Flavio Paiva Junqueira commented on ZOOKEEPER-107:

That's a great catch, Henry, the one related to having any new (perhaps 
invalid) follower being able to submit requests. When you start a new follower 
not in the configuration, do you run it  as a regular replica and let it find 
its way or you explicitly tell the follower to connect to the leader?

I'm not sure if we should discuss detail of the observer here or in the other 
jira, but I'm wondering how an observer is able to find the leader to connect. 
The default leader election uses identifiers to connect and form quorums, so 
I'm not sure a server not in the configuration would be able to determine which 
replica is the leader. I think we can do it with leader election 0, though, if 
a leader has been elected and is running

Are you planning on having observers as a separate feature, as per ZK-368? It 
would be great to have it, since you are going through the effort of 
implementing it already.

As for the message to observers containing the transaction, the advantage of 
having a special message (e.g., INFORM) is that we cut down the number of 
messages to observers: INFORM is essentially a COMMIT containing the request. 
If we don't change the protocol, then we can just have the leader sending a 
PROPOSAL to everyone, including the observers. As observers will receive the 
COMMIT as well, we have higher message complexity. For now, I'm good either 

> 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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