cluster

This property is true if the worker exited due to .disconnect(). If the worker exited any other way, it is false. If the worker has not exited, it is undefined.

The boolean worker.exitedAfterDisconnect allows distinguishing between voluntary and accidental exit, the primary may choose not to respawn a worker based on this value.

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

// kill worker
worker.kill();

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.

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.

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

events.EventEmitter

1. disconnect
2. error
3. exit
4. listening
5. message
6. online

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

Causes .exitedAfterDisconnect to be set.

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.

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.

import net from 'node:net';

if (cluster.isPrimary) {
  const worker = cluster.fork();
  let 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 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
    }
  });
}

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

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

import cluster from 'node:cluster';
import http from 'node:http';
import { availableParallelism } from 'node:os';
import process from 'node:process';

const numCPUs = availableParallelism();

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

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

  cluster.on('fork', (worker) => {
    console.log('worker is dead:', worker.isDead());
  });

  cluster.on('exit', (worker, code, signal) => {
    console.log('worker is dead:', worker.isDead());
  });
} else {
  // Workers can share any TCP connection. In this case, it is an HTTP server.
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end(`Current process\n ${process.pid}`);
    process.kill(process.pid);
  }).listen(8000);
}

This function will kill the worker. In the primary worker, it does this by disconnecting the worker.process, and once disconnected, killing with signal. In the worker, it does it by killing the process with signal.

The kill() function kills the worker process without waiting for a graceful disconnect, it has the same behavior as worker.process.kill().

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

In a worker, process.kill() exists, but it is not this function; it is kill().

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

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

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

This example will echo back all messages from the primary:

if (cluster.isPrimary) {
  const worker = cluster.fork();
  worker.send('hi there');

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

The addressType is one of:

• 4 (TCPv4)
• 6 (TCPv6)
• -1 (Unix domain socket)
• 'udp4' or 'udp6' (UDPv4 or UDPv6)

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

True if the process is not a primary (it is the negation of cluster.isPrimary).

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 either the first worker is spawned, or .setupPrimary() is called, whichever 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'.

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

This object is not intended to be changed or set manually.

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

import cluster from 'node:cluster';

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

A hash that stores the active worker objects, keyed by id field. This makes it easy to loop through all the workers. It is only available in the primary 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 the last 'disconnect' or 'exit' event is emitted.

import cluster from 'node:cluster';

for (const worker of Object.values(cluster.workers)) {
  worker.send('big announcement to all workers');
}

Spawn a new worker process.

This can only be called from the primary process.

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

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 .setupPrimary() is the env passed to .fork().

The defaults above apply to the first call only; the defaults for later calls are the current values at the time of cluster.setupPrimary() is called.

import cluster from 'node:cluster';

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

This can only be called from the primary process.

events.EventEmitter

1. disconnect
2. exit
3. fork
4. listening
5. message
6. online
7. setup

List of string arguments passed to the Node.js executable.

File path to worker file.

String arguments passed to worker.

Whether or not to send output to parent's stdio.

Configures the stdio of forked processes. Because the cluster module relies on IPC to function, this configuration must contain an 'ipc' entry. When this option is provided, it overrides silent. See child_prcess.spawn()'s stdio.

Sets the user identity of the process. (See setuid(2).)

Sets the group identity of the process. (See setgid(2).)

Sets inspector port of worker. This can be a number, or a function that takes no arguments and returns a number. By default each worker gets its own port, incremented from the primary's process.debugPort.

Specify the kind of serialization used for sending messages between processes. Possible values are 'json' and 'advanced'. See Advanced serialization for child_process for more details.

Current working directory of the worker process.

Hide the forked processes console window that would normally be created on Windows systems.