SOLR-16843: Replace timeNs by epochTimeNs in most of autoscaling #2679
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SOLR-16843
Description
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.
This timestamp cannot be re-used across JVM instances (either in another Solr node or same node after JVM restart).
Solution
For all timestamps that are either persisted at some point or used for event timestamps, use
getEpochTimeNs()instead ofgetTimeNs(). Values returned bygetEpochTimeNs()are absolute and can be safely compared.