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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> >> 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 <[email protected]> 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 < >> [email protected]> 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 <[email protected]> >> 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 >> >> >> > >> >
