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https://issues.apache.org/jira/browse/ZOOKEEPER-368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12730522#action_12730522
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Henry Robinson commented on ZOOKEEPER-368:
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Hi Mahadev -
To answer your question one by one:
* I think the read-scalability issue is an important use case. Observers are
also a natural way to deal with peers that try to connect but are not part of
the current view, which will be important for the dynamic membership patch. I
also would like to use them to publish proposals; for example they are a
convenient point to integrate a larger publish-subscribe system. Watches are
very useful for their intended purposes but are not ideal for listening to a
stream of proposals since they are cancelled once fired.
* Set peerType=observer in the configuration file to make a peer an observer.
In the dynamic membership patch, a follower that tries to connect while not in
the current view will become an observer until the view is changed to include
it.
* Yes, there have been no invasive changes in the Follower code (but some
restructuring to allow code re-use). All tests currently pass for me, which
while not conclusive proof, suggests that there have been no behavioural
changes and none are in by design.
* We must test in the same way we test the behaviour of Followers - their
ability to connect, to see proposals, to withstand stress. Some important
observer specific test cases will be:
** Ensuring an observer does not vote in an election or in a proposal (by
testing when an ensemble is not quorate but would be if the observer were a
follower)
** Making sure that an observer does not see messages it shouldn't (PROPOSALs
in particular)
** Ensuring that an observer does not become a leader.
> 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
>
>
> 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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