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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 Thu, 12 Jan 2017 02:13:16 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-261?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15819905#comment-15819905
] 

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

Github user enixon commented on a diff in the pull request:

    https://github.com/apache/zookeeper/pull/120#discussion_r95714094
  
    --- Diff: bin/zkServer-initialize.sh ---
    @@ -113,6 +113,8 @@ initialize() {
         else
             echo "No myid provided, be sure to specify it in $ZOO_DATADIR/myid if using non-standalone"
         fi
    +
    +    date > "$ZOO_DATADIR/initialize"
    --- End diff --
    
    True enough, `touch` is sufficient. Using `date` is an optimization I've included in other
scripts in the past as a way of sneaking a bit more information into an otherwise meaningless
file but in this context it's probably just confusing.


> 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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