Step-by-step mental model of how Raft works.

1 Core Idea (One-Line)

Raft keeps multiple servers in sync by electing one leader, and only the leader decides what gets written.

2 Roles in Raft

Every node is always in one of these roles:

At any time, there is at most one leader.

3 Leader Election (How a Leader Is Chosen)

Step-by-step

1. All nodes start as followers

2. If a follower doesn’t hear from a leader (heartbeat timeout):

   • It becomes a candidate

3. Candidate:

   • Increments its term
   • Asks others for votes

4. If it gets majority votes:

   • It becomes the leader

5. If not:

   • Election restarts with randomized timeouts

Majority is mandatory → prevents split-brain.

4 Log Replication (How Data Is Safely Written)

Writing data works like this:

1. Client sends request → Leader

2. Leader:

   • Appends entry to its log
   • Sends AppendEntries RPC to followers

3. Followers:

   • Write entry to their logs
   • Send ACK back

4. Once majority ACKs:

   • Leader commits the entry
   • Followers apply it to their state machine

Only committed entries are considered durable.

5 Terms (Raft’s Logical Clock)

Raft uses terms to maintain order.

If a node sees a higher term, it immediately becomes a follower.

6 Safety Guarantees (Why Raft Is Reliable)

Raft guarantees:

Leader Completeness

• If an entry is committed, all future leaders will have it

Log Matching

• If two logs have same index + term → logs before it are identical

No Split Brain

• Only one leader per term (majority rule)

7 Failure Scenarios (What Happens If…)

Leader crashes

• Followers stop receiving heartbeats
• New election starts
• New leader takes over

Follower crashes

• Leader continues (as long as majority exists)
• Follower catches up later

Network partition

• Side with majority continues
• Minority side becomes read-only

8 Why Raft Is Easier Than Paxos

Raft was designed to be understandable, unlike Paxos.

9 One Diagram Mental Model

Client
  |
  v
Leader  --->  Follower
  |            Follower
  |            Follower
  |
(commit after majority)

TL;DR

• Raft = Leader-based consensus
• Leader is elected by majority
• All writes go through leader
• Logs are replicated and committed safely
• Designed to be simple and predictable