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https://issues.apache.org/jira/browse/RATIS-748?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16969946#comment-16969946
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Lokesh Jain commented on RATIS-748:
-----------------------------------
This issue was found while debugging RATIS-727. One of the followers has the
following metadata -
flushIndex - 1371
commitIndex - 269
leaderCommitIndex - 1618
The follower flushIndex has reached 1370 but its commitIndex is still not
updated.
> 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
> Priority: Critical
>
> 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. 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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