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From "Henry Robinson (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-368) Observers
Date Wed, 08 Jul 2009 17:03:14 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12728797#action_12728797

Henry Robinson commented on ZOOKEEPER-368:

{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?}

Not really. I was working on the assumption that it should be the peer and not the leader
which decides whether or not it wants to be an observer or a follower. Since peers are identified
on the leader by their IP address, there's no way for the leader to tell the difference between
a follower or an observer from the same address. 

I was thinking of situations where a follower might not be able to, e.g., write to disk because
a partition was full and therefore wanted to indicate to the leader that it had effectively
failed while still receiving updates. If the leader sees that a follower has stopped replying
to pings (because they are queued behind a sync to disk), I think it currently disconnects
the follower; the follower might want to reconnect as an observer to do 'best effort' relaying
of updates to clients. 

I admit this is quite contrived! It's ok for this not to work as I describe, although I think
it will work as a side effect of the dynamic cluster stuff.

{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?}

Yep, absolutely. However as Flavio says the leader can rate limit state transfers if this
seems to be happening a lot. I would expect that log to disk be turned on for most observers,
but we can turn it off transparently to the leader if we just want to use an observer to,
say, publish updates to an rss feed or something and don't want to throw disk at the problem.
In fact, observers could even turn off syncing with the leader for certain use cases.

As soon as I find a little bit of bandwidth I'll get the new patch on here for discussion.

> 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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