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https://issues.apache.org/jira/browse/KUDU-567?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Mike Percy updated KUDU-567:
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Parent: KUDU-560
> Remove 'update_lock_' from consensus
> ------------------------------------
>
> Key: KUDU-567
> URL: https://issues.apache.org/jira/browse/KUDU-567
> Project: Kudu
> Issue Type: Sub-task
> Components: consensus
> Affects Versions: M4.5
> Reporter: David Alves
>
> We should remove 'update_lock_' from the consensus implementation, but in
> order to do so we need to use two watermarks.
> The last received watermark should reflect what has been received from the
> leader, the replica updates it when new messages arrive and the leader uses
> it to select which messages to send, but doesn't update global watermarks
> based on it.
> The last replicated watermark reflects exclusively what has been made durable
> at the replica, the leader uses this watermark to advance global watermarks.
> Removing 'update_lock_' presently would create a subtle bug. Here's the
> scenario:
> 1- Leader sends messages 1-10 to replica.
> 2- Replica takes all messages, prepares them and appends them to the log.
> 3- The log append takes a while, the rpc times out at the leader and the
> leader resends messages 1-10.
> 4- Messages are deduplicated at the replica making the leader's message
> "status-only", i.e. without anything for the replica to do, the replica would
> reply immediately with the last received watermark = 10 even though the log
> append from step 2 might not have completed yet.
> Having the two watermarks would allow the 2nd message to reply immediately to
> the leader without having the leader advance the watermarks more than it
> should.
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