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From "Raghu S (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-107) Allow dynamic changes to server cluster membership
Date Mon, 13 Jul 2009 15:49:15 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12730384#action_12730384
] 

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

Sorry to jump around bit, I thought I will mention this if we haven't already talked about
it. How do we plan to deal with a situation when a set of nodes can form a majority but  can't
form an ensemble because one or more peers have a grossly outdated configuration? Say an ensemble
of ABCDE moved to EFGHI while E was offline and only EFG are up? They form a majority but
can't form an ensemble since E doesn't know about any of the other servers yet?

One way to address this is to implement an out of band synchronization mechanism in which
E will realize that the ensemble has changed when F and G try to connect to E and have one
them synchronize E's logs since their last know zxids are ahead of E's. E can then attempt
to restart an election. Also, it is possible that F and G could see different ensembles (F
is a bit out dated, G is the most up to date), in which case E might first sync up form F
and then both E and F sync up form G if G comes online a bit later.

Any simpler solutions?

> 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
>            Assignee: Henry Robinson
>         Attachments: SimpleAddition.rtf
>
>
> Currently cluster membership is statically defined, adding/removing hosts to/from the
server cluster dynamically needs to be supported.

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