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https://issues.apache.org/jira/browse/ZOOKEEPER-1607?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13766740#comment-13766740
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Raul Gutierrez Segales commented on ZOOKEEPER-1607:
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Arrrg I guess dependent patches aren't applied :(
> Read-only Observer
> ------------------
>
> Key: ZOOKEEPER-1607
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1607
> Project: ZooKeeper
> Issue Type: Improvement
> Components: server
> Affects Versions: 3.4.3
> Reporter: Thawan Kooburat
> Attachments: persistent-read-only-for-observers.patch
>
>
> This feature reused some of the mechanism already provided by
> ReadOnlyZooKeeper (ZOOKEEPER-704) but implemented in a different way
> Goal: read-only clients should be able to connect to the observer or continue
> to read data from the observer event when there is an outage of underling
> quorum. This means that it is possible for the observer to provide 100% read
> uptime for read-only local session (ZOOKEEPER-1147)
> Implementation:
> The observer don't tear down itself when it lose connection with the leader.
> It only close the connection associated with non read-only sessions and
> global sessions. So the client can try other observer if this is a temporal
> failure.
> During the outage, the observer switch to read-only mode. All the pending and
> future write requests get will get NOT_READONLY error. Read-only state
> transition is sent to all session on that observer. The observer only accepts
> a new connection from a read-only client.
> When the observer is able to reconnect to the leader. It sends state
> transition (CONNECTED_STATE) to all current session. If it is able to
> synchronize with the leader using DIFF, the steam of txns is sent through the
> commit processor instead of applying to the DataTree directly to prevent
> raise condition between in-flight read requests (see ZOOKEEPER-1505). The
> client will receive watch events correctly and can start issuing write
> requests.
> However, if the observer is getting the snapshot. It need to drop all the
> connection since it cannot fire a watch correctly.
>
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