A helper script for managing a cluster of node worker servers

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0.2.19 years ago12 years agoMinified + gzip package size for cluster-master in KB


A module for taking advantage of the built-in cluster module in node v0.8 and above.
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")

// 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.

// 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



Set the cluster size to n. This will disconnect extra nodes and/or spin up new nodes, as needed. Done by default on restarts.


One by one, shut down nodes and spin up new ones. Callback is called when finished.


Gracefully shut down the worker nodes and then process.exit(0).


Forcibly shut down the worker nodes and then process.exit(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
  • 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.
  • 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