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From "Raghu S (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-107) Allow dynamic changes to server cluster membership
Date Thu, 07 May 2009 02:08:30 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12706697#action_12706697
] 

Raghu S commented on ZOOKEEPER-107:
-----------------------------------

I think there are some corner cases that may make the leader election impossible during a
node addition. Say the current config is A,B,C and the new config is A,B,C,D. When the leader
is trying to commit the new configuration, the power goes out and comes back on when only
A and B have logged the new configuration. Peer count in <A,B,C,D> = <4,4,3,3>
now. An election is not possible if C is down because A and B think the majority is 3 peers
and D can't participate in the election since it hasn't joined the cluster yet. 

It sounds like some out of band communication between an existing peer and a new peer is needed
to make this thing work. If a peer restarts or notices quorum loss and if the last logged
update is a node addition, the peer should try to contact the newly added server so that it
can push it's log to the new peer (if the new peer doesn't already have an up to date log)
and ask the new peer to restart. Until A or B do that in the above case, an election may not
be possible.

> Allow dynamic changes to server cluster membership
> --------------------------------------------------
>
>                 Key: ZOOKEEPER-107
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-107
>             Project: Zookeeper
>          Issue Type: Improvement
>          Components: server
>            Reporter: Patrick Hunt
>
> Currently cluster membership is statically defined, adding/removing hosts to/from the
server cluster dynamically needs to be supported.

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