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https://issues.apache.org/jira/browse/ZOOKEEPER-2845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16362505#comment-16362505
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ASF GitHub Bot commented on ZOOKEEPER-2845:
-------------------------------------------

Github user revans2 commented on the issue:

    https://github.com/apache/zookeeper/pull/453
  
    @anmolnar I will add some kind of a test.  I ran into a lot of issues with 
`testTxnAheadSnapInRetainDB`.  I could not get it to run correctly against 
master as it would always end up electing the original leader again and the 
test would fail, but not because it had reproduced the issue.  I finally just 
did development work based off of the [original 
patch](https://github.com/apache/zookeeper/compare/master...revans2:ZOOKEEPER-2845-updated-fix?expand=1)
 and verified that `testTxnAheadSnapInRetainDB` passed, or that if it failed it 
did so because of leader election.


> Data inconsistency issue due to retain database in leader election
> ------------------------------------------------------------------
>
>                 Key: ZOOKEEPER-2845
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2845
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: quorum
>    Affects Versions: 3.4.10, 3.5.3, 3.6.0
>            Reporter: Fangmin Lv
>            Assignee: Robert Joseph Evans
>            Priority: Critical
>
> In ZOOKEEPER-2678, the ZKDatabase is retained to reduce the unavailable time 
> during leader election. In ZooKeeper ensemble, it's possible that the 
> snapshot is ahead of txn file (due to slow disk on the server, etc), or the 
> txn file is ahead of snapshot due to no commit message being received yet. 
> If snapshot is ahead of txn file, since the SyncRequestProcessor queue will 
> be drained during shutdown, the snapshot and txn file will keep consistent 
> before leader election happening, so this is not an issue.
> But if txn is ahead of snapshot, it's possible that the ensemble will have 
> data inconsistent issue, here is the simplified scenario to show the issue:
> Let's say we have a 3 servers in the ensemble, server A and B are followers, 
> and C is leader, and all the snapshot and txn are up to T0:
> 1. A new request reached to leader C to create Node N, and it's converted to 
> txn T1 
> 2. Txn T1 was synced to disk in C, but just before the proposal reaching out 
> to the followers, A and B restarted, so the T1 didn't exist in A and B
> 3. A and B formed a new quorum after restart, let's say B is the leader
> 4. C changed to looking state due to no enough followers, it will sync with 
> leader B with last Zxid T0, which will have an empty diff sync
> 5. Before C take snapshot it restarted, it replayed the txns on disk which 
> includes T1, now it will have Node N, but A and B doesn't have it.
> Also I included the a test case to reproduce this issue consistently. 
> We have a totally different RetainDB version which will avoid this issue by 
> doing consensus between snapshot and txn files before leader election, will 
> submit for review.



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