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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-261) Reinitialized servers should not participate in leader election
Date Tue, 06 Dec 2016 23:28:58 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-261?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15727075#comment-15727075
] 

ASF GitHub Bot commented on ZOOKEEPER-261:
------------------------------------------

Github user enixon commented on the issue:

    https://github.com/apache/zookeeper/pull/120
  
    Thanks, @hanm , let's see if editing the correct string into the title suffices or if
I need to open up a new PR.


> Reinitialized servers should not participate in leader election
> ---------------------------------------------------------------
>
>                 Key: ZOOKEEPER-261
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-261
>             Project: ZooKeeper
>          Issue Type: Improvement
>          Components: leaderElection, quorum
>            Reporter: Benjamin Reed
>
> A server that has lost its data should not participate in leader election until it has
resynced with a leader. Our leader election algorithm and NEW_LEADER commit assumes that the
followers voting on a leader have not lost any of their data. We should have a flag in the
data directory saying whether or not the data is preserved so that the the flag will be cleared
if the data is ever cleared.
> Here is the problematic scenario: you have have ensemble of machines A, B, and C. C is
down. the last transaction seen by C is z. a transaction, z+1, is committed on A and B. Now
there is a power outage. B's data gets reinitialized. when power comes back up, B and C comes
up, but A does not. C will be elected leader and transaction z+1 is lost. (note, this can
happen even if all three machines are up and C just responds quickly. in that case C would
tell A to truncate z+1 from its log.) in theory we haven't violated our 2f+1 guarantee, since
A is failed and B still hasn't recovered from failure, but it would be nice if when we don't
have quorum that system stops working rather than works incorrectly if we lose quorum.



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