Mahadev konar commented on ZOOKEEPER-368:

thanks henry,

bq. See my earlier comments to Flavio - I think connecting Observers to 
Followers would be a significant change and therefore should wait for another 
JIRA. Most if not all the code would be on the Follower side, which would make 
me nervous about backwards compatibility.

- i just meant, that are you guys keeping that in mind in terms of some work 
that needs to be done right now to make that easier to do later (like: 
generalizing somethings for making it easy for observers to connect to 
followers later sometime)
- also one minor nit on QuorumPeerConfig. Should QuorumPeerConfig  just throw 
an exception and fail if peerType is specfied and it does not match either 
observer or participant? If not specified it goes to default of participant!... 
this is just to prevent any configuration mishaps.

> 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
> 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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