How To Write a Linux Daemon with Node.js on a VPS

The following is the initial project structure:

node-simple-http-daemon/
|
|-- README.md
|-- bin/
|   `-- node-simple-http-daemon
`-- lib/
    `-- app.js

Let’s start by creating a package.json file:

$ npm init

Install daemon module:

$ npm --save install daemon

And here is how the package.json should look like:

{
  "name": "node-simple-http-daemon",
  "version": "0.0.0",
  "description": "Simple HTTP Daemon written in Node.js",
  "main": "lib/app.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "start": "node lib/app.js"
  },
  "author": "Fardjad Davari",
  "license": "MIT",
  "dependencies": {
    "daemon": "~1.1.0"
  }
}

Note that we added a start script, So we can start the app with npm start command later.

For now, we’re going to create a simple HTTP server that starts listening on port 80 and responds with “Hello world” for every request.

Put the following in lib/app.js file:

/**
 * lib/app.js
 */

const PORT = 80;
const ADDRESS = '0.0.0.0';

var http = require('http');

var server = http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('Hello World\n');
});

server.listen(PORT, ADDRESS, function () {
    console.log('Server running at http://%s:%d/', ADDRESS, PORT);
    console.log('Press CTRL+C to exit');
});

You can start the VPS by running sudo npm start.

The following is the daemon executable code.

The following should be placed in bin/node-simple-http-daemon:

#!/usr/bin/env node

/**
 * bin/node-simple-http-daemon
 */

// Everything above this line will be executed twice
require('daemon')();

var cluster = require('cluster');

// Number of CPUs
var numCPUs = require('os').cpus().length;

/**
 * Creates a new worker when running as cluster master.
 * Runs the HTTP server otherwise.
 */
function createWorker() {
  if (cluster.isMaster) {
    // Fork a worker if running as cluster master
    var child = cluster.fork();

    // Respawn the child process after exit
    // (ex. in case of an uncaught exception)
    child.on('exit', function (code, signal) {
      createWorker();
    });
  } else {
    // Run the HTTP server if running as worker
    require('../lib/app');
  }
}

/**
 * Creates the specified number of workers.
 * @param  {Number} n Number of workers to create.
 */
function createWorkers(n) {
  while (n-- > 0) {
    createWorker();
  }
}

/**
 * Kills all workers with the given signal.
 * Also removes all event listeners from workers before sending the signal
 * to prevent respawning.
 * @param  {Number} signal
 */
function killAllWorkers(signal) {
  var uniqueID,
      worker;

  for (uniqueID in cluster.workers) {
    if (cluster.workers.hasOwnProperty(uniqueID)) {
      worker = cluster.workers[uniqueID];
      worker.removeAllListeners();
      worker.process.kill(signal);
    }
  }
}

/**
 * Restarts the workers.
 */
process.on('SIGHUP', function () {
  killAllWorkers('SIGTERM');
  createWorkers(numCPUs * 2);
});

/**
 * Gracefully Shuts down the workers.
 */
process.on('SIGTERM', function () {
  killAllWorkers('SIGTERM');
});

// Create two children for each CPU
createWorkers(numCPUs * 2);

Time to start our daemon! But before that, we should modify our app.js to handle SIGTERM.

Add the following to the end of the app.js file:

process.on('SIGTERM', function () {
  if (server === undefined) return;
  server.close(function () {
    // Disconnect from cluster master
    process.disconnect && process.disconnect();
  });
});

Make the daemon script executable:

$ chmod +x bin/node-simple-http-daemon

And run it (Make sure nothing else is running on port 80):

$ sudo bin/node-simple-http-daemon

Now, your daemon and it’s workers should be running in background. You can confirm that by sending an HTTP GET request via cURL:

$ curl 127.0.0.1

Before moving to the next step, let’s take a look at the process IDs:

$ ps -axf | grep [n]ode-simple-http-daemon | \
  awk '{ print "pid:"$2", parent-pid:"$3 }'

Sample output:

pid:33811, parent-pid:1
pid:33853, parent-pid:33811
pid:33856, parent-pid:33811
pid:33857, parent-pid:33811
pid:33858, parent-pid:33811
pid:33859, parent-pid:33811
pid:33860, parent-pid:33811
pid:33861, parent-pid:33811
pid:33862, parent-pid:33811

Note that the daemon is the process with parent-pid of 1.

Try restarting the workers by sending SIGHUP to the daemon:

$ kill -HUP 33811 # (replace 33811 with the daemon PID)

Now if you list the PIDs again, you should be able to see new worker processes with new PIDs. Amazing, isn’t it?

To stop the daemon, just run:

$ kill -TERM 33811 # (replace 33811 with the daemon PID)

We’re almost done. We only need to make workers drop their privileges after the VPS gets started.

Modify server.listen() callback in app.js so it reads:

server.listen(PORT, ADDRESS, function () {
    console.log('Server running at http://%s:%d/', ADDRESS, PORT);
    console.log('Press CTRL+C to exit');
    
    // Check if we are running as root
    if (process.getgid() === 0) {
      process.setgid('nobody');
      process.setuid('nobody');
    }
});

And that’s it for the daemon part.

Now you can install it system-wide:

$ sudo npm link