Distributed Consensus: Raft, Multi-Paxos & Zab

// Raft RequestVote RPC
struct RequestVoteArgs {
  term: int, candidateId: string, lastLogIndex: int, lastLogTerm: int
}

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// Raft Consensus State Machine & Leader Election Simulation
class RaftNode {
    constructor(id, peers) {
        this.id = id;
        this.peers = peers; // Array of peer node IDs
        this.currentTerm = 0;
        this.votedFor = null;
        this.state = 'FOLLOWER'; // FOLLOWER, CANDIDATE, LEADER
        this.log = [];
        this.commitIndex = 0;
        this.votesReceived = 0;
    }
 
    startElection() {
        this.state = 'CANDIDATE';
        this.currentTerm += 1;
        this.votedFor = this.id;
        this.votesReceived = 1; // Vote for self
 
        console.log(`[${this.id}] Timed out! Starting Election for Term ${this.currentTerm}...`);
 
        // Request votes from all peer nodes
        this.peers.forEach(peer => {
            peer.handleRequestVote(this.id, this.currentTerm, this.log.length - 1);
        });
    }
 
    handleRequestVote(candidateId, candidateTerm, candidateLastLog) {
        // Reject if candidate term is older
        if (candidateTerm > this.currentTerm) {
            this.currentTerm = candidateTerm;
            this.state = 'FOLLOWER';
            this.votedFor = null;
        }
 
        if (candidateTerm === this.currentTerm && (this.votedFor === null || this.votedFor === candidateId)) {
            this.votedFor = candidateId;
            console.log(`[${this.id}] Voted YES for Candidate ${candidateId} in Term ${candidateTerm}`);
            return true;
        }
        return false;
    }
 
    receiveVote() {
        this.votesReceived++;
        // Quorum condition: strictly greater than N / 2 votes
        const majority = Math.floor((this.peers.length + 1) / 2) + 1;
        if (this.votesReceived >= majority && this.state === 'CANDIDATE') {
            this.state = 'LEADER';
            console.log(`👑 [${this.id}] Achieved Quorum (${this.votesReceived} votes)! Promoted to CLUSTER LEADER for Term ${this.currentTerm}.`);
        }
    }
}
 
// Instantiate 3-node Raft Cluster
const nodeA = new RaftNode("Node_A", []);
const nodeB = new RaftNode("Node_B", []);
const nodeC = new RaftNode("Node_C", []);
 
nodeA.peers = [nodeB, nodeC];
nodeB.peers = [nodeA, nodeC];
nodeC.peers = [nodeA, nodeB];
 
console.log("=== Distributed Raft Consensus Engine ===");
nodeA.startElection();
nodeA.receiveVote(); // From Node_B
nodeA.receiveVote(); // From Node_C
console.log("✅ Raft cluster achieved consensus with guaranteed quorum safety!");