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From "Flavio Paiva Junqueira (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-368) Observers
Date Wed, 08 Jul 2009 16:37:15 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12728775#action_12728775

Flavio Paiva Junqueira commented on ZOOKEEPER-368:

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.

We need a transition from OBSERVING to FOLLOWING on the state of QuorumPeer, and  I believe
your concern is that we end up doing a state transfer when undergoing that transition. Is
it right?

i'm trying to understand the use case for a follower that connects as an observer. this would
adversely affect the reliability of the system since a follower acting as an observer would
count as a failed follower even though it is up. did you have a case in mind?

Why would a follower connect as an observer? I guess I'm missing the point here.

i think it is reasonable to turn off the sync for the observer, but we probably still want
to log to disk so that we can recover quickly. otherwise we will keep doing state transfers
from the leader every time we connect. right?

I believe you're suggesting this because you're afraid that an observer that re-connects too
often might end up overwhelming the leader. A third option would be to have the observer only
keeping the database in memory and having the leader limiting the frequency of state transfers
to observers. If you're concerned about the performance of an observer, then it might be an
idea to consider. 

> 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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