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https://issues.apache.org/jira/browse/FLINK-9107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16423682#comment-16423682
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ASF GitHub Bot commented on FLINK-9107:
---------------------------------------
Github user aljoscha commented on a diff in the pull request:
https://github.com/apache/flink/pull/5790#discussion_r178754487
--- Diff: docs/dev/stream/operators/process_function.md ---
@@ -269,4 +269,38 @@ override def onTimer(timestamp: Long, ctx:
OnTimerContext, out: Collector[OUT]):
}
{% endhighlight %}
</div>
-</div>
\ No newline at end of file
+</div>
+
+## Optimisations
+
+### Timer Coalescing
+
+Every timer registered at the `TimerService` via
`ctx.timerService().registerEventTimeTimer()` will
+be stored on heap and enqueued for execution. There is, however, a maximum
of one timer per key and
+timestamp at a millisecond resolution and thus, in the worst case, every
key may have a timer for
+each upcoming millisecond. Even if you do not do any processing for
outdated timers in `onTimer`
+(as above), this may put a significant burden on the Flink runtime.
+
+Since there is only one timer per key and timestamp, however, you may
coalesc timers by reducing the
--- End diff --
typo: coalesc -> coalesce
> Document timer coalescing for ProcessFunctions
> ----------------------------------------------
>
> Key: FLINK-9107
> URL: https://issues.apache.org/jira/browse/FLINK-9107
> Project: Flink
> Issue Type: Improvement
> Components: Documentation, Streaming
> Affects Versions: 1.3.0, 1.4.0, 1.5.0, 1.6.0
> Reporter: Nico Kruber
> Assignee: Nico Kruber
> Priority: Major
> Fix For: 1.5.0, 1.4.3, 1.3.4
>
>
> In a {{ProcessFunction}}, registering timers for each event via
> {{ctx.timerService().registerEventTimeTimer()}} using times like
> {{ctx.timestamp() + timeout}} will get a millisecond accuracy and may thus
> create one timer per millisecond which may lead to some overhead in the
> {{TimerService}}.
> This problem can be mitigated by using timer coalescing if the desired
> accuracy of the timer can be larger than 1ms. A timer firing at full seconds
> only, for example, can be realised like this:
> {code}
> coalescedTime = ((ctx.timestamp() + timeout) / 1000) * 1000;
> ctx.timerService().registerEventTimeTimer(coalescedTime);
> {code}
> As a result, only a single timer may exist for every second since we do not
> add timers for timestamps that are already there.
> This should be documented in the {{ProcessFunction}} docs.
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