zookeeper-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-2845) Data inconsistency issue due to retain database in leader election
Date Wed, 31 Jan 2018 20:26:00 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16347557#comment-16347557
] 

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

GitHub user revans2 opened a pull request:

    https://github.com/apache/zookeeper/pull/453

    ZOOKEEPER-2845: Send a SNAP if transactions cannot be verified.

    I will be creating a patch/pull request for 3.4 and 3.5 too, but I wanted to get a pull
request up for others to look at ASAP.
    
    I have a version of this based off of #310 at https://github.com/revans2/zookeeper/tree/ZOOKEEPER-2845-orig-test-patch
but the test itself is flaky.  Frequently leader election does not go as planned on the test
and it ends up failing but not because it ended up in an inconsistent state.
    
    I am happy to answer any questions anyone has about the patch.  

You can merge this pull request into a Git repository by running:

    $ git pull https://github.com/revans2/zookeeper ZOOKEEPER-2845-master

Alternatively you can review and apply these changes as the patch at:

    https://github.com/apache/zookeeper/pull/453.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #453
    
----
commit 0219b2c9e44527067cd5fed4b642729171721886
Author: Robert Evans <evans@...>
Date:   2018-01-29T20:27:10Z

    ZOOKEEPER-2845: Send a SNAP if transactions cannot be verified.

----


> 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.



--
This message was sent by Atlassian JIRA
(v7.6.3#76005)

Mime
View raw message