[jira] [Created] (RATIS-1161) Updating commit index of follower does not require term check.

["Glen Geng (Jira)" <ji...@apache.org>]

Glen Geng created RATIS-1161:
--------------------------------

             Summary: Updating commit index of follower does not require term check.
                 Key: RATIS-1161
                 URL: https://issues.apache.org/jira/browse/RATIS-1161
             Project: Ratis
          Issue Type: Improvement
          Components: server
    Affects Versions: 1.1.0
            Reporter: Glen Geng


This is a followup of Jira https://issues.apache.org/jira/browse/RATIS-748

 

According to the basic raft algorithm, the logic of advancing commit index for follower and leader is different.

*For follower:*

When receiving an AppendEntries Request, the logic is

 
{code:java}
If leaderCommit > commitIndex: set commitIndex = min(leaderCommit, index of last new entry)
{code}
*For leader:*

The majority commit should only be used on the log entry that created by current leader, thus need a term check.
{code:java}
If there exists an N such that N > commitIndex, a majority of matchIndex[i] ≥ N, and log[N].term == currentTerm: set commitIndex = N{code}
 

*For current ratis:*

We apply the advancing commit index logic of leader to follower, which will make follower's commit index to lag behind.
{code:java}
  /**
   * Update the last committed index.
   * @param majorityIndex the index that has achieved majority.
   * @param currentTerm the current term.
   * @return true if update is applied; otherwise, return false, i.e. no update required.
   */
  public boolean updateLastCommitted(long majorityIndex, long currentTerm) {
    try(AutoCloseableLock writeLock = writeLock()) {
      final long oldCommittedIndex = getLastCommittedIndex();
      final long newCommitIndex = Math.min(majorityIndex, getFlushIndex());
      if (oldCommittedIndex < newCommitIndex) {
        // Only update last committed index for current term. See §5.4.2 in
        // paper for details.
        final TermIndex entry = getTermIndex(newCommitIndex);
        if (entry != null && entry.getTerm() == currentTerm) {
          commitIndex.updateIncreasingly(newCommitIndex, traceIndexChange);
          return true;
        }
      }
    }
    return false;
  }
{code}
 

*Intention of this Jira*

We want to prove the we can safely revert the advancing commit index logic of follower.

 
 



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