cluster.default.Cluster

interface Cluster

• readonly isPrimary: boolean

  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.

• readonly isWorker: boolean

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

• readonly SCHED_NONE: number
• readonly SCHED_RR: number
• schedulingPolicy: number

  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'.

• readonly settings: ClusterSettings

  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.

• readonly worker?: Worker

  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}`);
  }

• Worker: typeof Worker

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

• readonly workers?: Dict<Worker>

  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');
  }

• [events.captureRejectionSymbol](

  error: Error,

  event: string | symbol,

  ...args: any[]

  ): void;

  The Symbol.for('nodejs.rejection') method is called in case a promise rejection happens when emitting an event and captureRejections is enabled on the emitter. It is possible to use events.captureRejectionSymbol in place of Symbol.for('nodejs.rejection').

  import { EventEmitter, captureRejectionSymbol } from 'node:events';
  
  class MyClass extends EventEmitter {
    constructor() {
      super({ captureRejections: true });
    }
  
    [captureRejectionSymbol](err, event, ...args) {
      console.log('rejection happened for', event, 'with', err, ...args);
      this.destroy(err);
    }
  
    destroy(err) {
      // Tear the resource down here.
    }
  }
• addListener<E extends keyof ClusterEventMap>(

  eventName: E,

  listener: (...args: ClusterEventMap[E]) => void

  ): this;

  Alias for emitter.on(eventName, listener).

  addListener(

  eventName: string | symbol,

  listener: (...args: any[]) => void

  ): this;

  Alias for emitter.on(eventName, listener).
• disconnect(

  callback?: () => void

  ): void;
• emit<E extends keyof ClusterEventMap>(

  eventName: E,

  ...args: ClusterEventMap[E]

  ): boolean;

  Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

  Returns true if the event had listeners, false otherwise.

  import { EventEmitter } from 'node:events';
  const myEmitter = new EventEmitter();
  
  // First listener
  myEmitter.on('event', function firstListener() {
    console.log('Helloooo! first listener');
  });
  // Second listener
  myEmitter.on('event', function secondListener(arg1, arg2) {
    console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
  });
  // Third listener
  myEmitter.on('event', function thirdListener(...args) {
    const parameters = args.join(', ');
    console.log(`event with parameters ${parameters} in third listener`);
  });
  
  console.log(myEmitter.listeners('event'));
  
  myEmitter.emit('event', 1, 2, 3, 4, 5);
  
  // Prints:
  // [
  //   [Function: firstListener],
  //   [Function: secondListener],
  //   [Function: thirdListener]
  // ]
  // Helloooo! first listener
  // event with parameters 1, 2 in second listener
  // event with parameters 1, 2, 3, 4, 5 in third listener

  emit(

  eventName: string | symbol,

  ...args: any[]

  ): boolean;

  Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

  Returns true if the event had listeners, false otherwise.

  import { EventEmitter } from 'node:events';
  const myEmitter = new EventEmitter();
  
  // First listener
  myEmitter.on('event', function firstListener() {
    console.log('Helloooo! first listener');
  });
  // Second listener
  myEmitter.on('event', function secondListener(arg1, arg2) {
    console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
  });
  // Third listener
  myEmitter.on('event', function thirdListener(...args) {
    const parameters = args.join(', ');
    console.log(`event with parameters ${parameters} in third listener`);
  });
  
  console.log(myEmitter.listeners('event'));
  
  myEmitter.emit('event', 1, 2, 3, 4, 5);
  
  // Prints:
  // [
  //   [Function: firstListener],
  //   [Function: secondListener],
  //   [Function: thirdListener]
  // ]
  // Helloooo! first listener
  // event with parameters 1, 2 in second listener
  // event with parameters 1, 2, 3, 4, 5 in third listener
• eventNames(): string | symbol[];

  Returns an array listing the events for which the emitter has registered listeners.

  import { EventEmitter } from 'node:events';
  
  const myEE = new EventEmitter();
  myEE.on('foo', () => {});
  myEE.on('bar', () => {});
  
  const sym = Symbol('symbol');
  myEE.on(sym, () => {});
  
  console.log(myEE.eventNames());
  // Prints: [ 'foo', 'bar', Symbol(symbol) ]

• fork(

  env?: any

  ): Worker;

  Spawn a new worker process.

  This can only be called from the primary process.

  @param env

  Key/value pairs to add to worker process environment.

• getMaxListeners(): number;

  Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to events.defaultMaxListeners.

• listenerCount<E extends keyof ClusterEventMap>(

  eventName: E,

  listener?: (...args: ClusterEventMap[E]) => void

  ): number;

  Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.

  @param eventName

  The name of the event being listened for

  @param listener

  The event handler function

  listenerCount(

  eventName: string | symbol,

  listener?: (...args: any[]) => void

  ): number;

  Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.

  @param eventName

  The name of the event being listened for

  @param listener

  The event handler function

• listeners<E extends keyof ClusterEventMap>(

  eventName: E

  ): (...args: ClusterEventMap[E]) => void[];

  Returns a copy of the array of listeners for the event named eventName.

  server.on('connection', (stream) => {
    console.log('someone connected!');
  });
  console.log(util.inspect(server.listeners('connection')));
  // Prints: [ [Function] ]

  listeners(

  eventName: string | symbol

  ): (...args: any[]) => void[];

  Returns a copy of the array of listeners for the event named eventName.

  server.on('connection', (stream) => {
    console.log('someone connected!');
  });
  console.log(util.inspect(server.listeners('connection')));
  // Prints: [ [Function] ]
• off<E extends keyof ClusterEventMap>(

  eventName: E,

  listener: (...args: ClusterEventMap[E]) => void

  ): this;

  Alias for emitter.removeListener().

  off(

  eventName: string | symbol,

  listener: (...args: any[]) => void

  ): this;

  Alias for emitter.removeListener().
• on<E extends keyof ClusterEventMap>(

  eventName: E,

  listener: (...args: ClusterEventMap[E]) => void

  ): this;

  Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

  server.on('connection', (stream) => {
    console.log('someone connected!');
  });

  Returns a reference to the EventEmitter, so that calls can be chained.

  By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

  import { EventEmitter } from 'node:events';
  const myEE = new EventEmitter();
  myEE.on('foo', () => console.log('a'));
  myEE.prependListener('foo', () => console.log('b'));
  myEE.emit('foo');
  // Prints:
  //   b
  //   a

  @param eventName

  The name of the event.

  @param listener

  The callback function

  on(

  eventName: string | symbol,

  listener: (...args: any[]) => void

  ): this;

  Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

  server.on('connection', (stream) => {
    console.log('someone connected!');
  });

  Returns a reference to the EventEmitter, so that calls can be chained.

  By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

  import { EventEmitter } from 'node:events';
  const myEE = new EventEmitter();
  myEE.on('foo', () => console.log('a'));
  myEE.prependListener('foo', () => console.log('b'));
  myEE.emit('foo');
  // Prints:
  //   b
  //   a

  @param eventName

  The name of the event.

  @param listener

  The callback function

• once<E extends keyof ClusterEventMap>(

  eventName: E,

  listener: (...args: ClusterEventMap[E]) => void

  ): this;

  Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

  server.once('connection', (stream) => {
    console.log('Ah, we have our first user!');
  });

  Returns a reference to the EventEmitter, so that calls can be chained.

  By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

  import { EventEmitter } from 'node:events';
  const myEE = new EventEmitter();
  myEE.once('foo', () => console.log('a'));
  myEE.prependOnceListener('foo', () => console.log('b'));
  myEE.emit('foo');
  // Prints:
  //   b
  //   a

  @param eventName

  The name of the event.

  @param listener

  The callback function

  once(

  eventName: string | symbol,

  listener: (...args: any[]) => void

  ): this;

  Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

  server.once('connection', (stream) => {
    console.log('Ah, we have our first user!');
  });

  Returns a reference to the EventEmitter, so that calls can be chained.

  By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

  import { EventEmitter } from 'node:events';
  const myEE = new EventEmitter();
  myEE.once('foo', () => console.log('a'));
  myEE.prependOnceListener('foo', () => console.log('b'));
  myEE.emit('foo');
  // Prints:
  //   b
  //   a

  @param eventName

  The name of the event.

  @param listener

  The callback function

• prependListener<E extends keyof ClusterEventMap>(

  eventName: E,

  listener: (...args: ClusterEventMap[E]) => void

  ): this;

  Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

  server.prependListener('connection', (stream) => {
    console.log('someone connected!');
  });

  Returns a reference to the EventEmitter, so that calls can be chained.

  @param eventName

  The name of the event.

  @param listener

  The callback function

  prependListener(

  eventName: string | symbol,

  listener: (...args: any[]) => void

  ): this;

  Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

  server.prependListener('connection', (stream) => {
    console.log('someone connected!');
  });

  Returns a reference to the EventEmitter, so that calls can be chained.

  @param eventName

  The name of the event.

  @param listener

  The callback function

• prependOnceListener<E extends keyof ClusterEventMap>(

  eventName: E,

  listener: (...args: ClusterEventMap[E]) => void

  ): this;

  Adds a one-time listener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

  server.prependOnceListener('connection', (stream) => {
    console.log('Ah, we have our first user!');
  });

  Returns a reference to the EventEmitter, so that calls can be chained.

  @param eventName

  The name of the event.

  @param listener

  The callback function

  prependOnceListener(

  eventName: string | symbol,

  listener: (...args: any[]) => void

  ): this;

  Adds a one-time listener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

  server.prependOnceListener('connection', (stream) => {
    console.log('Ah, we have our first user!');
  });

  Returns a reference to the EventEmitter, so that calls can be chained.

  @param eventName

  The name of the event.

  @param listener

  The callback function

• rawListeners<E extends keyof ClusterEventMap>(

  eventName: E

  ): (...args: ClusterEventMap[E]) => void[];

  Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

  import { EventEmitter } from 'node:events';
  const emitter = new EventEmitter();
  emitter.once('log', () => console.log('log once'));
  
  // Returns a new Array with a function `onceWrapper` which has a property
  // `listener` which contains the original listener bound above
  const listeners = emitter.rawListeners('log');
  const logFnWrapper = listeners[0];
  
  // Logs "log once" to the console and does not unbind the `once` event
  logFnWrapper.listener();
  
  // Logs "log once" to the console and removes the listener
  logFnWrapper();
  
  emitter.on('log', () => console.log('log persistently'));
  // Will return a new Array with a single function bound by `.on()` above
  const newListeners = emitter.rawListeners('log');
  
  // Logs "log persistently" twice
  newListeners[0]();
  emitter.emit('log');

  rawListeners(

  eventName: string | symbol

  ): (...args: any[]) => void[];

  Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

  import { EventEmitter } from 'node:events';
  const emitter = new EventEmitter();
  emitter.once('log', () => console.log('log once'));
  
  // Returns a new Array with a function `onceWrapper` which has a property
  // `listener` which contains the original listener bound above
  const listeners = emitter.rawListeners('log');
  const logFnWrapper = listeners[0];
  
  // Logs "log once" to the console and does not unbind the `once` event
  logFnWrapper.listener();
  
  // Logs "log once" to the console and removes the listener
  logFnWrapper();
  
  emitter.on('log', () => console.log('log persistently'));
  // Will return a new Array with a single function bound by `.on()` above
  const newListeners = emitter.rawListeners('log');
  
  // Logs "log persistently" twice
  newListeners[0]();
  emitter.emit('log');
• removeAllListeners<E extends keyof ClusterEventMap>(

  eventName?: E

  ): this;

  Removes all listeners, or those of the specified eventName.

  It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

  Returns a reference to the EventEmitter, so that calls can be chained.

  removeAllListeners(

  eventName?: string | symbol

  ): this;

  Removes all listeners, or those of the specified eventName.

  It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

  Returns a reference to the EventEmitter, so that calls can be chained.
• removeListener<E extends keyof ClusterEventMap>(

  eventName: E,

  listener: (...args: ClusterEventMap[E]) => void

  ): this;

  Removes the specified listener from the listener array for the event named eventName.

  const callback = (stream) => {
    console.log('someone connected!');
  };
  server.on('connection', callback);
  // ...
  server.removeListener('connection', callback);

  removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

  Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them from emit() in progress. Subsequent events behave as expected.

  import { EventEmitter } from 'node:events';
  class MyEmitter extends EventEmitter {}
  const myEmitter = new MyEmitter();
  
  const callbackA = () => {
    console.log('A');
    myEmitter.removeListener('event', callbackB);
  };
  
  const callbackB = () => {
    console.log('B');
  };
  
  myEmitter.on('event', callbackA);
  
  myEmitter.on('event', callbackB);
  
  // callbackA removes listener callbackB but it will still be called.
  // Internal listener array at time of emit [callbackA, callbackB]
  myEmitter.emit('event');
  // Prints:
  //   A
  //   B
  
  // callbackB is now removed.
  // Internal listener array [callbackA]
  myEmitter.emit('event');
  // Prints:
  //   A

  Because listeners are managed using an internal array, calling this will change the position indexes of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

  When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:

  import { EventEmitter } from 'node:events';
  const ee = new EventEmitter();
  
  function pong() {
    console.log('pong');
  }
  
  ee.on('ping', pong);
  ee.once('ping', pong);
  ee.removeListener('ping', pong);
  
  ee.emit('ping');
  ee.emit('ping');

  Returns a reference to the EventEmitter, so that calls can be chained.

  removeListener(

  eventName: string | symbol,

  listener: (...args: any[]) => void

  ): this;

  Removes the specified listener from the listener array for the event named eventName.

  const callback = (stream) => {
    console.log('someone connected!');
  };
  server.on('connection', callback);
  // ...
  server.removeListener('connection', callback);

  removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

  Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them from emit() in progress. Subsequent events behave as expected.

  import { EventEmitter } from 'node:events';
  class MyEmitter extends EventEmitter {}
  const myEmitter = new MyEmitter();
  
  const callbackA = () => {
    console.log('A');
    myEmitter.removeListener('event', callbackB);
  };
  
  const callbackB = () => {
    console.log('B');
  };
  
  myEmitter.on('event', callbackA);
  
  myEmitter.on('event', callbackB);
  
  // callbackA removes listener callbackB but it will still be called.
  // Internal listener array at time of emit [callbackA, callbackB]
  myEmitter.emit('event');
  // Prints:
  //   A
  //   B
  
  // callbackB is now removed.
  // Internal listener array [callbackA]
  myEmitter.emit('event');
  // Prints:
  //   A

  Because listeners are managed using an internal array, calling this will change the position indexes of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

  When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:

  import { EventEmitter } from 'node:events';
  const ee = new EventEmitter();
  
  function pong() {
    console.log('pong');
  }
  
  ee.on('ping', pong);
  ee.once('ping', pong);
  ee.removeListener('ping', pong);
  
  ee.emit('ping');
  ee.emit('ping');

  Returns a reference to the EventEmitter, so that calls can be chained.
• setMaxListeners(

  n: number

  ): this;

  By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.

  Returns a reference to the EventEmitter, so that calls can be chained.
• setupPrimary(

  settings?: ClusterSettings

  ): void;

  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.