Raft Distributed Consensus Algorithm

The Raft consensus algorithm is designed as an alternative to Paxos, achieving equivalence in fault tolerance and performance while decomposing the consensus problem into relatively independent sub-problems: leader election, log replication, and safety. Raft guarantees safety properties under all non-Byzantine failure conditions as long as a strict majority of nodes (quorum of size floor(N/2) + 1) remain operational and connected, enforcing a strong leader paradigm where log entries flow strictly from the leader to followers.

package raft

import (
	"sync"
	"time"
)

type NodeState int

const (
	Follower NodeState = iota
	Candidate
	Leader
)

type LogEntry struct {
	Term    int
	Command interface{}
}

type RaftNode struct {
	mu        sync.Mutex
	id        string
	peers     []string
	state     NodeState
	currentTerm int
	votedFor    string
	log         []LogEntry
	commitIndex int
	lastApplied int
	nextIndex   map[string]int
	matchIndex  map[string]int
	electionResetEvent time.Time
}

func (rf *RaftNode) startElection() {
	rf.mu.Lock()
	rf.state = Candidate
	rf.currentTerm++
	rf.votedFor = rf.id
	rf.electionResetEvent = time.Now()
	term := rf.currentTerm
	rf.mu.Unlock()

	// Broadcast RequestVote RPCs to all peers concurrently...
}

func TestSingleLeaderPerTerm(t *testing.T) {
	// Instantiate cluster of 5 Raft nodes
	// Induce network partition
	// Assert consensus safety invariants
}

func (rf *RaftNode) Snapshot(index int, snapshot []byte) {
	rf.mu.Lock()
	defer rf.mu.Unlock()
	// Trim log up to index and persist snapshot data atomically
}

Architectural Invariants & Formal Safety Properties

Raft enforces strict invariants to maintain safety across dynamic cluster topologies: Election Safety (at most one leader can be elected in a given term), Leader Append-Only (a leader never overwrites or truncates its own entries, it only appends new ones), Log Matching (if two logs contain an entry with the same index and term, then they are identical in all entries up through the given index), Leader Completeness (if a log entry is committed in a given term, that entry will be present in the logs of the leaders for all higher-numbered terms), and State Machine Safety (if a server has applied a log entry at a given index to its state machine, no other server will ever apply a different log entry for the same index).

RPC Specifications: AppendEntries and RequestVote

Communication between Raft nodes occurs exclusively via two primary remote procedure calls. RequestVote is invoked by candidates to gather votes, containing candidateId, term, lastLogIndex, and lastLogTerm; receivers reject votes if their own log is more up-to-date. AppendEntries is invoked by the leader to replicate log entries and provide heartbeat signals. It contains prevLogIndex and prevLogTerm, enabling the receiver to perform a consistency check. If the check fails, the follower rejects the entry, prompting the leader to decrement nextIndex and retry until consistency is established.

Cluster Membership Changes & Joint Consensus

Transitioning cluster configurations safely requires mitigating the risk of split-brain scenarios where two independent majorities could form simultaneously. Raft handles configuration changes either via single-server changes (where commit of configuration C_new requires majority agreement under C_new) or via Joint Consensus, where log entries are committed using a joint configuration requiring separate majorities from both the old configuration (C_old) and the new configuration (C_new) before transitioning to C_new exclusively.

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