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From "Benjamin Reed (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-368) Observers
Date Wed, 08 Jul 2009 15:33:14 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12728749#action_12728749
] 

Benjamin Reed commented on ZOOKEEPER-368:
-----------------------------------------

since henry is doing the work, i'm certainly willing to see how it goes with a new message.
the two disadvantages i see with this approach (of adding INFORM):
* at a high level the only difference between a follower and an observer is that its ACKs
do not get counted. i think this high level difference can translate into a simple modification
of the follower. (since we already have to make sure that the follower sees proposals before
commits, we shouldn't run into the problem you alluded to henry.)
* for the dynamic ensemble patch we want to be able to convert observers to followers easily
and quickly. flipping a switch at the observer to start sending ACKs and at the leader to
start acknowledging the ACKs would be an easy way to do the conversion.

oh and there already is a jira about going into read-only mode on disconnect.

> 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
>
>
> 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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