Pierre Salagnac created SOLR-16843:
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Summary: timestamp issue with autoscaling framework
Key: SOLR-16843
URL: https://issues.apache.org/jira/browse/SOLR-16843
Project: Solr
Issue Type: Bug
Security Level: Public (Default Security Level. Issues are Public)
Components: AutoScaling
Affects Versions: 8.11
Reporter: Pierre Salagnac
The framework does not exist anymore with Solr 9+, so this bug only impacts
Solr 8.x.
Triggers in autoscaling framework use timestamps returned by the JVM call
{{{}System.nanoTime(){}}}, but according to the Javadoc, this is NOT an
absolute timestamp. This is just a number relative to a random origin, and this
origin will change each time the JVM is restarted.
I figured out this impacts at least the following triggers (with basically the
same pattern),
- {{IndexSizeTrigger}}
- {{MetricTrigger}}
- {{SearchRateTrigger}}
These triggers want to fire an event when a certain condition (depending on
each trigger) is met for a certain period of time. They maintain a map with
[what, timestamp] entries to track a short term history, with the option to
remove an entry if the condition is not met anymore, so we don't trigger any
event.
Timestamps come from {{{}System.nanoTime(){}}}. So far so good as long as we
compare these timestamps to each others in the same JVM. Now, this map is
persisted in Zookeeper in case of an overseer change (written and read by
{{TriggerBase.saveState()}} and {{restoreState() }}). With an overseer change,
the {{nanoTime()}} origin is randomly moved to something else. Consequently,
all the persisted timestamps from the previous overseer cannot be compared with
the current JVM "clock".
This ends in triggers never being fired, or being fired without waiting for the
time configured.
Simple fix could be to always use {{TimeSource.getEpochTimeNs()}} instead of
{{getTimeNs()}} in autoscaling code.
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