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https://issues.apache.org/jira/browse/FLINK-6719?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16449756#comment-16449756
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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_r183708069
--- 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.
--- End diff --
Add a note that timers are synchronously checkpointed (regardless of the
configuration of the state backend). Hence, a large number of timers can
significantly increase checkpointing time. See optimizations section for advice
to reduce the number of timers.
> 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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