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
<mailto: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
<mailto: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
<mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto: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
>> >> >
