic predicate over the input it has seen. It emits a notification >> exactly once when the predicate is first satisfied. To be efficient, it >> will also need some form of summarization over the input seen. >> >> Notify >> .withSummarizer(combineFn) >> .withPredicate(summary -> ...) >> >> Something like that? The complexity is not much less than just writing a >> stateful DoFn directly, but the boilerplate is much less. >> >> Kenn >> >> On Thu, Nov 7, 2019 at 2:02 PM Steve Niemitz <sniem...@apache.org> wrote: >> >>> Interestingly enough, we just had a use case come up that I think could >>> have been solved by finishing triggers. >>> >>> Basically, we want to emit a notification when a certain threshold is >>> reached (in this case, we saw at least N elements for a given key), and >>> then never notify again within that window. As mentioned, we can >>> accomplish this using a stateful DoFn as mentioned above, but I thought it >>> was interesting that this just came up, and wanted to share. >>> >>> Maybe it'd be worth building something to simulate this into the SDK? >>> >>> On Mon, Nov 4, 2019 at 8:15 PM Kenneth Knowles <k...@apache.org> wrote: >>> >>>> By the way, adding this guard uncovered two bugs in Beam's Java >>>> codebase, luckily only benchmarks and tests. There were *no* non-buggy >>>> instances of a finishing trigger. They both declare allowed lateness that >>>> is never used. >>>> >>>> Nexmark query 10: >>>> >>>> // Clear fancy triggering from above. >>>> .apply( >>>> Window.<KV<Void, OutputFile>>into(...) >>>> .triggering(AfterWatermark.pastEndOfWindow()) >>>> // We expect no late data here, but we'll assume the >>>> worst so we can detect any. >>>> .withAllowedLateness(Duration.standardDays(1)) >>>> .discardingFiredPanes()) >>>> >>>> This is nonsensical: the trigger will fire once and close, never firing >>>> again. So the allowed lateness has no effect except to change counters from >>>> "dropped due to lateness" to "dropped due to trigger closing". The intent >>>> would appear to be to restore the default triggering, but it failed. >>>> >>>> PipelineTranslationTest: >>>> >>>> >>>> Window.<Long>into(FixedWindows.of(Duration.standardMinutes(7))) >>>> .triggering( >>>> AfterWatermark.pastEndOfWindow() >>>> >>>> .withEarlyFirings(AfterPane.elementCountAtLeast(19))) >>>> .accumulatingFiredPanes() >>>> .withAllowedLateness(Duration.standardMinutes(3L))); >>>> >>>> Again, the allowed lateness has no effect. This test is just to test >>>> portable proto round-trip. But still it is odd to write a nonsensical >>>> pipeline for this. >>>> >>>> Takeaway: experienced Beam developers never use this pattern, but they >>>> still get it wrong and create pipelines that would have data loss bugs >>>> because of it. >>>> >>>> Since there is no other discussion here, I will trust the community is >>>> OK with this change and follow Jan's review of my implementation of his >>>> idea. >>>> >>>> Kenn >>>> >>>> >>>> On Thu, Oct 31, 2019 at 4:06 PM Kenneth Knowles <k...@apache.org> >>>> wrote: >>>> >>>>> Opened https://github.com/apache/beam/pull/9960 for this idea. This >>>>> will alert users to broken pipelines and force them to alter them. >>>>> >>>>> Kenn >>>>> >>>>> On Thu, Oct 31, 2019 at 2:12 PM Kenneth Knowles <k...@apache.org> >>>>> wrote: >>>>> >>>>>> On Thu, Oct 31, 2019 at 2:11 AM Jan Lukavský <je...@seznam.cz> wrote: >>>>>> >>>>>>> Hi Kenn, >>>>>>> >>>>>>> does there still remain some use for trigger to finish? If we don't >>>>>>> drop >>>>>>> data, would it still be of any use to users? If not, would it be >>>>>>> better >>>>>>> to just remove the functionality completely, so that users who use >>>>>>> it >>>>>>> (and it will possibly break for them) are aware of it at compile >>>>>>> time? >>>>>>> >>>>>>> Jan >>>>>>> >>>>>> >>>>>> Good point. I believe there is no good use for a top-level trigger >>>>>> finishing. As mentioned, the intended uses aren't really met by triggers, >>>>>> but are met by stateful DoFn. >>>>>> >>>>>> Eugene's bug even has this title :-). We could not change any >>>>>> behavior but just reject pipelines with broken top-level triggers. This >>>>>> is >>>>>> probably a better solution. Because if a user has a broken trigger, the >>>>>> new >>>>>> behavior is probably not enough to magically fix their pipeline. They are >>>>>> better off knowing that they are broken and fixing it. >>>>>> >>>>>> And at that point, there is a lot of dead code and my PR is really >>>>>> just cleaning it up as a simplification. >>>>>> >>>>>> Kenn >>>>>> >>>>>> >>>>>> >>>>>>> On 10/30/19 11:26 PM, Kenneth Knowles wrote: >>>>>>> > Problem: a trigger can "finish" which causes a window to "close" >>>>>>> and >>>>>>> > drop all remaining data arriving for that window. >>>>>>> > >>>>>>> > This has been discussed many times and I thought fixed, but it >>>>>>> seems >>>>>>> > to not be fixed. It does not seem to have its own Jira or thread >>>>>>> that >>>>>>> > I can find. But here are some pointers: >>>>>>> > >>>>>>> > - data loss bug: >>>>>>> > >>>>>>> https://lists.apache.org/thread.html/ce413231d0b7d52019668765186ef27a7ffb69b151fdb34f4bf80b0f@%3Cdev.beam.apache.org%3E >>>>>>> > - user hitting the bug: >>>>>>> > >>>>>>> https://lists.apache.org/thread.html/28879bc80cd5c7ef1a3e38cb1d2c063165d40c13c02894bbccd66aca@%3Cuser.beam.apache.org%3E >>>>>>> > - user confusion: >>>>>>> > >>>>>>> https://lists.apache.org/thread.html/2707aa449c8c6de1c6e3e8229db396323122304c14931c44d0081449@%3Cuser.beam.apache.org%3E >>>>>>> > - thread from 2016 on the topic: >>>>>>> > >>>>>>> https://lists.apache.org/thread.html/5f44b62fdaf34094ccff8da2a626b7cd344d29a8a0fff6eac8e148ea@%3Cdev.beam.apache.org%3E >>>>>>> > >>>>>>> > In theory, trigger finishing was intended for users who can get >>>>>>> their >>>>>>> > answers from a smaller amount of data and then drop the rest. In >>>>>>> > practice, triggers aren't really expressive enough for this. >>>>>>> Stateful >>>>>>> > DoFn is the solution for these cases. >>>>>>> > >>>>>>> > I've opened https://github.com/apache/beam/pull/9942 which makes >>>>>>> the >>>>>>> > following changes: >>>>>>> > >>>>>>> > - when a trigger says it is finished, it never fires again but >>>>>>> data >>>>>>> > is still kept >>>>>>> > - at GC time the final output will be emitted >>>>>>> > >>>>>>> > As with all bugfixes, this is backwards-incompatible (if your >>>>>>> pipeline >>>>>>> > relies on buggy behavior, it will stop working). So this is a >>>>>>> major >>>>>>> > change that I wanted to discuss on dev@. >>>>>>> > >>>>>>> > Kenn >>>>>>> > >>>>>>> >>>>>>
