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https://issues.apache.org/jira/browse/ZOOKEEPER-1549?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14051688#comment-14051688
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Hongchao Deng commented on ZOOKEEPER-1549:
------------------------------------------
I think the root of the problem is in this function:
```
public long restore(DataTree dt, Map<Long, Integer> sessions,
PlayBackListener listener) throws IOException {
...
while (true) {
// iterator points to
// the first valid txn when initialized
hdr = itr.getHeader();
...
processTransaction(hdr,dt,sessions, itr.getTxn());
...
listener.onTxnLoaded(hdr, itr.getTxn());
if (!itr.next())
break;
}
...
```
Instead of processing all transactions in the log, keeping an index of
committedUpTo and processing until that makes more sense. Am I correct??
> 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
>
> Attachments: ZOOKEEPER-1549-3.4.patch, ZOOKEEPER-1549-learner.patch,
> case.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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