How to fix Error: Worker already exists with id in Node.js

const cluster = require('cluster');
const http = require('http');
const os = require('os');

if (cluster.isPrimary) {
  const numCPUs = os.cpus().length;

  // CORRECT: Let cluster auto-assign IDs
  for (let i = 0; i < numCPUs; i++) {
    const worker = cluster.fork();
    console.log(`Worker forked with ID: ${worker.id}`);
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.id} (PID ${worker.process.pid}) died`);
  });
} else {
  const server = http.createServer((req, res) => {
    res.writeHead(200);
    res.end(`Hello from worker ${cluster.worker.id}\n`);
  });

  server.listen(8000);
}

// WRONG: Attempting to manually set IDs
const workers = {};
for (let i = 0; i < 4; i++) {
  const worker = cluster.fork();
  worker.id = i; // ERROR: worker.id is read-only!
  workers[i] = worker;
}

// CORRECT: Use auto-assigned IDs
const workers = {};
for (let i = 0; i < 4; i++) {
  const worker = cluster.fork();
  workers[worker.id] = worker; // Use the assigned ID
}

const cluster = require('cluster');

if (cluster.isPrimary) {
  // Track which workers we've already forked
  const forkingWorkers = new Set();

  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.id} exited`);

    // Remove from tracking
    forkingWorkers.delete(worker.id);

    // Only fork replacement if not already doing so
    if (!forkingWorkers.has(worker.id)) {
      console.log(`Respawning worker to replace ${worker.id}`);
      forkingWorkers.add(worker.id);

      const newWorker = cluster.fork();

      // The new worker will get the NEXT available ID, not the old one
      newWorker.on('online', () => {
        console.log(`New worker ${newWorker.id} is online`);
      });
    }
  });

  // Initial worker creation
  for (let i = 0; i < 2; i++) {
    cluster.fork();
  }
}

const cluster = require('cluster');
let forking = false;

if (cluster.isPrimary) {
  cluster.on('exit', (worker, code, signal) => {
    // WRONG: No guard against multiple calls
    // cluster.fork(); // Could be called multiple times

    // CORRECT: Use a guard to prevent duplicate forks
    if (!forking && code !== 0 && !worker.exitedAfterDisconnect) {
      forking = true;

      const newWorker = cluster.fork();

      newWorker.on('online', () => {
        console.log(`Worker ${newWorker.id} online`);
        forking = false; // Reset guard
      });

      // Timeout in case worker never comes online
      setTimeout(() => {
        forking = false;
      }, 5000);
    }
  });

  cluster.fork();
}

const cluster = require('cluster');

// CORRECT: Check isPrimary early
if (cluster.isPrimary) {
  console.log(`Primary ${process.pid} started`);

  // Initialization code runs once
  for (let i = 0; i < 4; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker) => {
    console.log(`Worker ${worker.id} died`);
  });
} else {
  // Worker code
  console.log(`Worker ${cluster.worker.id} started`);
}

// ✓ No cluster setup code here that might run twice

const cluster = require('cluster');
const http = require('http');

if (cluster.isPrimary) {
  console.log(`Primary ${process.pid} starting`);

  for (let i = 0; i < 2; i++) {
    const worker = cluster.fork();
    console.log(`✓ Forked worker with ID: ${worker.id}, PID: ${worker.process.pid}`);
  }

  cluster.on('online', (worker) => {
    console.log(`✓ Worker ${worker.id} is online`);
  });

  cluster.on('exit', (worker, code, signal) => {
    console.log(`✗ Worker ${worker.id} exited (code: ${code}, signal: ${signal})`);
  });

  // List active workers
  setInterval(() => {
    const workerIds = Object.keys(cluster.workers).map(id => cluster.workers[id].id);
    console.log(`Active worker IDs: [${workerIds.join(', ')}]`);
  }, 5000);
} else {
  const server = http.createServer((req, res) => {
    res.writeHead(200);
    res.end(`Worker ${cluster.worker.id}\n`);
  });

  server.listen(3000 + cluster.worker.id);
  console.log(`Worker ${cluster.worker.id} listening on port ${3000 + cluster.worker.id}`);
}