Raft Leader Election

When servers start up, they begin as followers.
A server remains in follower state as long as it receives valid RPCs from a leader or candidate.
Leaders send periodic heartbeats (AppendEntries RPCs that carry no log entries) to all followers in order to maintain their authority.
If a follower receives no communication over a period of time called the election timeout, then it assumes there is no viable leader and begins an election to choose a new leader.
To begin an election, a follower increments its current term and transitions to candidate state.
It then votes for itself and issues RequestVote RPCs in parallel to each of the other servers in the cluster.
A candidate continues to be a candidate until one of three things happens:
- it wins the election
  - receives votes from a majority of the servers in the full cluster for the same term.
- another server establishes itself as leader
  - When waiting for vote, some may have already become leader (or previous leader came back to live)
  - restriction: If the leader’s term (included in its RPC) is at least as large as the candidate’s current term
- a period of time goes by with no winner.
  - timeout and start new RPC
  - uses randomized election timeouts

Guarantees:
- all the committed entries from previous terms are present on each new leader from the moment of its election
  - log entries only flow in one direction, from leaders to followers, and leaders never overwrite existing entries in their logs.
A candidate must contact a majority of the cluster in order to be elected
Each server will vote for at most one candidate in a given term, on a first-come-first-served basis
up to date: compare the index and term of the last entries in the logs.
- diff terms: log with later term is more up to date
- same term, longer log is up-to-date

Fish Touching🐟🎣