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From Kien Truong <duckientru...@gmail.com>
Subject Re: Timer coalescing necessary?
Date Fri, 13 Oct 2017 13:36:39 GMT

Thanks for the explanation.

Because timer callback and normal execution are not guarantee to be 
concurrent-safe, if we have multiple timers with the same timestamp, are 
all of them run before the normal execution resume or are they 
interleaved with normal execution?

Also may I ask how often are the ProcessingTimeService fired ?

Best regards,


On 10/13/2017 7:48 PM, Aljoscha Krettek wrote:
> Hi,
> This is slightly different for processing-time and event-time triggers.
> First, event-time triggers: there are two data structures, a PriorityQueue (which is
implemented as a heap) of timers that is sorted by timestamp, a set of registered timers that
is used for deduplication. When adding a timer, we first check whether it already exists (using
the set) and then add it to the queue. Whenever we receive a watermark we poll from the timer
queue as long as the timestamp of the top timer is <= the watermark. We remote the timer
from the set and call the user callback.
> For processing-time triggers it's very similar, except that we use a ProcessingTimeService
instead of the watermark for advancing time. We always have one "physical" processing-time
timer set at the ProcessingTimeService. When this fires we follow the same procedure as for
event-time and then register a new "physical" timer for the next lowest processing-time timer.
> In you case this would mean 3 separate internal timers, but a timer is only a timestamp
and a key (and a namespace).
> Best,
> Aljoscha
>> On 13. Oct 2017, at 13:56, Kien Truong <duckientruong@gmail.com> wrote:
>> Hi Aljoscha,
>> Could you clarify how the timer system works right now ?
>> For example, let's say I have a function F, with 3 keys that are registered to execute
at processing time T.
>> Would Flink maintain a single internal timer at time T, then run the callback on
all 3 keys when it's triggered ? Or there'd be 3 internal timers that will be triggered separately
at time T  ?
>> Best regards,
>> Kien
>> On 10/13/2017 6:43 PM, Aljoscha Krettek wrote:
>>> Hi,
>>> If you have multiple timers per key, then coalescing can make sense to reduce
the burden on the timer system. Coalescing them across different keys would not be possible
right now.
>>> Best,
>>> Aljoscha
>>>> On 13. Oct 2017, at 06:37, Kien Truong <duckientruong@gmail.com>
>>>>   wrote:
>>>> Hi,
>>>> We are having a streaming job where we use timers to implement key timeout
for stateful functions. Should we implement coalescing logic to reduce the number of timer
trigger, or it is not necessary with Flink?
>>>> Best regards,
>>>> Kien

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