What is cluster module?

The `cluster` module in Node.js allows you to create child processes (workers) that share server ports, enabling your application to take full advantage of multi-core systems and improve its robustness and scalability.

What is the `cluster` Module?

Node.js applications are single-threaded by default. While this simplifies concurrency, it means a single Node.js process cannot fully utilize all CPU cores available on a machine. The cluster module addresses this limitation by allowing a master process to fork multiple worker processes that can share the same server port.

Each worker process runs an independent instance of your application. When a request comes in, the operating system (or a load balancer) distributes it among the available worker processes.

Why Use `cluster`?

• Multi-core Utilization: Effectively uses all CPU cores on a server, leading to better performance for CPU-bound tasks.
• Improved Resilience: If one worker process crashes due to an unhandled error, other workers remain active, and the master process can fork a new worker to replace the failed one, preventing a complete application downtime.
• Scalability: Allows your application to handle a greater number of concurrent requests by distributing the load across multiple processes.

How It Works (Master-Worker Model)

The cluster module operates on a master-worker model:

• Master Process: This is the primary process that runs first. Its main responsibility is to create and manage worker processes. It typically doesn't handle client requests directly.
• Worker Processes: These are child processes forked by the master. Each worker runs a copy of the application code and can handle incoming requests independently. They share the same server port thanks to inter-process communication (IPC) with the master.

Example Usage

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

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

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

  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died`);
    // Optional: Respawn the worker
    cluster.fork();
  });
} else {
  // Workers can share any TCP connection
  // In this case, it is an HTTP server
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end(`Hello from worker ${process.pid} and served!\n`);
  }).listen(8000);

  console.log(`Worker ${process.pid} started`);
}

Key Properties and Events

• cluster.isMaster (or cluster.isPrimary from Node.js v15.0.0+): A boolean indicating if the current process is the master (primary) process.
• cluster.fork(): Used by the master to create a new worker process.
• cluster.workers: An object containing references to all active worker processes, indexed by their ID.
• cluster.on('exit', ...): Event emitted by the master when a worker process dies. Useful for logging or respawning workers.
• worker.on('message', ...) / worker.send(message): For inter-process communication (IPC) between master and workers.