Henry Robinson commented on ZOOKEEPER-368:

I've written a proposal document here: 
http://wiki.apache.org/hadoop/ZooKeeper/Observers - comments, edits, additions 
all very welcome. 

Gustavo - good catches. There is some cruft in this patch from the work on 
dynamic membership that I've been doing. Before this patch gets committed I 
shall need to remove it. In particular the VIEWCHANGE messages are completely 
gone. However I hope there's not too much obscuring the intent of this patch. 

Your points about security are very well made - I would suggest a whitelist for 
now (which gives the same level of security as Followers enjoy), follower by a 
separate JIRA to look at a better way to secure the data. I've made some 
comments on the wiki page - please do chime in with suggestions as I'm no kind 
of authority on security. 

Flavio - I think that would be a significant change (would have to add a lot of 
code from Leader into Follower, particularly because Observers can issue 
proposals) and one whose cost / benefits need to be worked out separately. I'd 
prefer to get Observers in first, and then we can look at load-balancing them 
if necessary. How does that sound?

> 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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