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https://issues.apache.org/jira/browse/ZOOKEEPER-368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12775956#action_12775956
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Patrick Hunt commented on ZOOKEEPER-368:
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Henry, I don't see any docs for this in src/docs. I suggest that you start a
new document (new xml file) for this feature, it should explain why/how(torun)
at the very
least -- so that potential users can come up to speed.
Flavio, could you also review the comments on this JIRA as part of your commit
review? We should make sure that either all of the issues are addressed,
or at the very least new JIRAs are created (Henry could you do this?) for the
pending items so that we don't lose the comments/concerns/issues that have been
identified
previously (this is a major new/visible feature so I think it warrants the
extra time/effort).
> 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: obs-refactor.patch, observer-refactor.patch, observers
> sync benchmark.png, observers.patch, ZOOKEEPER-368.patch,
> ZOOKEEPER-368.patch, ZOOKEEPER-368.patch, ZOOKEEPER-368.patch,
> ZOOKEEPER-368.patch, 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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