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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 Tue, 13 Mar 2012 20:28:43 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-8163?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13228668#comment-13228668
] 

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

Hi Hari,

The creation is not atomic, but the creation of the "info" znode is done after the creation
of the "lock" znode. So, only one NN can try to create an "info" node at a time (since only
one can hold the lock at a time)

The only other possibility is that the NN creates the "lock" znode, then the session expires,
and then creates the "info" znode. But this isn't possible since, if the session expired,
the creation of the "info" node would fail with an exception.

Do you have a specific error scenario in mind that I'm not covering?
                
> 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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