On Thu, Jun 20, 2019 at 8:03 PM Jan Lukavský <[email protected]> wrote:
> Hi Reuven, > > > I would be cautious changing this. Being able to put multiple timers in > the same bundle saves a lot, and if we force them to all run separate > through ReduceFnRunner we risk regressing performance of some pipelines. > > I understand your point. The issue here is that, the current behavior is > at least ... unexpected. There might be one different conceptual approach > to that: > > a) if a bundle contains timers for several distinct timestamps (say T1 > and T2), then it implies, that timer T1 is effectively not fired at time > T1, but at time T2 - that is due to the fact, that logically, the time > hopped discretely from some previous time T0 to T2 without any "stopping > by". Hence, it should be invalid to setup timer for any time lower than T2 > . > But that is exactly how time advances. Watermarks often don't move smoothly, as a single old element can hold up the watermark. When that element is finished, the watermark can jump forward in time, triggering many timers. > b) the time will move smoothly (or, millisecond precision smoothly), but > that implies, that there cannot be more distinct timers inside single > bundle. > > If we don't want to take path b), we are probably left with path a) (as > doing nothing seems weird, because it breaks one invariant, that time can > only move forward). > I'm not sure how this breaks that invariant. The input watermark has only moved forward, as should be true fo the output watermark. The output watermark is help up by watermark holds in the step, which usually means that the output watermark is already being help to the earliest pending timer. > Option a) can be done - we might add something like `getInputWatermark()` > and `getOutputWatermark()` to `DoFn.OnTimerContext`, and throw exception > when user tries to setup timer for time before input watermark. > Effectively, that way we will let the user know, that his timer was set to > time T1, but was fired at T2. But, that seems to be breaking change, > unfortunately. > > What do you think? > > Jan > On 6/20/19 5:29 PM, Reuven Lax wrote: > > > > On Thu, Jun 20, 2019 at 3:08 PM Jan Lukavský <[email protected]> wrote: > >> Hi, >> >> this problem seems to be harder than I thought. I have a somewhat working >> code in [1], but there are still failing some tests (now tests for >> ReduceFnRunner), but I'm not sure, if the problem is not in the tests, so >> that my current behavior is actually correct. Let me explain the problem: >> >> - let's have a fixed window with allowed lateness of 1 ms >> >> - let's add two elements into the window (on time), no late elements >> >> - now, ReduceFnRunner with default trigger will set *two* timers - one >> for window.maxTimestamp() and second for window.maxTimestamp() + >> allowedLateness >> >> - the previous implementation fired *both* timers at once (within single >> call to ReduceFnRunner#onTimers), but now it fires twice - once for the >> first timer and second for the other >> > > I would be cautious changing this. Being able to put multiple timers in > the same bundle saves a lot, and if we force them to all run separate > through ReduceFnRunner we risk regressing performance of some pipelines. > > >> - the result of this is that although in both cases only single pane is >> emitted, in my branch the fired pane doesn't have the `isLast` flag set >> (that is because the window is not yet garbage collected - waiting for late >> data - but the second time it is not fired, because no late data arrived) >> >> Would anyone know what is actually the correct behavior regarding the >> PaneInfo.isLast? I suppose there are only two options - either two panes >> can come with isLast flag (both end-of-window and late), or it might be >> possible, that no pane will marked with this flag (because no late pane is >> fired). >> >> Jan >> >> [1] https://github.com/apache/beam/pull/8815 >> >> >> On 6/10/19 6:26 PM, Jan Lukavský wrote: >> >> It seems to me that watermark hold cannot change it (currently), because >> in the current implementation timers fire according to input watermark, but >> watermark holds apply to output watermark. If I didn't miss anything. >> >> Dne 10. 6. 2019 18:15 napsal uživatel Lukasz Cwik <[email protected]> >> <[email protected]>: >> >> I see. Is there a missing watermark hold for timers less then T2? >> >> On Mon, Jun 10, 2019 at 9:08 AM Jan Lukavský <[email protected]> wrote: >> >> Yes, there is no difference between GC and user timers in this case. I >> think the problem is simply that when watermark moves from time T1 to T2, >> DirectRunner fires all timers that fire until T2, but that can create new >> timers for time between T1 and T2, and these will be fired later, although >> should have been fired before T2. >> >> Jan >> On 6/10/19 5:48 PM, Kenneth Knowles wrote: >> >> Reading your Jira, I believe this problem will manifest without the >> interaction of user timers and GC. Interesting case. It surrounds whether a >> runner makes a timer available or fires it prior to the bundle being >> committed. >> >> I have commented elsewhere about this part, quoting the Jira: >> >> > have experimented with this a little and have not yet figured out what >> the correct solution should be. What I tried: >> > 1) hold input watermark for min(setup timers) >> > 2) fire timers based not on input watermark, but on output watermark >> (output watermark is held by min timer stamp) >> >> Neither of these quite works. What we need is a separate "element input >> watermark" and "timer input watermark". The overall input watermark that >> drives GC is the min of these. The output watermark is also held to this >> overall input watermark. User timers fire according to the element input >> watermark. >> >> Kenn >> >> On Mon, Jun 10, 2019 at 8:44 AM Lukasz Cwik <[email protected]> wrote: >> >> Jan are you editing the implementation of how timers work within the >> DirectRunner or are trying to build support for time sorted input on top of >> the Beam model for timers? >> Because I think you will need to do the former. >> >> On Mon, Jun 10, 2019 at 8:41 AM Jan Lukavský <[email protected]> wrote: >> >> Hm, that would probably work, thanks! >> >> But, should the timers behave like that? I'm trying to fix tris by >> introducing a sequence of watermarks >> >> inputs watermark -> timer watermark -> output watermark >> >> as suggested in the JIRA, and it actually seems to be working as >> expected. It even cleans some code paths, but I'm debugging some strange >> behavior this exposed - >> `WatermarkHold.watermarkHoldTagForTimestampCombiner` seems to have stopped >> clearing itself after this change and some Pipelines therefore stopped >> working. I'm little lost why this happened. I can push code I have if >> anyone interested. >> >> Jan >> On 6/10/19 5:32 PM, Lukasz Cwik wrote: >> >> We hit an instance of this problem before and solved it rescheduling the >> GC timer again if there was a conflicting timer that was also meant to fire. >> >> On Mon, Jun 10, 2019 at 8:17 AM Jan Lukavský <[email protected]> wrote: >> >> For a single key. I'm getting into collision of timerId >> `__StatefulParDoGcTimerId` (StatefulDoFnRunner) and my timerId for flushing >> sorted elements in implementation of @RequiresTimeSortedInput. The timers >> are being swapped at the end of input (but it can happen anywhere near end >> of window), which results in state being cleared before it gets flushed, >> which means data loss. >> >> Jan >> On 6/10/19 5:08 PM, Reuven Lax wrote: >> >> Do you mean for a single key or across keys? >> >> On Mon, Jun 10, 2019, 5:11 AM Jan Lukavský <[email protected]> wrote: >> >> Hi, >> >> I have come across issue [1], where I'm not sure how to solve this in >> most elegant way. >> >> Any suggestions? >> >> Thanks, >> >> Jan >> >> [1] https://issues.apache.org/jira/browse/BEAM-7520 >> >> >> >> It seems to me that watermark hold cannot change it (currently), because in >> the current implementation timers fire according to input watermark, but >> watermark holds apply to output watermark. If I didn't miss anything. >> >> Dne 10. 6. 2019 18:15 napsal uživatel Lukasz Cwik <[email protected]> >> <[email protected]>: >> >> I see. Is there a missing watermark hold for timers less then T2? >> >> On Mon, Jun 10, 2019 at 9:08 AM Jan Lukavský <[email protected]> >> <[email protected]> wrote: >> >> Yes, there is no difference between GC and user timers in this case. I think >> the problem is simply that when watermark moves from time T1 to T2, >> DirectRunner fires all timers that fire until T2, but that can create new >> timers for time between T1 and T2, and these will be fired later, although >> should have been fired before T2. >> >> Jan >> >> On 6/10/19 5:48 PM, Kenneth Knowles wrote: >> >> Reading your Jira, I believe this problem will manifest without the >> interaction of user timers and GC. Interesting case. It surrounds whether a >> runner makes a timer available or fires it prior to the bundle being >> committed. >> >> I have commented elsewhere about this part, quoting the Jira: >> >> >> have experimented with this a little and have not yet figured out what the >> correct solution should be. What I tried: >> 1) hold input watermark for min(setup timers) >> 2) fire timers based not on input watermark, but on output watermark (output >> watermark is held by min timer stamp) >> >> Neither of these quite works. What we need is a separate "element input >> watermark" and "timer input watermark". The overall input watermark that >> drives GC is the min of these. The output watermark is also held to this >> overall input watermark. User timers fire according to the element input >> watermark. >> >> Kenn >> >> On Mon, Jun 10, 2019 at 8:44 AM Lukasz Cwik <[email protected]> >> <[email protected]> wrote: >> >> Jan are you editing the implementation of how timers work within the >> DirectRunner or are trying to build support for time sorted input on top of >> the Beam model for timers? >> Because I think you will need to do the former. >> >> On Mon, Jun 10, 2019 at 8:41 AM Jan Lukavský <[email protected]> >> <[email protected]> wrote: >> >> Hm, that would probably work, thanks! >> >> But, should the timers behave like that? I'm trying to fix tris by >> introducing a sequence of watermarks >> >> inputs watermark -> timer watermark -> output watermark >> >> as suggested in the JIRA, and it actually seems to be working as expected. >> It even cleans some code paths, but I'm debugging some strange behavior this >> exposed - `WatermarkHold.watermarkHoldTagForTimestampCombiner` seems to have >> stopped clearing itself after this change and some Pipelines therefore >> stopped working. I'm little lost why this happened. I can push code I have >> if anyone interested. >> >> Jan >> >> On 6/10/19 5:32 PM, Lukasz Cwik wrote: >> >> We hit an instance of this problem before and solved it rescheduling the GC >> timer again if there was a conflicting timer that was also meant to fire. >> >> On Mon, Jun 10, 2019 at 8:17 AM Jan Lukavský <[email protected]> >> <[email protected]> wrote: >> >> For a single key. I'm getting into collision of timerId >> `__StatefulParDoGcTimerId` (StatefulDoFnRunner) and my timerId for flushing >> sorted elements in implementation of @RequiresTimeSortedInput. The timers >> are being swapped at the end of input (but it can happen anywhere near end >> of window), which results in state being cleared before it gets flushed, >> which means data loss. >> >> Jan >> >> On 6/10/19 5:08 PM, Reuven Lax wrote: >> >> Do you mean for a single key or across keys? >> >> On Mon, Jun 10, 2019, 5:11 AM Jan Lukavský <[email protected]> >> <[email protected]> wrote: >> >> Hi, >> >> I have come across issue [1], where I'm not sure how to solve this in >> most elegant way. >> >> Any suggestions? >> >> Thanks, >> >> Jan >> >> [1] https://issues.apache.org/jira/browse/BEAM-7520 >> >>
