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https://issues.apache.org/jira/browse/ZOOKEEPER-368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12728778#action_12728778
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Flavio Paiva Junqueira commented on ZOOKEEPER-368:
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The jira on read-only mode is ZOOKEEPER-40. Thanks, Ben.
> Observers
> ---------
>
> Key: ZOOKEEPER-368
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-368
> Project: Zookeeper
> Issue Type: New Feature
> Components: quorum
> Reporter: Flavio Paiva Junqueira
> Assignee: Henry Robinson
> Attachments: ZOOKEEPER-368.patch, ZOOKEEPER-368.patch
>
>
> Currently, all servers of an ensemble participate actively in reaching
> agreement on the order of ZooKeeper transactions. That is, all followers
> receive proposals, acknowledge them, and receive commit messages from the
> leader. A leader issues commit messages once it receives acknowledgments from
> a quorum of followers. For cross-colo operation, it would be useful to have a
> third role: observer. Using Paxos terminology, observers are similar to
> learners. An observer does not participate actively in the agreement step of
> the atomic broadcast protocol. Instead, it only commits proposals that have
> been accepted by some quorum of followers.
> One simple solution to implement observers is to have the leader forwarding
> commit messages not only to followers but also to observers, and have
> observers applying transactions according to the order followers agreed upon.
> In the current implementation of the protocol, however, commit messages do
> not carry their corresponding transaction payload because all servers
> different from the leader are followers and followers receive such a payload
> first through a proposal message. Just forwarding commit messages as they
> currently are to an observer consequently is not sufficient. We have a couple
> of options:
> 1- Include the transaction payload along in commit messages to observers;
> 2- Send proposals to observers as well.
> Number 2 is simpler to implement because it doesn't require changing the
> protocol implementation, but it increases traffic slightly. The performance
> impact due to such an increase might be insignificant, though.
> For scalability purposes, we may consider having followers also forwarding
> commit messages to observers. With this option, observers can connect to
> followers, and receive messages from followers. This choice is important to
> avoid increasing the load on the leader with the number of observers.
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