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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-2845) Data inconsistency issue due to retain database in leader election
Date Tue, 13 Feb 2018 12:41:00 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16362233#comment-16362233
] 

ASF GitHub Bot commented on ZOOKEEPER-2845:
-------------------------------------------

Github user revans2 commented on the issue:

    https://github.com/apache/zookeeper/pull/453
  
    Thank you to everyone who reviewed the patch, but with the help of Fangmin Lv I found
one case that the original patch didn't cover.  I have reworked the patch to cover that case,
but to do so I had to take a completely different approach.
    
    I think this is a better approach because it reuses a lot of the code that was originally
run to load the database from disk.  So now instead of reloading the entire database from
disk, we apply all of the uncommitted transactions in the log to the in memory database. 
This should put it in exactly the same state as if we had cleared the data and reloaded it
from disk, but with much less overhead.


> 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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