Henry Robinson updated ZOOKEEPER-368:

    Attachment: ZOOKEEPER-368.patch

New patch - now that the refactor has gone in, Hudson should be able to give 
this the once over.

Findbugs is 0 for me, patch applies against trunk and tests pass. 

The only restriction with this patch is that Observers only work with the 
vanilla LeaderElection protocol. This is because they need a responder thread 
to run so that they can query votes from the ensemble, and this doesn't happen 
if electionAlg>0. I have a patch nearly done to start the responderThread for 
every leader election algorithm, but it's not as simple as it might seem: we 
need a TCP responder thread, a new port to run it on and a possible race 
condition with LETest sorted out first. I've done most of this, but adding 
those to this patch would just overcomplicate things. An exception will be 
thrown if you try to start a cluster w/o electionAlg=0 (and there's a test for 

That aside, I'd be grateful for comments and feedback, as I think this patch is 
very nearly good to go. 

> 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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