Henry Robinson updated ZOOKEEPER-368:

    Attachment: ZOOKEEPER-368.patch

Docs update: added explicit mention of need to set electionAlg=0. 

> Observers: core functionality 
> ------------------------------
>                 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: obs-refactor.patch, observer-refactor.patch, observers 
> sync benchmark.png, observers.patch, ZOOKEEPER-368.patch, 
> ZOOKEEPER-368.patch, ZOOKEEPER-368.patch, ZOOKEEPER-368.patch, 
> ZOOKEEPER-368.patch, ZOOKEEPER-368.patch, ZOOKEEPER-368.patch, 
> ZOOKEEPER-368.patch, ZOOKEEPER-368.patch, ZOOKEEPER-368.patch
> Edit (Henry Robinson/henryr) 12/11/09:
> This JIRA specifically concerns the implementation of non-voting peers called 
> Observers, their documentation and their tests. 
> Explicit goals are 1. not breaking any current ZK functionality, 2. enabling 
> at least one deployment scenario involving Observers, 3. documentation 
> describing how to use the feature and 4. tests validating the correct 
> behaviour of 2. 
> Non goals of this JIRA are 1. performance optimizations specific to 
> Observers, 2. compatibility with every feature of ZooKeeper (in particular 
> all leader election protocols), which are both to be addressed in future 
> JIRAs. 
> See http://wiki.apache.org/hadoop/ZooKeeper/Observers for more detail of use 
> cases, proposed design and usage.
> See http://wiki.apache.org/hadoop/ZooKeeper/Observers/ReviewGuide for a brief 
> commentary on the current 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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