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https://issues.apache.org/jira/browse/ZOOKEEPER-1549?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13566971#comment-13566971
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Thawan Kooburat commented on ZOOKEEPER-1549:
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Ah sorry, I forgot to included your test case in ZOOKEEPER-1558. I can add that
one too. Jacky's test is more like end-to-end test. I think we should have one
for this patch, but it would be better if the test don't rely too much on
ZooKeeper internals since it will be hard to maintain the code moving forward.
The reason I create the first version on 3.4 is because we still have pending
ZOOKEEPER-107 on trunk which may require major rework if that patch get
committed.
I also am aiming to support rolling upgrade for both 3.4.5 -> 3.4.6 and 3.4.x
-> 3.5. I am not sure about 3.3.x -> 3.4.6 since some backward compatibility
with respect to Zab Pre 1.0 is removed. However, the backward compatibility
stuff that need to be retained is allowing the learner to handle outstanding
proposals during synchronization phase. This is not related to the protocol but
it just the implementation of the leader. I haven't changed the leader to get
rid of this behavior in this patch.
> Data inconsistency when follower is receiving a DIFF with a dirty snapshot
> --------------------------------------------------------------------------
>
> Key: ZOOKEEPER-1549
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1549
> Project: ZooKeeper
> Issue Type: Bug
> Components: quorum
> Affects Versions: 3.4.3
> Reporter: Jacky007
> Assignee: Thawan Kooburat
> Priority: Blocker
> Fix For: 3.5.0, 3.4.6
>
> Attachments: case.patch, ZOOKEEPER-1549-3.4.patch,
> ZOOKEEPER-1549-learner.patch
>
>
> the trunc code (from ZOOKEEPER-1154?) cannot work correct if the snapshot is
> not correct.
> here is scenario(similar to 1154):
> Initial Condition
> 1. Lets say there are three nodes in the ensemble A,B,C with A being the
> leader
> 2. The current epoch is 7.
> 3. For simplicity of the example, lets say zxid is a two digit number,
> with epoch being the first digit.
> 4. The zxid is 73
> 5. All the nodes have seen the change 73 and have persistently logged it.
> Step 1
> Request with zxid 74 is issued. The leader A writes it to the log but there
> is a crash of the entire ensemble and B,C never write the change 74 to their
> log.
> Step 2
> A,B restart, A is elected as the new leader, and A will load data and take a
> clean snapshot(change 74 is in it), then send diff to B, but B died before
> sync with A. A died later.
> Step 3
> B,C restart, A is still down
> B,C form the quorum
> B is the new leader. Lets say B minCommitLog is 71 and maxCommitLog is 73
> epoch is now 8, zxid is 80
> Request with zxid 81 is successful. On B, minCommitLog is now 71,
> maxCommitLog is 81
> Step 4
> A starts up. It applies the change in request with zxid 74 to its in-memory
> data tree
> A contacts B to registerAsFollower and provides 74 as its ZxId
> Since 71<=74<=81, B decides to send A the diff.
> Problem:
> The problem with the above sequence is that after truncate the log, A will
> load the snapshot again which is not correct.
> In 3.3 branch, FileTxnSnapLog.restore does not call listener(ZOOKEEPER-874),
> the leader will send a snapshot to follower, it will not be a problem.
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