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A module for taking advantage of the built-in `cluster` module in node v0.8+, enables rolling worker restarts, resizing, repl, events, configurable timeouts, debug method. Extended version of cluster-master

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A module for taking advantage of the built-in cluster module in node v0.8+, enables rolling worker restarts, resizing, repl, events, configurable timeouts, debug method.

Modified from Isaac's original version cluster-master adding:

  • events
  • repl config, help, docs
  • configurable timeouts
  • exports debug method for ability to write to all REPLs and console

Note: I had provided these changes as pull requests back to the original author, but after waiting for 10 months, I will now provide this as an alternative module

Your main server.js file uses this module to fire up a cluster of workers. Those workers then do the actual server stuff (using, express, tako, raw node, whatever; any TCP/TLS/HTTP/HTTPS server would work.)

This module provides some basic functionality to keep a server running. As the name implies, it should only be run in the master module, not in any cluster workers.

var clusterMaster = require("cluster-master-ext")

// most basic usage: just specify the worker
// Spins up as many workers as you have CPUs
// Note that this is VERY WRONG for a lot of multi-tenanted
// VPS environments where you may have 32 CPUs but only a
// 256MB RSS cap or something. ie. specify the size to
// have what makes sense

// more advanced usage.  Specify configs.
// in real life, you can only actually call clusterMaster() once.
clusterMaster({ exec: "worker.js" // script to run
              , size: 5 // number of workers
              , env: { SOME: "environment_vars" }
              , args: [ "--deep", "doop" ]
              , silent: true
              , signals: false
              , onMessage: function (msg) {
                  console.error("Message from %s %j"
                               , this.uniqueID
                               , msg)

// methods

// graceful rolling restart

// graceful shutdown

// not so graceful shutdown

// listen to events to additional cleanup or shutdown
  .on('resize', function (clusterSize) { })
  .on('restart', function () { })
  .on('quit', function () { })
  .on('quitHard', function () { });


Use from github or via npm

npm install cluster-master-ext



Set the cluster size to n. This will disconnect extra nodes and/or spin up new nodes, as needed. Done by default on restarts. Fires resize event with new clusterSize just before performing the resize.


One by one, shut down nodes and spin up new ones. Callback is called when finished. Fires restart event just before performing restart.


Gracefully shut down the worker nodes and then process.exit(0). Fires quit event just before performing the shutdown.


Forcibly shut down the worker nodes and then process.exit(1). Fires quitHard event just before performing hard shut down.


Retrieve the clusterMaster EventEmitter to be able to listen to clusterMaster events. This emitter is also returned from the original clusterMaster() constructor.

clusterMaster.debug(arg1, arg2, ...)

Arguments passed to debug are formatted with util.format and output to stdout and any REPL's.

clusterMaster.debug('The number one is %s', 1);


The exec, env, argv, and silent configs are passed to the cluster.fork() call directly, and have the same meaning.

  • exec - The worker script to run

  • env - Envs to provide to workers

  • argv - Additional args to pass to workers.

  • silent - Boolean, default=false. Do not share stdout/stderr

  • size - Starting cluster size. Default = CPU count

  • signals - Boolean, default=true. Set up listeners to:

    • SIGHUP - restart
    • SIGINT - quit (control-c)
    • SIGABRT - quitHard
  • onMessage - Method that gets called when workers send a message to the parent. Called in the context of the worker, so you can reply by looking at this.

  • stopTimeout - Time in milliseconds to wait for worker to stop before forcefully killing the process during restart or resize, default 5000 (5 seconds)

  • skepticTimeout - Time in milliseconds to wait for worker to live before shutting previous worker down during restart, default 2000 (2 seconds)

  • silenceDebug - if true, then silences the normal console debug messages, default false (output will still continue to repls regardless)

  • aliveEvent - the cluster event to wait for to consider the child process to be alive, set to online for non http workers, default listening

  • replHelp - Array of additional text lines to add to repl help command

var config = {
  replHelp: [
      'process     - access node.js process',
      '.break      - interrupt current command'
  • replContext - Object of additional properties or functions to add to the REPL context
var config = {
  replContext: {
    foo: fooObject  // adds foo to the REPL which exposes the fooObject
  • repl - where to have REPL listen, defaults to env.CLUSTER_MASTER_REPL || 'cluster-master-socket'
    • if repl is null or false - REPL is disabled and will not be started
    • if repl is string path - REPL will listen on unix domain socket to this path
    • if repl is an integer port - REPL will listen on TCP
    • if repl is an object with address and port, then REPL will listen on TCP address:PORT

Examples of configuring repl

var config = { repl: false }                       // disable REPL
var config = { repl: '/tmp/cluster-master-sock' }  // unix domain socket
var config = { repl: 3001 }                        // tcp socket
var config = { repl: { address: '', port: 3002 }}  // tcp

Note: be careful when using TCP for your REPL since anyone on the network can connect to your REPL (no security). So either disable the REPL or use a unix domain socket which requires local access (or ssh access) to the server.


Cluster-master provides a REPL into the master process so you can inspect the state of your cluster. By default the REPL is accessible by a socket written to the root of the directory, but you can override it with the CLUSTER_MASTER_REPL environment variable. You can access the REPL with nc or socat like so:

nc -U ./cluster-master-socket

# OR

socat ./cluster-master-socket stdin

The REPL provides you with access to these objects or functions:

  • help - display these commands
  • repl - access the REPL
  • resize(n) - resize the cluster to n workers
  • restart(cb) - gracefully restart workers, cb is optional
  • stop() - gracefully stop workers and master
  • kill() - forcefully kill workers and master
  • cluster - node.js cluster module
  • size - current cluster size
  • connections - number of REPL connections to master
  • workers - current workers
  • select(fld) - map of id to field (from workers)
  • pids - map of id to pids
  • ages - map of id to worker ages
  • states - map of id to worker states
  • debug(a1) - output a1 to stdout and all REPLs
  • sock - this REPL socket'
  • .exit - close this connection to the REPL


clusterMaster emits events on clusterMaster.emitter() when its methods are called which allows you to respond and do additional cleanup right before the action is carried out.

  • debug - fired when debug() is called to output messages, listener ex: fn(msg, args, ...)
  • disconnect - fired before worker is to be disconnected, listener ex: fn(worker)
  • resize - fired on clusterMaster.resize(n), listener ex: fn(clusterSize)
  • restart - fired on clusterMaster.restart(), listener ex: fn(oldWorkers) restartComplete - fired when restart is completed
  • quit - fired on clusterMaster.quit()
  • quitHard - fired on clusterMaster.quitHard()



Authors and Contributors

  • Isaac Z. Schlueter, author of the original project cluster-master
  • Jeff Barczewski
  • Sean McCullough


A module for taking advantage of the built-in `cluster` module in node v0.8+, enables rolling worker restarts, resizing, repl, events, configurable timeouts, debug method. Extended version of cluster-master



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