Could Raft elect a leader with uncommitted log?

Question

Suppose a cluster of 5 nodes(ABCDE), node-A is elected leader at the beginning, and while leader-A issues AppendEntries RPCs to the follower(BCDE) to replicate log entry(log-X)，only node-B receives and returns success, at this point leader-A crashes.

If node C(or D or E) wins the next leader election then everything's fine, because only node B has log-X, and that means log-X is not committed.

My question is, could node-B (which has the highest term and longest log) win the next leader election? If so, will node-B spread log-X to other nodes?

Answer 1

Yes B could win the election, if it does become leader then the first thing it does it to create a log entry with the new term in its log, and start replicating its log to all the followers. As B's log includes log-X, if all goes well, eventually the log-X entry will be replicated & considered committed.

If node C wins the election, then when it becomes leader, it won't have the log-X entry, and it'll end up overwriting that entry on Node B.

See section 5.4.2 of the raft paper for more details.

Also this means you can't treat failure as meaning the attempted entry definitely doesn't exist, only that the caller doesn't know the outcome. Section 8 has some suggestions for handling this.