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https://issues.apache.org/jira/browse/FLINK-4421?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16095699#comment-16095699
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mingleizhang commented on FLINK-4421:
-------------------------------------

Hi, Stephan. Thanks for reporting this. I would like to know, Under what 
circumstances will lead to clock updates ? that is what you said can lead to 
negative duration. And I would like to try read {{Introduce a Clock utility for 
monotonous system timestamps}}.

Like today I run a code like {{System.currentTimeMillis()}} return 12345. 
Tomorrow, I run {{System.currentTimeMillis()}} return 12145. So, It can lead to 
negative duration.

> Make clocks and time measurements monotonous
> --------------------------------------------
>
>                 Key: FLINK-4421
>                 URL: https://issues.apache.org/jira/browse/FLINK-4421
>             Project: Flink
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Stephan Ewen
>            Priority: Minor
>
> Currently, many places use {{System.currentTimeMillis()}} to acquire 
> timestamps or measure time intervals.
> Since this relies on the system clock, and the system clock is not 
> necessarily monotonous (in the presence of clock updates), this can lead to 
> negative duration and decreasing timestamps where increasing timestamps are 
> expected.



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