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Henry Robinson commented on ZOOKEEPER-368: ------------------------------------------ Hi Falvio / Patrick - Thanks for your comments! Design document - there's a brief writeup at http://wiki.apache.org/hadoop/ZooKeeper/Observers which very broadly covers the design. I will update it when I get a moment to do so. User documentation - yes, will do, already on my to do list. There is a section in the above wiki page that will be a good start. Quorums - yes, it should work with all mechanisms. The only caveat is that it only works with the simple LeaderElection protocol, which presumes a majority quorum approach (there are lines where votes > quorum.size() / 2 is hardcoded rather than using the verifier - I think this is the source of at least one of the to-dos). Debug messages: ugh, sorry about that. Will update the patch to build against trunk shortly and remove those messages. > 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. -- This message is automatically generated by JIRA. - You can reply to this email to add a comment to the issue online.