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Henry Robinson commented on ZOOKEEPER-368: ------------------------------------------ > i'm very sensitive to the work already done issue! i've totally been there. :) As long as we get the right solution, it's all good. The amount of new code is an issue. I think we have to just make a judgement call. Most of the Observer code is shared between Observers and Followers, so I think that existing tests would exercise a lot of the code. I will finish up a tightened version of the patch in the next day or so that I think can be considered for commit (in terms of code quality) and post that, and I will also write something on the wiki page about precisely what has changed so that we have something concrete to discuss. The current patch is really only a proof-of-concept. For example: the VIEWCHANGE message was a hangover from the dynamic ensemble change stuff (since removed), and certainly shouldn't be in the patch. Now I've got the hang of git branches, keeping things separate is a lot easier to do... I'm up for a conference call to discuss this - I'm also in SF for a week next week, so maybe we can meet in person at last and talk this over :) > 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, > 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.