gianm edited a comment on issue #7595: Optimize overshadowed segments computation URL: https://github.com/apache/incubator-druid/pull/7595#issuecomment-494654440 > For the concern about DruidCoordinatorRuleRunner seeing a stale view, do you foresee issues with that acting on an older view of the overshadowed segments? It's not just a view of overshadowed segments, right? The entire timeline would be an older snapshot. I think as long as it's not 'surprisingly rolled back' then it is OK. Meaning: no run should use an older snapshot than a prior run. Within any given coordinator leadership epoch, the snapshot at time T will be newer than the snapshot at previous times < T. So avoiding surprising rollbacks really boils down to avoiding a situation where the _new leader_ uses an older snapshot than the _old leader_. I think the best way to avoid this is to make sure that any snapshot used in a given leadership epoch must have been taken after the start of that epoch. (If it was from prior to the start of the epoch, it might be out of date and using it might lead to a surprising rollback.)
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