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https://issues.apache.org/jira/browse/FLINK-6719?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16449752#comment-16449752
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ASF GitHub Bot commented on FLINK-6719:
---------------------------------------
Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/5887#discussion_r183710303
--- Diff: docs/dev/stream/operators/process_function.md ---
@@ -322,3 +322,26 @@ ctx.timerService.registerEventTimeTimer(coalescedTime)
{% endhighlight %}
</div>
</div>
+
+### Fault Tolerance
+
+Timers registered within `ProcessFunction` are fault tolerant.
+
+Timers registered within `ProcessFunction` will be checkpointed by Flink.
Upon restoring, timers that are checkpointed
+from the previous job will be restored on whatever new instance is
responsible for that key.
+
+#### Processing Time Timers
+
+For processing timer timers, note that the firing time of a timer is an
absolute value of when to fire.
+
+What this means is that if a checkpointed timer’s firing processing
timestamp is t (which is basically the registering
+time + configured trigger time), then it will also fire at processing
timestamp t on the new instance. Therefore, you
+should be aware of out-of-sync clocks between the 2 instances.
+
+Another thing to note is that if the restored job isn’t running at t (when
the timer is supposed to fire), then on restore,
+that timer is fired immediately.
+
+#### Event Time Timers
+
+For event time timers, given that Flink does not checkpoint watermarks, a
restored event time timer will fire when the
--- End diff --
The fact that Flink doesn't checkpoint watermarks is not really related to
and does not affect the behavior of timers. It is useful information but I
don't think we need to mention it here.
It's sufficient to mention that et-timer fire when the wm passes them.
> Add details about fault-tolerance of timers to ProcessFunction docs
> -------------------------------------------------------------------
>
> Key: FLINK-6719
> URL: https://issues.apache.org/jira/browse/FLINK-6719
> Project: Flink
> Issue Type: Improvement
> Components: DataStream API, Documentation
> Affects Versions: 1.5.0
> Reporter: Tzu-Li (Gordon) Tai
> Assignee: Bowen Li
> Priority: Major
> Fix For: 1.6.0, 1.5.1
>
>
> The fault-tolerance of timers is a frequently asked questions on the mailing
> lists. We should add details about the topic in the {{ProcessFunction}} docs.
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