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https://issues.apache.org/jira/browse/ZOOKEEPER-368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12779186#action_12779186
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Flavio Paiva Junqueira commented on ZOOKEEPER-368:
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I have just made sure that the docs compile, and after Henry's response, I
don't have further comments. + 1, good job, Henry!
> Observers: core functionality
> ------------------------------
>
> 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
> Fix For: 3.3.0
>
> 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,
> ZOOKEEPER-368.patch, ZOOKEEPER-368.patch, ZOOKEEPER-368.patch
>
>
> Edit (Henry Robinson/henryr) 12/11/09:
> This JIRA specifically concerns the implementation of non-voting peers called
> Observers, their documentation and their tests.
> Explicit goals are 1. not breaking any current ZK functionality, 2. enabling
> at least one deployment scenario involving Observers, 3. documentation
> describing how to use the feature and 4. tests validating the correct
> behaviour of 2.
> Non goals of this JIRA are 1. performance optimizations specific to
> Observers, 2. compatibility with every feature of ZooKeeper (in particular
> all leader election protocols), which are both to be addressed in future
> JIRAs.
> See http://wiki.apache.org/hadoop/ZooKeeper/Observers for more detail of use
> cases, proposed design and usage.
> See http://wiki.apache.org/hadoop/ZooKeeper/Observers/ReviewGuide for a brief
> commentary on the current 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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