[jira] [Assigned] (KUDU-2048) consensus: Only evict a replica is a majority is up to date

["Todd Lipcon (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/KUDU-2048?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Todd Lipcon reassigned KUDU-2048:
---------------------------------

    Assignee: Todd Lipcon

> consensus: Only evict a replica is a majority is up to date
> -----------------------------------------------------------
>
>                 Key: KUDU-2048
>                 URL: https://issues.apache.org/jira/browse/KUDU-2048
>             Project: Kudu
>          Issue Type: Bug
>          Components: consensus, recovery
>    Affects Versions: 1.4.0
>            Reporter: Mike Percy
>            Assignee: Todd Lipcon
>
> In the context of replica eviction and 3-2-3 recovery, we currently have a "hacky" rule that states that evicting down to less than 2 replicas in a config is prohibited. However we don't currently check to see, when evicting, whether that would leave the config with less than a majority of caught-up voters.
> That means, for example, that if we have a config of 3 replicas { A, B, C } and B falls behind, so is currently undergoing a tablet copy, and C goes offline then the algorithm will evict C. However, since A is the only up-to-date replica, this leaves the config in a state where nothing can commit until B is done copying. Even worse, if B is killed or has an error, then we are left in a state that requires manual recovery.
> Consider adding an additional rule that states that to evict a node, we must have a majority of up-to-date replicas that are recently active. This will help prevent certain problem scenarios like the above from occurring.



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