How to fix Error: Primary cluster worker is not listening (listening server required) in Node.js

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isPrimary) {
  // PRIMARY: Fork workers, don't listen
  console.log('Primary process starting...');
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log('Worker died, restarting...');
    cluster.fork();
  });
} else {
  // WORKER: Create and listen on server
  const server = http.createServer((req, res) => {
    res.writeHead(200);
    res.end('Hello from worker ' + process.pid);
  });

  server.listen(8000, () => {
    console.log('Worker listening on port 8000');
  });
}

// WRONG: No server listening in workers
if (!cluster.isPrimary) {
  const consumer = createRabbitMQConsumer(); // Not a listening server
  consumer.on('message', handler);
  // Error: No listening server, cluster.on('listening') never fires
}

// WRONG: Primary listening instead of workers
if (cluster.isPrimary) {
  const server = http.createServer(...);
  server.listen(8000); // Should be in worker block
  cluster.fork();
}

// CORRECT: Workers have listening servers
if (!cluster.isPrimary) {
  const server = http.createServer(...);
  server.listen(8000); // Workers listen, primary doesn't
}

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

if (cluster.isPrimary) {
  // Fork workers for background processing
  for (let i = 0; i < os.cpus().length; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker) => {
    console.log('Worker died, restarting...');
    cluster.fork(); // Restart dead workers
  });

  // Don't use cluster.on('listening') for non-server workloads
} else {
  // Worker: Create a message consumer, NOT a listening server
  const consumer = new RabbitMQConsumer();

  consumer.start()
    .then(() => console.log('Consumer started'))
    .catch((err) => {
      console.error('Consumer failed:', err);
      process.exit(1);
    });
}

// test-cluster.js
const cluster = require('cluster');
const http = require('http');
const os = require('os');

const PORT = process.env.PORT || 8000;

if (cluster.isPrimary) {
  console.log('Primary process ' + process.pid + ' starting...');

  // Fork workers
  for (let i = 0; i < os.cpus().length; i++) {
    cluster.fork();
  }

  // Listen for 'listening' event from workers
  cluster.on('listening', (worker, address) => {
    console.log('Worker ' + worker.process.pid + ' listening on port ' + address.port);
  });

  cluster.on('exit', (worker, code, signal) => {
    console.log('Worker ' + worker.process.pid + ' exited');
    cluster.fork(); // Respawn
  });
} else {
  // Worker: Create a listening server
  const server = http.createServer((req, res) => {
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    res.end('Response from worker ' + process.pid + '\n');
  });

  server.listen(PORT, () => {
    console.log('Worker ' + process.pid + ' listening on port ' + PORT);
  });
}

// Modern (Node 16+)
if (cluster.isPrimary) {
  // Fork workers
} else {
  // Worker code
}

// Older versions (deprecated)
if (cluster.isMaster) {
  // This still works but is deprecated
}