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https://issues.apache.org/jira/browse/ZOOKEEPER-2845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16347557#comment-16347557
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ASF GitHub Bot commented on ZOOKEEPER-2845:
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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.
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