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a grossly outdated configuration?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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