Question

In the Raft consensus algorithm, what specific condition must a candidate meet regarding its log before it can be granted a vote by a follower?

Accepted Answer

In the Raft consensus algorithm, a follower grants its vote to a candidate only if the candidate’s log is at least as up-to-date as the follower’s own log. Up-to-date is defined by comparing the index and term of the last entries in the logs. First, the follower compares the terms of the last log entries. If the candidate’s last log entry has a higher term than the follower’s last entry, the candidate’s log is considered more up-to-date. If the terms are the same, the follower checks the length of the logs. If the candidate’s log is at least as long as the follower’s log, meaning the candidate’s last index is greater than or equal to the follower’s last index, the candidate is considered up-to-date. If either condition is met, the candidate’s log contains all committed entries from previous terms, ensuring the safety of the replicated state machine. If the candidate’s log is shorter or has a lower term at the last entry, the follower denies the vote, preventing the candidate from becoming leader and potentially overwriting committed data.

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In the Raft consensus algorithm, what specific condition must a candidate meet regarding its log before it can be granted a vote by a follower?