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https://issues.apache.org/jira/browse/ZOOKEEPER-928?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12930780#action_12930780
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Vishal K commented on ZOOKEEPER-928:
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Hi Flavio,
I was aware of that. However, this is not the case of idefinite TCP IO hang. If
the leader hangs (e.g., software deadlock in ZooKeeper) its TCP connection will
remain active. The follower will not see a socket timeout. Now, how can the
follower determine if the leader is down?
> Follower should stop following and start FLE if it does not receive pings
> from the leader
> -----------------------------------------------------------------------------------------
>
> Key: ZOOKEEPER-928
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-928
> Project: Zookeeper
> Issue Type: Bug
> Components: quorum, server
> Affects Versions: 3.3.2
> Reporter: Vishal K
> Priority: Critical
> Fix For: 3.3.3, 3.4.0
>
>
> In Follower.followLeader() after syncing with the leader, the follower does:
> while (self.isRunning()) {
> readPacket(qp);
> processPacket(qp);
> }
> It looks like it relies on socket timeout expiry to figure out if the
> connection with the leader has gone down. So a follower *with no cilents*
> may never notice a faulty leader if a Leader has a software hang, but the TCP
> connections with the peers are still valid. Since it has no cilents, it won't
> hearbeat with the Leader. If majority of followers are not connected to any
> clients, then FLE will fail even if other followers attempt to elect a new
> leader.
> We should keep track of pings received from the leader and see if we havent
> seen
> a ping packet from the leader for (syncLimit * tickTime) time and give up
> following the
> leader.
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