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From "Henry Robinson (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-107) Allow dynamic changes to server cluster membership
Date Tue, 07 Jul 2009 23:59:14 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12728429#action_12728429

Henry Robinson commented on ZOOKEEPER-107:

That's definitely one way to do it. The other side to that argument is to keep the message
complexity down, especially if we can envisage use cases with lots of Observers. A connection
to a remote Observer might be more likely to violate the FIFO requirement of ZK connections;
having a single-message protocol makes it easier to deal with this case (not a correctness
issue of Observers, just annoying if PROPOSALs arrive after COMMITs for some reason). I think
that's a marginal issue though. My preference is for INFORM messages as this completely separates
Observer logic from Follower logic and doesn't add much complexity to the code. 

The Observer also has to take care not to participate in leader elections. I think Observers
also need to announce themselves as such to the Leader, to enable the case where a Follower
wishes to connect as an Observer temporarily (otherwise the Leader will think the Observer
to be a Follower and use it as part of a quorum). Also if the leader can distinguish between
followers and observers then it can treat both differently (e.g. through batching multiple
INFORMs or allowing observers to lag by prioritising follower traffic). 

Keeping Observers as special-case Followers would simplify the code for the observers patch
(I've got a new version nearly ready to submit, just fixing some tests). However, it would
mean that Observers are harder to customise - for example, there's no persistence requirement
for an Observer and so some of the RequestProcessors can be optionally removed or replaced
by something that only asynchronously writes to disk. Keeping them lightweight has been a
goal. My feeling was that I was introducing too many 'if (amObserver()) {...}' branches to
an already fairly hard to follow bit of code (in particular Follower.followLeader). Breaking
the functionality into two separate classes seems to have made things cleaner.

Regarding Observers being able to issue proposals; I don't have a problem with that, should
be reasonably easy to add. 

> 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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