Key: ZOOKEEPER-368
             Project: Zookeeper
          Issue Type: New Feature
          Components: quorum
            Reporter: Flavio Paiva Junqueira

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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