Raft Overview: Roles, Terms and RPCs

Designed to be understandable

Raft (Ongaro and Ousterhout) is a consensus algorithm for managing a replicated log, designed explicitly for understandability. It splits the problem into leader election, log replication and safety. At any moment each server is a follower (passive, answers RPCs), a candidate (trying to become leader) or the leader (handles all client requests and replicates the log). Time is divided into terms, numbered with consecutive integers. Each term begins with an election and has at most one leader. Terms act as a logical clock: every RPC carries the sender's term; a server that sees a higher term updates its own and becomes a follower, and a request with a stale term is rejected. Only two RPCs are needed for the core protocol: RequestVote, sent by candidates, and AppendEntries, sent by the leader both to replicate entries and, when empty, as a heartbeat. Each server persists currentTerm, votedFor and its log before answering.

Raft server roles

A follower that hears no heartbeat becomes a candidate; a candidate that wins a majority becomes leader; any server that sees a higher term returns to follower.

The term rule

Every RPC handler starts with the same check. A leader that sees term 7 while it is in term 5 steps down immediately; a message from an older term is rejected.

class Node:
    def __init__(self, name):
        self.name, self.term, self.role = name, 0, "follower"

    def on_message(self, msg_term):
        if msg_term > self.term:          # newer term seen: step down
            self.term, self.role = msg_term, "follower"
            return "updated"
        if msg_term < self.term:          # stale sender: reject
            return "reject (stale term)"
        return "ok"

n = Node("n1")
n.term, n.role = 5, "leader"
print(n.on_message(4), n.role, n.term)
print(n.on_message(7), n.role, n.term)

Output:

reject (stale term) leader 5
updated follower 7

Persist before you reply

currentTerm, votedFor and log entries must be on stable storage before a server responds to an RPC. Everything else (commitIndex, lastApplied, nextIndex, matchIndex) is volatile and can be rebuilt after a restart.

त्वरित जाँच: A Raft leader in term 5 receives an AppendEntries response carrying term 8. What does it do?

• Ignore it, since leaders outrank followers
• Send a heartbeat to reassert leadership
• Start a new election in term 6
• Update its term to 8 and become a follower