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 

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. 

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