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https://issues.apache.org/jira/browse/ZOOKEEPER-2846?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16695420#comment-16695420
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Michael K. Edwards commented on ZOOKEEPER-2846:
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Does this need to be addressed (or release noted) for 3.5.5?
> Leader follower sync with on disk txns can possibly leads to data
> inconsistency
> -------------------------------------------------------------------------------
>
> Key: ZOOKEEPER-2846
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2846
> Project: ZooKeeper
> Issue Type: Bug
> Components: quorum
> Affects Versions: 3.4.10, 3.5.3, 3.6.0
> Reporter: Fangmin Lv
> Priority: Critical
>
> On disk txn sync could cause data inconsistency if the current leader just
> had a snap sync before it became leader, and then having diff sync with its
> followers may synced the txns gap on disk. Here is scenario:
> Let's say S0 - S3 are followers, and S4 is leader at the beginning:
> 1. Stop S2 and send one more request
> 2. Stop S3 and send more requests to the quorum to let S3 have a snap sync
> with S4 when it started up
> 3. Stop S4 and S3 became the new leader
> 4. Start S2 and had a diff sync with S3, now there are gaps in S2
> Attached the test case to verify the issue. Currently, there is no efficient
> way to check the gap in txn files is a real gap or due to Epoch change. We
> need to add that support, but before that, it would be safer to disable the
> on disk txn leader-follower sync.
> Another two scenarios which could cause the same issue:
> (Scenario 1) Server A, B, C, A is leader, the others are followers:
> 1). A synced to disk, but the other 2 restarted before receiving the
> proposal
> 2). B and C formed quorum, B is leader, and committed some requests
> 3). A looking again, and sync with B, B won't able to trunc A but send snap
> instead, and leaves the extra txn in A's txn file
> 4). A became new leader, and someone else has a diff sync with A it will
> have the extra txn
> (Scenario 2) Diff sync with committed txn, will only apply to data tree but
> not on disk txn file, which will also leave hole in it and lead to data
> inconsistency issue when syncing with learners.
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