[jira] [Resolved] (ZOOKEEPER-1131) Transactions can be dropped because leader election uses last committed zxid instead of last acknowledged/received zxid

["Alexander Shraer (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ZOOKEEPER-1131?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Alexander Shraer resolved ZOOKEEPER-1131.
-----------------------------------------

    Resolution: Not A Problem

> Transactions can be dropped because leader election uses last committed zxid instead of last acknowledged/received zxid
> -----------------------------------------------------------------------------------------------------------------------
>
>                 Key: ZOOKEEPER-1131
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1131
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: leaderElection, server
>    Affects Versions: 3.4.0
>            Reporter: Alexander Shraer
>
> Suppose we have 3 servers - A, B, C which have seen the same number of commits. 
> - A is the leader and it sends out a new proposal.
> - B doesn't receive the proposal, but A and C receive and ACK it
> - A commits the proposal, but fails before anyone else sees the commit.
> - B and C start leader election. 
> - since both B and C saw the same number of commits, if B has a higher server-id than C, leader election will elect B. Then, the last transaction will be truncated from C's log, which is a bug since it was acked by a majority.
>   
> This happens since servers propose their last committed zxid in leader election, and not their last received / acked zxid (this is not being tracked, AFAIK). See method
> FastLeaderElection.getInitLastLoggedZxid(), which calls QuorumPeer.getLastLoggedZxid(), which is supposed to return the last logged Zxid, but instead calls zkDb.getDataTreeLastProcessedZxid() which returns the last committed zxid.

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