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https://issues.apache.org/jira/browse/ZOOKEEPER-368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12724574#action_12724574
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Henry Robinson commented on ZOOKEEPER-368:
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That's a good suggestion, and would work. 

I wanted Observers to follow the same bootstrap code as Followers - and 
therefore have a LOOKING state and not depend on a Follower - which could be 
lagging and perhaps, therefore, never finding a leader - for leader resolution.

LE doesn't actually need to know about Observers, the point is more to make 
sure that Observers don't know anything about LE. This means just making sure 
that Observers don't try and initiate elections or vote in them. 

As a more general point, I'm trying to reuse a lot of code between Observers 
and Followers, as they are extremely similar in their behaviour. What do people 
think about the need to break out Observer into its own class, and probably 
inherit from a base Peer set of classes where the code needs to be shared? I'm 
in two minds - it's cleaner to do, but will increase the complexity of 
following the code. 



> 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
>         Attachments: 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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