Stephan Ewen created FLINK-4423:
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Summary: Introduce a Clock utility for monotonous system timestamps
Key: FLINK-4423
URL: https://issues.apache.org/jira/browse/FLINK-4423
Project: Flink
Issue Type: Sub-task
Reporter: Stephan Ewen
I suggest to introduce a {{Clock}} class that provides a
{{currentTimeMillis()}} function that calls {{System.currentTimeMillis()}} but
also remembers the max returned timestamp so far. That way it would never
return decreasing timestamps.
In the presence of clock backwards adjustments, the appearance would be that
time stands still for a while, until the clock has caught up with the previous
timestamp.
Since we don't rely on this for measuring timeouts, but only for logging /
visualization / etc (see [FLINK-4422]) it should not mess up any distributed
system behavior.
We would use this in places like the {{ExecutionGraph}}, where we record
timestamps for state transitions. That way, the utilities that derive charts
and times from the status timestamps would not be thrown off if timestamps were
decreasing when expected increasing.
The same holds for ingestion time timestamps and for processing time triggers.
NOTE: I would like some other opinions on that - it is a somewhat delicate
matter.
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