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https://issues.apache.org/jira/browse/ZOOKEEPER-2418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15263211#comment-15263211
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Nicholas Wolchko commented on ZOOKEEPER-2418:
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Transactions are applied based on receiving a quorum of acks. The
SyncRequestProcessor being stalled prevents the leader from acking proposals,
but committing is based only on receiving enough acks. If there are a quorum of
followers who have acked the proposal, the leader will continue to commit the
transaction even though it didn't ack it itself.
In my example the leader has applied all transactions from 1-1000 to its data
tree even though the SyncRequestProcessor is stalled.
> txnlog diff sync can skip sending some transactions to followers
> ----------------------------------------------------------------
>
> Key: ZOOKEEPER-2418
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2418
> Project: ZooKeeper
> Issue Type: Bug
> Components: server
> Affects Versions: 3.5.1
> Reporter: Nicholas Wolchko
> Assignee: Nicholas Wolchko
> Priority: Critical
> Original Estimate: 168h
> Remaining Estimate: 168h
>
> If the leader is having disk issues so that its on disk txnlog is behind the
> in memory commit log, it will send a DIFF that is missing the transactions in
> between the two.
> Example:
> There are 5 hosts in the cluster. 1 is the leader. 5 is disconnected.
> We commit up to zxid 1000.
> At zxid 450, the leader's disk stalls, but we still commit transactions
> because 2,3,4 are up and acking writes.
> At zxid 1000, the txnlog on the leader has 1-450 and the commit log has
> 500-1000.
> Then host 5 regains its connection to the cluster and syncs with the leader.
> It will receive a DIFF containing zxids 1-450 and 500-1000.
> This is because queueCommittedProposals in the LearnerHandler just queues
> everything within its zxid range. It doesn't give an error if there is a gap
> between peerLastZxid and the iterator it is queueing from.
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