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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 Wed, 07 Dec 2016 06:47:59 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-261?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15727911#comment-15727911
] 

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_r91234951
  
    --- Diff: src/java/main/org/apache/zookeeper/server/persistence/FileTxnSnapLog.java ---
    @@ -132,6 +137,9 @@ public FileTxnSnapLog(File dataDir, File snapDir) throws IOException
{
     
             txnLog = new FileTxnLog(this.dataDir);
             snapLog = new FileSnap(this.snapDir);
    +
    +        autoCreateDB = Boolean.parseBoolean(System.getProperty(ZOOKEEPER_DB_AUTOCREATE,
    --- End diff --
    
    I included `ZOOKEEPER_DB_AUTOCREATE` to allow users to opt out of the feature until they're
ready to update their ensemble management tooling to support creating the new file. Is that
in accord with Zookeeper style?
    
    On the question of style, `ZOOKEEPER_DB_AUTOCREATE_DEFAULT` exists purely because `ZOOKEEPER_DATADIR_AUTOCREATE_DEFAULT`
exists above it in the file. If including the defaults as static constants isn't Zookeeper
style then I'm happy to replace it with a string literal in the constructor.


> 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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