nodejs/doc/api/cluster.markdown

Cluster

Stability: 1 - Experimental

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

The cluster module allows you to easily create a network of processes that all share server ports.

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

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

  cluster.on('exit', function(worker, code, signal) {
    console.log('worker ' + worker.process.pid + ' died');
  });
} else {
  // Workers can share any TCP connection
  // In this case its a HTTP server
  http.createServer(function(req, res) {
    res.writeHead(200);
    res.end("hello world\n");
  }).listen(8000);
}

Running node 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

This feature was introduced recently, and may change in future versions. Please try it out and provide feedback.

Also 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.

When you call server.listen(...) in a worker, it serializes the arguments and passes the request to the master process. If the master process already has a listening server matching the worker's requirements, then it passes the handle to the worker. If it does not already have a listening server matching that requirement, then it will create one, and pass the handle to the child.

This causes potentially surprising behavior in three edge cases:

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.

When multiple processes are all accept()ing on the same underlying resource, the operating system load-balances across them very efficiently. 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. Node 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.

cluster.settings

• {Object}
  • exec {String} file path to worker file. (Default=__filename)
  • args {Array} string arguments passed to worker. (Default=process.argv.slice(2))
  • silent {Boolean} whether or not to send output to parent's stdio. (Default=false)

All settings set by the .setupMaster is stored in this settings object. This object is not supposed to be changed or set manually, by you.

cluster.isMaster

• {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

• {Boolean}

This boolean flag is true if the process is a worker forked from a master. If the process.env.NODE_UNIQUE_ID is set to a value, then isWorker is true.

Event: 'fork'

• worker {Worker object}

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

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

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

Event: 'online'

• worker {Worker object}

After forking a new worker, the worker should respond with a online message. When the master receives a online message it will emit such event. The difference between 'fork' and 'online' is that fork is emitted when the master tries to fork a worker, and 'online' is emitted when the worker is being executed.

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

Event: 'listening'

• worker {Worker object}
• address {Object}

When calling listen() from a worker, a 'listening' event is automatically assigned to the server instance. When the server is listening a message is send to the master where the 'listening' event is emitted.

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', function(worker, address) {
  console.log("A worker is now connected to " + address.address + ":" + address.port);
});

Event: 'disconnect'

• worker {Worker object}

When a workers IPC channel has disconnected this event is emitted. This will happen when the worker dies, usually after calling .destroy().

When calling .disconnect(), there may be a delay between the disconnect and exit events. This event can be used to detect if the process is stuck in a cleanup or if there are long-living connections.

cluster.on('disconnect', function(worker) {
  console.log('The worker #' + worker.id + ' has disconnected');
});

Event: 'exit'

• worker {Worker object}
• code {Number} the exit code, if it exited normally.
• signal {String} the name of the signal (eg. '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', function(worker, code, signal) {
  var exitCode = worker.process.exitCode;
  console.log('worker ' + worker.process.pid + ' died ('+exitCode+'). restarting...');
  cluster.fork();
});

Event: 'setup'

• worker {Worker object}

When the .setupMaster() function has been executed this event emits. If .setupMaster() was not executed before fork() this function will call .setupMaster() with no arguments.

cluster.setupMaster([settings])

• settings {Object}
  • exec {String} file path to worker file. (Default=__filename)
  • args {Array} string arguments passed to worker. (Default=process.argv.slice(2))
  • silent {Boolean} whether or not to send output to parent's stdio. (Default=false)

setupMaster is used to change the default 'fork' behavior. The new settings are effective immediately and permanently, they cannot be changed later on.

Example:

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

cluster.fork([env])

• env {Object} Key/value pairs to add to child process environment.
• return {Worker object}

Spawn a new worker process. This can only be called from the master process.

cluster.disconnect([callback])

• callback {Function} called when all workers are disconnected and handlers are closed

When calling this method, all workers will commit a graceful suicide. When they are disconnected all internal handlers will be closed, allowing the master process to die graceful if no other event is waiting.

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

cluster.worker

• {Object}

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

var 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

• {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.

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

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.

worker.id

• {String}

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.process

• {ChildProcess object}

All workers are created using child_process.fork(), the returned object from this function is stored in process.

See: Child Process module

worker.suicide

• {Boolean}

This property is a boolean. It is set when a worker dies after calling .destroy() or immediately after calling the .disconnect() method. Until then it is undefined.

worker.send(message, [sendHandle])

• message {Object}
• sendHandle {Handle object}

This function is equal to the send methods provided by child_process.fork(). In the master you should use this function to send a message to a specific worker. However in a worker you can also use process.send(message), since this is the same function.

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', function(msg) {
    process.send(msg);
  });
}

worker.destroy()

This function will kill the worker, and inform the master to not spawn a new worker. The boolean suicide lets you distinguish between voluntary and accidental exit.

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

// destroy worker
worker.destroy();

worker.disconnect()

When calling this function the worker will no longer accept new connections, but they will be handled by any other listening worker. Existing connection will be allowed to exit as usual. When no more connections exist, the IPC channel to the worker will close allowing it to die graceful. When the IPC channel is closed the disconnect event will emit, this is then followed by the exit event, there is emitted when the worker finally die.

Because there might be long living connections, it is useful to implement a timeout. This example ask the worker to disconnect and after 2 seconds it will destroy the server. An alternative would be to execute worker.destroy() after 2 seconds, but that would normally not allow the worker to do any cleanup if needed.

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

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

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

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

  server.listen(8000);

  server.on('close', function() {
    // cleanup
  });

  process.on('message', function(msg) {
    if (msg === 'force kill') {
      server.destroy();
    }
  });
}

Event: 'message'

• message {Object}

This event is the same as the one provided by child_process.fork(). In the master you should use this event, however 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:

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

if (cluster.isMaster) {

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

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

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

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

} else {

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

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

Event: 'online'

Same as the cluster.on('online') event, but emits only when the state change on the specified worker.

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

Event: 'listening'

• address {Object}

Same as the cluster.on('listening') event, but emits only when the state change on the specified worker.

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

Event: 'disconnect'

Same as the cluster.on('disconnect') event, but emits only when the state change on the specified worker.

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

Event: 'exit'

• code {Number} the exit code, if it exited normally.
• signal {String} the name of the signal (eg. 'SIGHUP') that caused the process to be killed.

Emitted by the individual worker instance, when the underlying child process is terminated. See child_process event: 'exit'.

var worker = cluster.fork();
worker.on('exit', function(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!");
  }
});