This happens when the watermark hops forward. In practice whenever there is any backlog, this is the normal mode of operation.
On Fri, Jun 28, 2019, 12:42 AM Lukasz Cwik <lc...@google.com> wrote: > Earlier it was said that performance was poor if we moved to a model where > we prevented multiple timer firings. Since timer firings are per key, can > you provide details of what use case has multiple user timer firings per > key? > > On Thu, Jun 27, 2019 at 4:34 PM Reuven Lax <re...@google.com> wrote: > >> The watermark holds (which is how the timer holds up the watermark today, >> as there is no timer watermark) is per key. Usually the input watermark >> making a "hop" is not a problem, in fact it's the normal state of affairs. >> >> On Fri, Jun 28, 2019 at 1:08 AM Lukasz Cwik <lc...@google.com> wrote: >> >>> Thanks Reuven and Jan. >>> >>> Since timers are per key, wouldn't it be that the timer watermark should >>> also be per key for a StatefulDoFn and hence we would still be able to fire >>> multiple timers (at most one per key) and still have good performance even >>> when the input watermark makes a "hop"? >>> >>> >>> On Thu, Jun 27, 2019 at 3:43 PM Jan Lukavský <je...@seznam.cz> wrote: >>> >>>> It would be possible to have "timer watermark", between input and >>>> output watermark, so that input watermark >= timer watermark >= output >>>> watermark, but it turns out, that doing so implies that we fire timers only >>>> for single instant (because until the timer is fired and processed, the >>>> "timer watermark" is on hold). >>>> On 6/28/19 12:40 AM, Jan Lukavský wrote: >>>> >>>> At least the implementation in DirectRunner fires timers according to >>>> input watemark. Holding the timer up to output watermark causes deadlocks, >>>> because timers fired at time T might clear watermark hold for the same >>>> time. >>>> On 6/27/19 11:55 PM, Reuven Lax wrote: >>>> >>>> I believe that timers correspond to watermark holds, which hold up the >>>> output watermark, not the input watermark. >>>> >>>> On Thu, Jun 27, 2019 at 11:21 PM Lukasz Cwik <lc...@google.com> wrote: >>>> >>>>> I'm confused as to why it is valid to advance the watermark to T3 in >>>>> the original scenario. >>>>> >>>>> T1 and T2 should be treated as inputs to the function and hold the >>>>> input watermark hence T1 should fire and if it doesn't produce any new >>>>> timers before T2, then T2 should fire since the watermark will now advance >>>>> to T2. The only time you would have multiple watermark timers fire as part >>>>> of the same bundle is if they were distinct timers both set to the same >>>>> time. >>>>> >>>>> I have some examples[1] documented in the modelling, scheduling, and >>>>> executing timers doc. >>>>> >>>>> 1: >>>>> https://docs.google.com/document/d/1GRL88rKLHbMR0zJnBHYwM4xtj66VYlB112EWVUFcGB0/edit#heading=h.fzptl5h0vi9k >>>>> >>>>> >>>>> On Wed, Jun 26, 2019 at 6:40 AM Reuven Lax <re...@google.com> wrote: >>>>> >>>>>> Earlier than the input watermark only applies to event time timers, >>>>>> but the above problem holds for processing time timers as well. >>>>>> >>>>>> On Wed, Jun 26, 2019, 1:50 PM Robert Bradshaw <rober...@google.com> >>>>>> wrote: >>>>>> >>>>>>> Yeah, it wouldn't be optimal performance-wise, but I think it's good >>>>>>> to keep the bar for a correct SDK low. Might still be better than >>>>>>> sending one timer per bundle, and you only pay the performance if >>>>>>> timers are set earlier than the input watermark (and there was a >>>>>>> timer >>>>>>> firing in this range). (How often this happens probably varies a lot >>>>>>> in practice.) >>>>>>> >>>>>>> On Wed, Jun 26, 2019 at 2:33 PM Reuven Lax <re...@google.com> wrote: >>>>>>> > >>>>>>> > This would have a lot of performance problems (especially since >>>>>>> there is user code that caches within a bundle, and invalidates the >>>>>>> cache >>>>>>> at the end of every bundle). However this would be a valid "lazy" >>>>>>> implementation. >>>>>>> > >>>>>>> > On Wed, Jun 26, 2019 at 2:29 PM Robert Bradshaw < >>>>>>> rober...@google.com> wrote: >>>>>>> >> >>>>>>> >> Note also that a "lazy" SDK implementation would be to simply >>>>>>> return >>>>>>> >> all the timers (as if they were new timers) to runner once a >>>>>>> timer set >>>>>>> >> (before or at the last requested timer in the bundle) is >>>>>>> encountered. >>>>>>> >> E.g. Suppose we had timers T1, T3, T5 in the bundle. On firing >>>>>>> T1, we >>>>>>> >> set T2 and delete T3. The SDK could then claim that a timers were >>>>>>> >> (again) set at T3, T5, then set one at at T2 and deleted at T3 and >>>>>>> >> then be done with the bundle (not actually process T3 and T5). >>>>>>> (One >>>>>>> >> way to think about this is that timers are actually bundle splits >>>>>>> into >>>>>>> >> a bundle of "done" and "future" work.) A more intelligent SDK >>>>>>> could, >>>>>>> >> of course, process the whole bundle by tracking modifications to >>>>>>> the >>>>>>> >> to-be-fired timers itself rather than requiring a trip through the >>>>>>> >> runner. >>>>>>> >> >>>>>>> >> On Wed, Jun 26, 2019 at 1:51 PM Reuven Lax <re...@google.com> >>>>>>> wrote: >>>>>>> >> > >>>>>>> >> > I like this option the best. It might be trickier to implement, >>>>>>> but seems like it would be the most consistent solution. >>>>>>> >> > >>>>>>> >> > Another problem it would solve is the following: let's say a >>>>>>> bundle arrives containing timers T1 and T2, and while processing T1 the >>>>>>> user code deletes T2 (or resets it to a time in the far future). I'm >>>>>>> actually not sure what we do today, but I'm a bit afraid that we will go >>>>>>> ahead and fire T2 since it's already in the bundle, which is clearly >>>>>>> incorrect. The SDK needs to keep track of this and skip T2 in order to >>>>>>> solve this, which is the same sort of work needed to implement Robert's >>>>>>> suggestion. >>>>>>> >> > >>>>>>> >> > Reuven >>>>>>> >> > >>>>>>> >> > On Wed, Jun 26, 2019 at 12:28 PM Robert Bradshaw < >>>>>>> rober...@google.com> wrote: >>>>>>> >> >> >>>>>>> >> >> Another option, that is nice from an API perspective but >>>>>>> places a >>>>>>> >> >> burden on SDK implementers (and possibly runners), is to >>>>>>> maintain the >>>>>>> >> >> ordering of timers by requiring timers to be fired in order, >>>>>>> and if >>>>>>> >> >> any timers are set to fire them immediately before processing >>>>>>> later >>>>>>> >> >> timers. In other words, if T1 sets T2 and modifies T3, these >>>>>>> would >>>>>>> >> >> take effect (locally, the runner may not even know about T2) >>>>>>> before T3 >>>>>>> >> >> was processed. >>>>>>> >> >> >>>>>>> >> >> On Wed, Jun 26, 2019 at 11:13 AM Jan Lukavský <je...@seznam.cz> >>>>>>> wrote: >>>>>>> >> >> > >>>>>>> >> >> > Hi, >>>>>>> >> >> > >>>>>>> >> >> > I have mentioned an issue I have come across [1] on several >>>>>>> other >>>>>>> >> >> > threads, but it probably didn't attract the attention that >>>>>>> it would desire. >>>>>>> >> >> > >>>>>>> >> >> > I will try to restate the problem here for clarity: >>>>>>> >> >> > >>>>>>> >> >> > - on runners that use concept of bundles (the original >>>>>>> issue mentions >>>>>>> >> >> > DirectRunner, but it will probably apply for other runners, >>>>>>> which use >>>>>>> >> >> > bundles, as well), the workflow is as follows: >>>>>>> >> >> > >>>>>>> >> >> > a) process elements in bundle >>>>>>> >> >> > >>>>>>> >> >> > b) advance watermark >>>>>>> >> >> > >>>>>>> >> >> > c) process timers >>>>>>> >> >> > >>>>>>> >> >> > d) continue to next bundle >>>>>>> >> >> > >>>>>>> >> >> > - the issue with this is that when we are initially at >>>>>>> time T0, set >>>>>>> >> >> > two timers for T1 and T3, then advance watermark to T3 (or >>>>>>> beyond), the >>>>>>> >> >> > timers will fire (correctly) in order T1, T3, but if timer >>>>>>> at T1 sets >>>>>>> >> >> > another timer for T2, then this timer will be fired in next >>>>>>> bundle (and >>>>>>> >> >> > therefore after T3) >>>>>>> >> >> > >>>>>>> >> >> > - this causes issues mostly with race conditions in window >>>>>>> GC timers >>>>>>> >> >> > and user timers (and users don't have any way to solve that!) >>>>>>> >> >> > >>>>>>> >> >> > - note that the same applies when one timer tries to reset >>>>>>> timer that >>>>>>> >> >> > is already in the current bundle >>>>>>> >> >> > >>>>>>> >> >> > I have investigated a way of solving this by running timers >>>>>>> only for >>>>>>> >> >> > single timestamp (instant) at each bundle, but as Reuven >>>>>>> pointed out, >>>>>>> >> >> > that could regress performance (mostly by delaying firing of >>>>>>> timers, >>>>>>> >> >> > that could have fired). Options I see: >>>>>>> >> >> > >>>>>>> >> >> > 1) either set the OnTimerContext#timestamp() to current >>>>>>> input >>>>>>> >> >> > watermark (not the time that user actually set the timer), or >>>>>>> >> >> > >>>>>>> >> >> > 2) add OnTimerContext#getCurrentInputWatermark() and >>>>>>> disallow setting >>>>>>> >> >> > (or resetting) timers for time between >>>>>>> OnProcessContext#timestamp and >>>>>>> >> >> > OnProcessContext#getCurrentInputWatermark(), by throwing an >>>>>>> exception >>>>>>> >> >> > >>>>>>> >> >> > 3) any other option? >>>>>>> >> >> > >>>>>>> >> >> > Option 1) seems to be broken by design, as it can result in >>>>>>> corrupt data >>>>>>> >> >> > (emitted with wrong timestamp, which is even somewhat >>>>>>> arbitrary), I'm >>>>>>> >> >> > including it just for completeness. Option 2) is breaking >>>>>>> change, that >>>>>>> >> >> > can result in PIpeline failures (although the failures will >>>>>>> happen on >>>>>>> >> >> > Pipelines, that are probably already broken). >>>>>>> >> >> > >>>>>>> >> >> > Although I have come with a workaround in the work where I >>>>>>> originally >>>>>>> >> >> > come across this issue, I think that this is generally >>>>>>> serious and >>>>>>> >> >> > should be dealt with. Mostly because when using user-facing >>>>>>> APIs, there >>>>>>> >> >> > are no workarounds possible, today. >>>>>>> >> >> > >>>>>>> >> >> > Thanks for discussion! >>>>>>> >> >> > >>>>>>> >> >> > Jan >>>>>>> >> >> > >>>>>>> >> >> > [1] https://issues.apache.org/jira/browse/BEAM-7520 >>>>>>> >> >> > >>>>>>> >>>>>>
