Hi Piotr / Divye,
Very thanks for the discussion!
First IMO it seems we have reached the consensus
on the high-level API: Most operators should usually
have only one reasonable action to the pending timers
on termination, thus we could let the operators to
implement its own actions with the low-level interface
provided. The only exception is the ProcessFunction, with
which users might register customized timers, thus users
might also defines the actions on termination (If I have
misunderstandings here, please correct me).
For the low-level API, I could get the benefits with the callback
options: since in most cases an operator has only one action
to all the timers, its a waste for us to store the same flag for
all the timers, also with a lot of code / state format changes.
But since it is enough for most users to simply trigger / cacnel
the timers, it would be redundant for users to implement the logic
twice. Thus perhaps we might combine the benefits of the two options:
We might have a separate interface
public interface TimerHandlersOnTermination {
void processPendingTimer(Timer timer, long currentTime);
}
public class Timer {
long getRegisteredTimestamp();
void trigger();
void waitFor();
void cancel();
}
Then if an operator have implemented the TimerHandlersOnTermination
interface, on termination we could call processPendingTimer(xx) for every
pending timers. Users might simply trigger / waitFor / cancel it, or execute
some other logics if needed.
Then for the ProcessFunction we might have a similar interface to
processPendingTimer, except we might need to provide Context / Collector
to the ProcessFunction.
Do you think this would be a good direction?
Also @Piotr
> I don't see a problem here. Interface doesn't have to reflect that, only
> the runtime must set the correct key context before executing the handler
> dealing with the processing time timers at the end of input/time.
Sorry I might not make it very clear here. I think the difficulty with supported
setting the currentKey is a special issue for the callback options (no matter
what
the interface is) since it allows users to execute logic other than the one
registered
with the timers.
The complexity comes from that currently we have two level of TimerServices:
The ProcessingTimerService (there is no key) and InternalTimerService (with
key).
Currently only ProcessingTimerService is exposed to the runtime and
InternalTimerService is much more a utility to implement the operator.
Then with the current code, the runtime could only access to
ProcessingTimerService on termination. If we only executes some predefined
actions, we do not need to worry about the implementation of
InternalTimerService and just execute the registered timers. But if we allow
users to execute arbitrary logic, we need to be also aware of the
InternalTimerServices and parse the key from the timers stored in it.
I think we should always have method to overcome this issue,
but to support the callback options would be more complex.
Best,
Yun Gao
------------------------------------------------------------------
From:Divye Kapoor <dkap...@pinterest.com.INVALID>
Send Time:2022 Nov. 24 (Thu.) 08:50
To:dev <dev@flink.apache.org>
Cc:Xenon Development Team <xenon-...@pinterest.com>
Subject:Re: Re: [DISCUSS] FLIP-269: Properly Handling the Processing Timers on
Job Termination
Sounds good. Looks like we're on the same page.
Thanks!
Divye
On Wed, Nov 23, 2022 at 2:41 AM Piotr Nowojski <pnowoj...@apache.org> wrote:
> Hi Divye
>
> I think we are mostly on the same page. Just to clarify/rephrase:
>
> > One thing to think about - on EOF “trigger immediately” will mean that
> the
> > asynchronous wait timeout timers will also fire - which is undesirable
>
> I didn't mean to fire all timers immediately in all of the built-in
> operators. Just that each built-in operator can have a hard coded way
> (without a way for users to change it) to handle those timers. Windowed
> operators would trigger the lingering timers (flush outputs),
> AsyncWaitOperator could just ignore them. The same way users could register
> EOF timer handlers in the ProcessFunction as Dawid Wysakowicz proposed, we
> (as flink developers) could use the same mechanism to implement any
> behaviour we want for the built-in operators. There should be no need to
> add any separate mechanism.
>
> Best,
> Piotrek
>
> śr., 23 lis 2022 o 08:21 Divye Kapoor <dkap...@pinterest.com.invalid>
> napisał(a):
>
> > Thanks Yun/Piotrek,
> >
> > Some brief comments inline below.
> >
> > On Tue, Nov 22, 2022 at 1:37 AM Piotr Nowojski <pnowoj...@apache.org>
> > wrote:
> >
> > > Hi,
> > >
> > > All in all I would agree with Dawid's proposal.
> >
> > +1
> >
> > We can add the flexibility
> > > of how to deal with the timers in the low level API via adding a
> handler
> > -
> > > if someone needs to customize it, he will always have a workaround.
> Note
> > > after giving it more thought, I agree that registering some handlers is
> > > better than overloading the register timer method and modifying the
> > timer's
> > > state.
> >
> > +1.
> >
> > >
> > >
> > > At the same time, we can force the most sensible semantic that we think
> > for
> > > the couple of built-in operators, which should be pretty
> straightforward
> > > (either ignore the timers, or fire them at once). I agree there might
> be
> > > some edge cases, that theoretically user might want to wait for the
> timer
> > > to fire naturally, but:
> > > 1. I'm not sure how common in practice this will be. If not at all,
> then
> > > why should we be complicating the API/system?
> >
> > That’s fair.
> > However, the specifics are very important here.
> >
> > One thing to think about - on EOF “trigger immediately” will mean that
> the
> > asynchronous wait timeout timers will also fire - which is undesirable
> > (because they are racing with the last async call). However, the issue is
> > cleanly resolved by waiting for the timer to be canceled when the last
> > event is processed. (“Wait for” case).
> >
> > Ignoring the timer has the least justification. Registering the handler
> as
> > per Dawid’s proposal and having that handler unregister the timers on EOF
> > makes best sense. This solution also unifies the trigger immediately case
> > as that handler can reregister the timers for early termination.
> >
> > The proposal:
> > 1. Operator receives EOF
> > 2. EOF timer handler triggers
> > 3. EOF handler adjusts the registered timers for early trigger or ignore.
> > If wait-for behavior is desired, timers are not changed. This is
> controlled
> > in client code.
> > 4. Operator waits for all timers to drain/trigger. (“Always”). There is
> no
> > special handling for ignore/early trigger.
> > 5. Operator allows job to proceed with shutdown.
> >
> > The only api change needed is an EOF handler.
> > The other agreement we need is that “Wait for” is the desired behavior in
> > processing time and that processing time is fundamentally different from
> > event time in this respect.
> > (I have changed my thinking since the last mail).
> >
> > 2. We can always expand the API in the future, and let the user override
> > > the default built-in behaviour of the operators via some setter on the
> > > stream transformation (`SingleOutputStreamOperator`), or via some
> custom
> > > API DSL style in each of the operators separately.
> >
> >
> > This is not required. See above.
> >
> >
> > >
> > > Re forcing the same semantics for processing time timers as for event
> > time
> > > ones - this is tempting, but indeed I see a possibility that users need
> > to
> > > adhere to some external constraints when using processing time.
> >
> > +1. As above, we should consider the 2 cases fundamentally different in
> > this area.
> >
> > Re: Yun -
> >
> > > b) Another issue is that what if users use timers with different
> > > > termination actions in the same
> > > > operator / UDF? For example, users use some kind of timeout (like
> > throws
> > > > exception if some thing
> > > > not happen after some other thing), and also some kind of window
> > > > aggregation logic. In this case,
> > > > without additional tags, users might not be able to distinguish
> which
> > > > timer should be canceled and
> > > > which time should be triggered ?
> >
> > as above. The EOF handler makes the choice.
> >
> > >
> > > > 4. How could these scenarios adjust their APIs ?
> > > > From the current listed scenarios, I'm more tend to that as @Dawid
> > > > pointed out, there might be only
> > > > one expected behavior for each scenario, thus it does not seems to
> > need
> > > > to allow users to adjust the
> > > > behavior. Thus @Divye may I have a double confirmation currently do
> we
> > > > have explicit scenarios that
> > > > is expected to change the different behaviors for the same scenario?
> >
> > Wait-for behavior is probably the only expected behavior and any
> > alterations should be from the EOF handler managing the registered
> timers.
> >
> > >
> > > > Besides @Divye from the listed scenarios, I have another concern for
> > > > global configuration is that for
> > > > one job, different operators seems to still might have different
> > > expected
> > > > behaviors. For example, A
> > > > job using both Window operator and AsyncWaitOperator might have
> > > different
> > > > requirements for timers
> > > > on termination?
> >
> >
> > Thank you for raising this case. This changed my thinking. Based on your
> > point, we should try and align on the “Wait-for” with EOF handler
> proposal.
> > I’m withdrawing the “single-runtime-config” proposal.
> >
> > Best,
> > Divye
> >
>
