[jira] Updated: (ZOOKEEPER-159) Cover two corner cases of leader election

["Flavio Paiva Junqueira (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ZOOKEEPER-159?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Flavio Paiva Junqueira updated ZOOKEEPER-159:
---------------------------------------------

    Status: Patch Available  (was: Open)

> Cover two corner cases of leader election
> -----------------------------------------
>
>                 Key: ZOOKEEPER-159
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-159
>             Project: Zookeeper
>          Issue Type: Bug
>          Components: leaderElection
>            Reporter: Flavio Paiva Junqueira
>             Fix For: 3.0.0
>
>         Attachments: ZOOKEEPER-159.patch
>
>
> In the default implementation of leader election, there are two undesirable cases that need to be covered:
> 1- If there is a leader elected and this leader is supported by at least  quorum of peers, then it can happen that one peer disconnects from the leader, and initiates a new leader election. As it is a new leader election, we increment the logical clock of this peer, and according to the current implementation, this peer won't accept any vote from a peer with a lower LE turn (corresponds to the value of the logical clock of the voting peer). The attached patch corrects this problem by allowing a peer to go back to a previous epoch in the case a majority votes for a leader, and the peer also receives a vote from the leader. This feature allows a peer to correct a false suspicion of the current leader;
> 2- If a peer  advances to a new turn before others, then it may end up voting for a peer that either does not have the highest server id or the the highest zxid. The attached patch fixes this problem by resetting the vote a peer when it updates its logical clock upon receiving a notification with a higher turn value.

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