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# Cluster#

Stability: 2 - Stable

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 easy creation of child processes that all share server ports.

    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`);
      });
    } 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);

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

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

    $ node server.js
    Master 3596 is running
    Worker 4324 started
    Worker 4520 started
    Worker 6056 started
    Worker 5644 started

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.
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. To listen on a unique port, generate a port number based on the cluster worker ID.

Node.js does not provide routing logic. It is, therefore important to design an application 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 a 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, however. It is the application's responsibility to manage the worker pool based on its own needs.

Although a primary use case for the `cluster` module is networking, it can also be used for other use cases requiring worker processes.

## Class: Worker#

Added in: v0.7.0

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'#

Added in: v0.7.7

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

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

### Event: 'error'#

Added in: v0.7.3

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

Within a worker, `process.on('error')` may also be used.

### Event: 'exit'#

Added in: v0.11.2

- `code` <number> The exit code, if it exited normally.
- `signal` <string> The name of the signal (e.g. `'SIGHUP'`) that caused the process to be killed.

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'#

Added in: v0.7.0

- `address` <Object>

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'#

Added in: v0.7.0

- `message` <Object>
- `handle` <undefined> | <Object>

Similar to the `'message'` event of `cluster`, but specific to this worker.

Within a worker, `process.on('message')` may also be used.

See `process` event: `'message'`.

Here is an example using the message system. It keeps a count in the master process of the number of HTTP requests received by the workers:

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

    if (cluster.isMaster) {

      // Keep track of http requests
      let 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 (let i = 0; i < numCPUs; i++) {
        cluster.fork();
      }

      for (const id in cluster.workers) {
        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'#

Added in: v0.7.0

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()#

History

Version | Changes
--- | ---
v7.3.0 | This method now returns a reference to `worker`.
v0.7.7 | Added in: v0.7.7

- Returns: <cluster.Worker> A reference to `worker`.

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 `.exitedAfterDisconnect` 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) {
      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 net = require('net');
      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
        }
      });
    }

### worker.exitedAfterDisconnect#

Added in: v6.0.0

- <boolean>

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

The boolean `worker.exitedAfterDisconnect` allows distinguishing 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.exitedAfterDisconnect === true) {
        console.log('Oh, it was just voluntary – no need to worry');
      }
    });

    // kill worker
    worker.kill();

### worker.id#

Added in: v0.8.0

- <number>

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()#

Added in: v0.11.14

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 has been created. It is disconnected after the `'disconnect'` event is emitted.

### worker.isDead()#

Added in: v0.11.14

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'])#

Added in: v0.9.12

- `signal` <string> Name of the kill signal to send to the worker process.

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`.

Because `kill()` attempts to gracefully disconnect the worker process, it is susceptible to waiting indefinitely for the disconnect to complete. For example, if the worker enters an infinite loop, a graceful disconnect will never occur. If the graceful disconnect behavior is not needed, use `worker.process.kill()`.

Causes `.exitedAfterDisconnect` 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#

Added in: v0.7.0

- <ChildProcess>

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 `.exitedAfterDisconnect` is not `true`. This protects against accidental disconnection.

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

History

Version | Changes
--- | ---
v4.0.0 | The `callback` parameter is supported now.
v0.7.0 | Added in: v0.7.0

- `message` <Object>
- `sendHandle` <Handle>
- `callback` <Function>
- Returns: <boolean>

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

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

## Event: 'disconnect'#

Added in: v0.7.9

- `worker` <cluster.Worker>

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'#

Added in: v0.7.9

- `worker` <cluster.Worker>
- `code` <number> The exit code, if it exited normally.
- `signal` <string> The name of the signal (e.g. `'SIGHUP'`) that caused the process to be killed.

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'#

Added in: v0.7.0

- `worker` <cluster.Worker>

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

    const 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'#

Added in: v0.7.0

- `worker` <cluster.Worker>
- `address` <Object>

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'#

History

Version | Changes
--- | ---
v6.0.0 | The `worker` parameter is passed now; see below for details.
v2.5.0 | Added in: v2.5.0

- `worker` <cluster.Worker>
- `message` <Object>
- `handle` <undefined> | <Object>

Emitted when the cluster master receives a message from any worker.

See `child_process` event: `'message'`.

Before Node.js v6.0, this event emitted only the message and the handle, but not the worker object, contrary to what the documentation stated.

If support for older versions is required but a worker object is not required, it is possible to work around the discrepancy by checking the number of arguments:

    cluster.on('message', (worker, message, handle) => {
      if (arguments.length === 2) {
        handle = message;
        message = worker;
        worker = undefined;
      }
      // ...
    });

## Event: 'online'#

Added in: v0.7.0

- `worker` <cluster.Worker>

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'#

Added in: v0.7.1

- `settings` <Object>

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])#

Added in: v0.7.7

- `callback` <Function> Called when all workers are disconnected and handles are closed.

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])#

Added in: v0.6.0

- `env` <Object> Key/value pairs to add to worker process environment.
- Returns: <cluster.Worker>

Spawn a new worker process.

This can only be called from the master process.

## cluster.isMaster#

Added in: v0.8.1

- <boolean>

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#

Added in: v0.6.0

- <boolean>

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

## cluster.schedulingPolicy#

Added in: v0.11.2

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 `cluster.setupMaster()` 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'`.

## cluster.settings#

History

Version | Changes
--- | ---
v9.5.0 | The `cwd` option is supported now.
v9.4.0 | The `windowsHide` option is supported now.
v8.2.0 | The `inspectPort` option is supported now.
v6.4.0 | The `stdio` option is supported now.
v0.7.1 | Added in: v0.7.1

- <Object>
  - `execArgv` <string[]> List of string arguments passed to the Node.js executable. Default: `process.execArgv`.
  - `exec` <string> File path to worker file. Default: `process.argv[1]`.
  - `args` <string[]> String arguments passed to worker. Default: `process.argv.slice(2)`.
  - `cwd` <string> Current working directory of the worker process. Default: `undefined` (inherits from parent process).
  - `silent` <boolean> Whether or not to send output to parent's stdio. Default: `false`.
  - `stdio` <Array> 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`.
  - `uid` <number> Sets the user identity of the process. (See `setuid(2)`.)
  - `gid` <number> Sets the group identity of the process. (See `setgid(2)`.)
  - `inspectPort` <number> | <Function> 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 master's `process.debugPort`.
  - `windowsHide` <boolean> Hide the forked processes console window that would normally be created on Windows systems. Default: `false`.

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

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

## cluster.setupMaster([settings])#

History

Version | Changes
--- | ---
v6.4.0 | The `stdio` option is supported now.
v0.7.1 | Added in: v0.7.1

- `settings` <Object> See `cluster.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.

    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#

Added in: v0.7.0

- <Object>

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#

Added in: v0.7.0

- <Object>

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 (const id in cluster.workers) {
        callback(cluster.workers[id]);
      }
    }
    eachWorker((worker) => {
      worker.send('big announcement to all workers');
    });

Using the worker's unique id is the easiest way to locate the worker.

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