A single instance of Node.js runs in a single thread. To take advantage of multi-core systems the user will sometimes want to launch a cluster of Node.js processes to handle the load.

The cluster module allows you to easily create child processes that all share server ports.

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

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

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`);
  });
} 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 world\n');
  }).listen(8000);
}

Running Node.js will now share port 8000 between the workers:

$ NODE_DEBUG=cluster node server.js
23521,Master Worker 23524 online
23521,Master Worker 23526 online
23521,Master Worker 23523 online
23521,Master Worker 23528 online

Please note that, on Windows, it is not yet possible to set up a named pipe server in a worker.

How It Works

The worker processes are spawned using the child_process.fork() method, so that they can communicate with the parent via IPC and pass server handles back and forth.

The cluster module supports two methods of distributing incoming connections.

The first one (and the default one on all platforms except Windows), is the round-robin approach, where the master process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.

The second approach is where the master process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly.

The second approach should, in theory, give the best performance. In practice however, distribution tends to be very unbalanced due to operating system scheduler vagaries. Loads have been observed where over 70% of all connections ended up in just two processes, out of a total of eight.

Because server.listen() hands off most of the work to the master process, there are three cases where the behavior between a normal Node.js process and a cluster worker differs:

1. server.listen({fd: 7}) Because the message is passed to the master, file descriptor 7 in the parent will be listened on, and the handle passed to the worker, rather than listening to the worker's idea of what the number 7 file descriptor references.
2. server.listen(handle) Listening on handles explicitly will cause the worker to use the supplied handle, rather than talk to the master process. If the worker already has the handle, then it's presumed that you know what you are doing.
3. server.listen(0) Normally, this will cause servers to listen on a random port. However, in a cluster, each worker will receive the same "random" port each time they do listen(0). In essence, the port is random the first time, but predictable thereafter. If you want to listen on a unique port, generate a port number based on the cluster worker ID.

There is no routing logic in Node.js, or in your program, and no shared state between the workers. Therefore, it is important to design your program such that it does not rely too heavily on in-memory data objects for things like sessions and login.

Because workers are all separate processes, they can be killed or re-spawned depending on your program's needs, without affecting other workers. As long as there are some workers still alive, the server will continue to accept connections. If no workers are alive, existing connections will be dropped and new connections will be refused. Node.js does not automatically manage the number of workers for you, however. It is your responsibility to manage the worker pool for your application's needs.

Class: Worker

A Worker object contains all public information and method about a worker. In the master it can be obtained using cluster.workers. In a worker it can be obtained using cluster.worker.

Event: 'disconnect'

Similar to the cluster.on('disconnect') event, but specific to this worker.

cluster.fork().on('disconnect', () => {
  // Worker has disconnected
});

Event: 'error'

This event is the same as the one provided by child_process.fork().

In a worker you can also use process.on('error').

Event: 'exit'

Similar to the cluster.on('exit') event, but specific to this worker.

const worker = cluster.fork();
worker.on('exit', (code, signal) => {
  if( signal ) {
    console.log(`worker was killed by signal: ${signal}`);
  } else if( code !== 0 ) {
    console.log(`worker exited with error code: ${code}`);
  } else {
    console.log('worker success!');
  }
});

Event: 'listening'

Similar to the cluster.on('listening') event, but specific to this worker.

cluster.fork().on('listening', (address) => {
  // Worker is listening
});

It is not emitted in the worker.

Event: 'message'

Similar to the cluster.on('message') event, but specific to this worker.

This event is the same as the one provided by child_process.fork().

In a worker you can also use process.on('message').

As an example, here is a cluster that keeps count of the number of requests in the master process using the message system:

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

if (cluster.isMaster) {

  // Keep track of http requests
  var numReqs = 0;
  setInterval(() => {
    console.log('numReqs =', numReqs);
  }, 1000);

  // Count requests
  function messageHandler(msg) {
    if (msg.cmd && msg.cmd == 'notifyRequest') {
      numReqs += 1;
    }
  }

  // Start workers and listen for messages containing notifyRequest
  const numCPUs = require('os').cpus().length;
  for (var i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  Object.keys(cluster.workers).forEach((id) => {
    cluster.workers[id].on('message', messageHandler);
  });

} else {

  // Worker processes have a http server.
  http.Server((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');

    // notify master about the request
    process.send({ cmd: 'notifyRequest' });
  }).listen(8000);
}

Event: 'online'

Similar to the cluster.on('online') event, but specific to this worker.

cluster.fork().on('online', () => {
  // Worker is online
});

It is not emitted in the worker.

worker.disconnect()

In a worker, this function will close all servers, wait for the 'close' event on those servers, and then disconnect the IPC channel.

In the master, an internal message is sent to the worker causing it to call .disconnect() on itself.

Causes .suicide to be set.

Note that after a server is closed, it will no longer accept new connections, but connections may be accepted by any other listening worker. Existing connections will be allowed to close as usual. When no more connections exist, see server.close(), the IPC channel to the worker will close allowing it to die gracefully.

The above applies only to server connections, client connections are not automatically closed by workers, and disconnect does not wait for them to close before exiting.

Note that in a worker, process.disconnect exists, but it is not this function, it is disconnect.

Because long living server connections may block workers from disconnecting, it may be useful to send a message, so application specific actions may be taken to close them. It also may be useful to implement a timeout, killing a worker if the 'disconnect' event has not been emitted after some time.

if (cluster.isMaster) {
  var worker = cluster.fork();
  var timeout;

  worker.on('listening', (address) => {
    worker.send('shutdown');
    worker.disconnect();
    timeout = setTimeout(() => {
      worker.kill();
    }, 2000);
  });

  worker.on('disconnect', () => {
    clearTimeout(timeout);
  });

} else if (cluster.isWorker) {
  const net = require('net');
  var server = net.createServer((socket) => {
    // connections never end
  });

  server.listen(8000);

  process.on('message', (msg) => {
    if(msg === 'shutdown') {
      // initiate graceful close of any connections to server
    }
  });
}

worker.id

Each new worker is given its own unique id, this id is stored in the id.

While a worker is alive, this is the key that indexes it in cluster.workers

worker.isConnected()

This function returns true if the worker is connected to its master via its IPC channel, false otherwise. A worker is connected to its master after it's been created. It is disconnected after the 'disconnect' event is emitted.

worker.isDead()

This function returns true if the worker's process has terminated (either because of exiting or being signaled). Otherwise, it returns false.

worker.kill([signal='SIGTERM'])

This function will kill the worker. In the master, it does this by disconnecting the worker.process, and once disconnected, killing with signal. In the worker, it does it by disconnecting the channel, and then exiting with code 0.

Causes .suicide to be set.

This method is aliased as worker.destroy() for backwards compatibility.

Note that in a worker, process.kill() exists, but it is not this function, it is kill.

worker.process

All workers are created using child_process.fork(), the returned object from this function is stored as .process. In a worker, the global process is stored.

See: Child Process module

Note that workers will call process.exit(0) if the 'disconnect' event occurs on process and .suicide is not true. This protects against accidental disconnection.

worker.send(message[, sendHandle][, callback])

Send a message to a worker or master, optionally with a handle.

In the master this sends a message to a specific worker. It is identical to ChildProcess.send().

In a worker this sends a message to the master. It is identical to process.send().

This example will echo back all messages from the master:

if (cluster.isMaster) {
  var worker = cluster.fork();
  worker.send('hi there');

} else if (cluster.isWorker) {
  process.on('message', (msg) => {
    process.send(msg);
  });
}

worker.suicide

Set by calling .kill() or .disconnect(), until then it is undefined.

The boolean worker.suicide lets you distinguish between voluntary and accidental exit, the master may choose not to respawn a worker based on this value.

cluster.on('exit', (worker, code, signal) => {
  if (worker.suicide === true) {
    console.log('Oh, it was just suicide\' – no need to worry').
  }
});

// kill worker
worker.kill();

Event: 'disconnect'

Emitted after the worker IPC channel has disconnected. This can occur when a worker exits gracefully, is killed, or is disconnected manually (such as with worker.disconnect()).

There may be a delay between the 'disconnect' and 'exit' events. These events can be used to detect if the process is stuck in a cleanup or if there are long-living connections.

cluster.on('disconnect', (worker) => {
  console.log(`The worker #${worker.id} has disconnected`);
});

Event: 'exit'

When any of the workers die the cluster module will emit the 'exit' event.

This can be used to restart the worker by calling .fork() again.

cluster.on('exit', (worker, code, signal) => {
  console.log('worker %d died (%s). restarting...',
    worker.process.pid, signal || code);
  cluster.fork();
});

See child_process event: 'exit'.

Event: 'fork'

When a new worker is forked the cluster module will emit a 'fork' event. This can be used to log worker activity, and create your own timeout.

var timeouts = [];
function errorMsg() {
  console.error('Something must be wrong with the connection ...');
}

cluster.on('fork', (worker) => {
  timeouts[worker.id] = setTimeout(errorMsg, 2000);
});
cluster.on('listening', (worker, address) => {
  clearTimeout(timeouts[worker.id]);
});
cluster.on('exit', (worker, code, signal) => {
  clearTimeout(timeouts[worker.id]);
  errorMsg();
});

Event: 'listening'

After calling listen() from a worker, when the 'listening' event is emitted on the server, a 'listening' event will also be emitted on cluster in the master.

The event handler is executed with two arguments, the worker contains the worker object and the address object contains the following connection properties: address, port and addressType. This is very useful if the worker is listening on more than one address.

cluster.on('listening', (worker, address) => {
  console.log(
    `A worker is now connected to ${address.address}:${address.port}`);
});

The addressType is one of:

• 4 (TCPv4)
• 6 (TCPv6)
• -1 (unix domain socket)
• "udp4" or "udp6" (UDP v4 or v6)

Event: 'message'

Emitted when any worker receives a message.

See child_process event: 'message'.

Event: 'online'

After forking a new worker, the worker should respond with an online message. When the master receives an online message it will emit this event. The difference between 'fork' and 'online' is that fork is emitted when the master forks a worker, and 'online' is emitted when the worker is running.

cluster.on('online', (worker) => {
  console.log('Yay, the worker responded after it was forked');
});

Event: 'setup'

Emitted every time .setupMaster() is called.

The settings object is the cluster.settings object at the time .setupMaster() was called and is advisory only, since multiple calls to .setupMaster() can be made in a single tick.

If accuracy is important, use cluster.settings.

cluster.disconnect([callback])

Calls .disconnect() on each worker in cluster.workers.

When they are disconnected all internal handles will be closed, allowing the master process to die gracefully if no other event is waiting.

The method takes an optional callback argument which will be called when finished.

This can only be called from the master process.

cluster.fork([env])

Spawn a new worker process.

This can only be called from the master process.

cluster.isMaster

True if the process is a master. This is determined by the process.env.NODE_UNIQUE_ID. If process.env.NODE_UNIQUE_ID is undefined, then isMaster is true.

cluster.isWorker

True if the process is not a master (it is the negation of cluster.isMaster).

cluster.schedulingPolicy

The scheduling policy, either cluster.SCHED_RR for round-robin or cluster.SCHED_NONE to leave it to the operating system. This is a global setting and effectively frozen once you spawn the first worker or call cluster.setupMaster(), whatever comes first.

SCHED_RR is the default on all operating systems except Windows. Windows will change to SCHED_RR once libuv is able to effectively distribute IOCP handles without incurring a large performance hit.

cluster.schedulingPolicy can also be set through the NODE_CLUSTER_SCHED_POLICY environment variable. Valid values are "rr" and "none".

cluster.settings

After calling .setupMaster() (or .fork()) this settings object will contain the settings, including the default values.

It is effectively frozen after being set, because .setupMaster() can only be called once.

This object is not supposed to be changed or set manually, by you.

cluster.setupMaster([settings])

setupMaster is used to change the default 'fork' behavior. Once called, the settings will be present in cluster.settings.

Note that:

• any settings changes only affect future calls to .fork() and have no effect on workers that are already running
• The only attribute of a worker that cannot be set via .setupMaster() is the env passed to .fork()
• the defaults above apply to the first call only, the defaults for later calls is the current value at the time of cluster.setupMaster() is called

Example:

const cluster = require('cluster');
cluster.setupMaster({
  exec: 'worker.js',
  args: ['--use', 'https'],
  silent: true
});
cluster.fork(); // https worker
cluster.setupMaster({
  exec: 'worker.js',
  args: ['--use', 'http']
});
cluster.fork(); // http worker

This can only be called from the master process.

cluster.worker

A reference to the current worker object. Not available in the master process.

const cluster = require('cluster');

if (cluster.isMaster) {
  console.log('I am master');
  cluster.fork();
  cluster.fork();
} else if (cluster.isWorker) {
  console.log(`I am worker #${cluster.worker.id}`);
}

cluster.workers

A hash that stores the active worker objects, keyed by id field. Makes it easy to loop through all the workers. It is only available in the master process.

A worker is removed from cluster.workers after the worker has disconnected and exited. The order between these two events cannot be determined in advance. However, it is guaranteed that the removal from the cluster.workers list happens before last 'disconnect' or 'exit' event is emitted.

// Go through all workers
function eachWorker(callback) {
  for (var id in cluster.workers) {
    callback(cluster.workers[id]);
  }
}
eachWorker((worker) => {
  worker.send('big announcement to all workers');
});

Should you wish to reference a worker over a communication channel, using the worker's unique id is the easiest way to find the worker.

socket.on('data', (id) => {
  var worker = cluster.workers[id];
});