node/doc/api/cluster.markdown

Cluster

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('death', function(worker) {
    console.log('worker ' + worker.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 server.js
Worker 2438 online
Worker 2437 online

cluster.isMaster

This boolean flag is 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 isMaster is true.

cluster.isWorker

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 different efined isWorker is true.

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 you own timeout.

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

cluster.on('fork', function (worker) {
  timeouts[worker.uniqueID] = setTimeout(errorMsg, 2000);
});
cluster.on('listening', function (worker) {
  clearTimeout(timeouts[worker.uniqueID]);
});
cluster.on('death', function (worker) {
  clearTimeout(timeouts[worker.uniqueID]);
  errorMsg();
});

Event: 'online'

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'

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.

cluster.on('listening', function (worker) {
  console.log("We are now connected");
});

Event: 'death'

When any of the workers die the cluster module will emit the 'death' event. This can be used to restart the worker by calling fork() again.

cluster.on('death', function(worker) {
  console.log('worker ' + worker.pid + ' died. restart...');
  cluster.fork();
});

cluster.fork([env])

Spawn a new worker process. This can only be called from the master process. The function takes an optional env object. The properties in this object will be added to the process environment in the worker.

cluster.workers

In the cluster all living worker objects are stored in this object by there uniqueID as the key. This makes it easy to loop through all living workers.

// Go through all workers
function eachWorker(callback) {
  for (var uniqueID in cluster.workers) {
    callback(cluster.workers[uniqueID]);
  }
}
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 uniqueID is the easiest way to find the worker.

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

Worker

This 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.uniqueID

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

Worker.process

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

Worker.send(message, [sendHandle])

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. To know the difference between suicide and accidentally death a suicide boolean is set to true.

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

// destroy worker
worker.destroy();

Worker.suicide

This property is a boolean. It is set when a worker dies, until then it is undefined. It is true if the worker was killed using the .destroy() method, and false otherwise.

Event: message

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

  // Start workers and listen for messages containing notifyRequest
  cluster.autoFork();
  Object.keys(cluster.workers).forEach(function (uniqueID) {
    cluster.workers[uniqueID].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) {
  // Worker is online
};

Event: listening

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

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

Event: death

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

cluster.fork().on('death', function (worker) {
  // Worker has died
};