Henry Robinson commented on ZOOKEEPER-368:

These are all great points. 

Addressing Patrick's points:

1. No - the ensemble doesn't 'know' about the observers (although the Leader 
maintains FollowerHandlers for them) so doesn't notice when they leave.
2. I need to look further into JMX - I don't know a lot about it. Currently 
Observers do pretty much the same thing as Followers.
3. Probably need to update 'stat'; I'll look at the others.
4. I've been trying to do this but need to do a second pass to say I'm happy 
with it.
5. Yes, for the time being any observer can connect. It would be very easy to 
whitelist say ranges of observer IPs in the config file - in fact we probably 
want to do that for all peers. 

I'll be more formal about testing and b/w compatibility although I'll 
appreciate input on both. I'll try and write a few words for a wiki page - I've 
already started work on the documentation but don't want to go too far before 
we agree on the general approach.

> 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
> 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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