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https://issues.apache.org/jira/browse/RATIS-748?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16973685#comment-16973685
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Tsz-wo Sze commented on RATIS-748:
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> ... The follower does not require the check of entry.getTerm() == currentTerm
> and its commitIndex can be updated to min(majorityIndex, getFlushIndex()). It
> has already verified the entries in the appendEntriesAsync call.
There may be an optimization that the follower do not check term. But how
could it hurt if it checks?
> This can lead to the follower commit being updated in bursts and can lead to
> failure of watch requests.
This is probably related to my previous question. Could you describe in more
details on how could the commit be updated in bursts?
> Follower might not update its commit index
> ------------------------------------------
>
> Key: RATIS-748
> URL: https://issues.apache.org/jira/browse/RATIS-748
> Project: Ratis
> Issue Type: Bug
> Reporter: Lokesh Jain
> Assignee: Lokesh Jain
> Priority: Critical
> Attachments: RATIS-748.001.patch, RATIS-748.002.patch
>
>
> While updating the commit index, the follower checks whether majority index
> is present in the raft log. There can be cases where leader is ahead of the
> follower and follower does not have the entry corresponding to the
> majorityIndex. In such cases the follower commit index is not updated. Below
> is the corresponding code snippet.
> {code:java}
> public boolean updateLastCommitted(long majorityIndex, long currentTerm) {
> try(AutoCloseableLock writeLock = writeLock()) {
> final long oldCommittedIndex = getLastCommittedIndex();
> if (oldCommittedIndex < majorityIndex) {
> // Only update last committed index for current term. See §5.4.2 in
> // paper for details.
> final TermIndex entry = getTermIndex(majorityIndex);
> if (entry != null && entry.getTerm() == currentTerm) {
> final long newCommitIndex = Math.min(majorityIndex, getFlushIndex());
> if (newCommitIndex > oldCommittedIndex) {
> commitIndex.updateIncreasingly(newCommitIndex, traceIndexChange);
> }
> return true;
> }
> }
> }
> return false;
> }{code}
> This function RaftLog#updateLastCommitted is also used by follower to update
> its commit index. The follower does not require the check of entry.getTerm()
> == currentTerm and its commitIndex can be updated to min(majorityIndex,
> getFlushIndex()). It has already verified the entries in the
> appendEntriesAsync call.
> This can lead to the follower commit being updated in bursts and can lead to
> failure of watch requests.
> cc [~shashikant] [~szetszwo]
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