Yeah that looks like what I had in mind too.  I think the most useful
notification output would be a KV of (K, summary)?

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 Lukavsk√Ĺ <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
>>>>>> >
>>>>>>
>>>>>

Reply via email to