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From "Todd Lipcon (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-8163) Improve ActiveStandbyElector to provide hooks for fencing old active
Date Wed, 14 Mar 2012 20:34:38 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-8163?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13229589#comment-13229589
] 

Todd Lipcon commented on HADOOP-8163:
-------------------------------------

bq. The question I had was how is the info znode creation prevented when the client does not
have the ephemeral lock znode? Is this ensured in the zk client or at the zookeeper?

This is ensured by ZooKeeper. The only reason the ephemeral node would disappear is if the
session was expired. This means the leader has marked the session as such -- and thus, you
can no longer issue commands under that same session.

To be sure, I just double checked with Pat Hunt from the ZK team. Apparently there was a rare
race condition bug ZOOKEEPER-1208 fixed in 3.3.4/3.4.0 about this exact case: https://issues.apache.org/jira/browse/ZOOKEEPER-1208?focusedCommentId=13149787&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13149787
... but since Hadoop will probably need the krb5 auth from ZK 3.4, it seems a reasonable requirement
to need at least that version.
                
> Improve ActiveStandbyElector to provide hooks for fencing old active
> --------------------------------------------------------------------
>
>                 Key: HADOOP-8163
>                 URL: https://issues.apache.org/jira/browse/HADOOP-8163
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: ha
>    Affects Versions: 0.24.0, 0.23.3
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: hadoop-8163.txt
>
>
> When a new node becomes active in an HA setup, it may sometimes have to take fencing
actions against the node that was formerly active. This JIRA extends the ActiveStandbyElector
which adds an extra non-ephemeral node into the ZK directory, which acts as a second copy
of the active node's information. Then, if the active loses its ZK session, the next active
to be elected may easily locate the unfenced node to take the appropriate actions.

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