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 >> >> >
