This is how PhD explain the need for prevoting stage:
One downside of Raft's leader election algorithm is that a server that
has been partitioned from the cluster is likely to cause a disruption
when it regains connectivity. When a server is partitioned, it will
not receive heartbeats. It will soon increment its term to start
an election, although it won't be able to collect enough votes to
become leader. When the server regains connectivity sometime later, its
larger term number will propagate to the rest of the cluster (either
through the server's RequestVote requests or through its AppendEntries
response). This will force the cluster leader to step down, and a new
election will have to take place to select a new leader.
Prevoting stage is addressing that. In the Prevote algorithm, a
candidate only increments its term if it first learns from a majority of
the cluster that they would be willing to grant the candidate their votes
(if the candidate's log is sufficiently up-to-date, and the voters have
not received heartbeats from a valid leader for at least a baseline
election timeout).
The Prevote algorithm solves the issue of a partitioned server disrupting
the cluster when it rejoins. While a server is partitioned, it won't
be able to increment its term, since it can't receive permission
from a majority of the cluster. Then, when it rejoins the cluster, it
still won't be able to increment its term, since the other servers
will have been receiving regular heartbeats from the leader. Once the
server receives a heartbeat from the leader itself, it will return to
the follower state(in the same term).
In our implementation we have "stable leader" extension that prevents
spurious RequestVote to dispose an active leader, but AppendEntries with
higher term will still do that, so prevoting extension is also required.
* scylla-dev/raft-prevote-v5:
raft: store leader and candidate state in state variant
raft: add boost tests for prevoting
raft: implement prevoting stage in leader election
raft: reset the leader on entering candidate state
raft: use modern unordered_set::contains instead of find in become_candidate
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