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Scaling Node

Scaling Node

After working through this guide you should be able to

  • set up cluster.js to run several node instances

1 Node and the Command Line

2 Arguments

The array process.argv contains information about how the program was started:

node fs.js file.txt

The contents of the array might be:

['/Users/b/.nvm/versions/node/v5.9.0/bin/node',  '/Users/b/code/fs.js', 'file.txt' ]

3 Standard Input and Standard Output channels

are streams in node:

process.stdout.write("Hello World\n"); 
process.stderr.write("you're doing it wrong");
console.log("this", true, 42, that);

node app.js > out.txt 2> err.txt

4 environment variables

5 process id

console.log(`process ${process.pid}`)

Or find it on the commandline:

$ ps
  PID TTY           TIME CMD
71422 ttys000    0:00.05 bash
20877 ttys002    0:00.11 node app.js

6 Sending and reacting to signals

process.on( "SIGINT", function() {
  console.log(`stopping`);
  process.exit();
});

Send SIGINT by pressing CTRL-C Send SIGINT with kill 20877

SIGTERM is used by docker, always handle it!

process.on( "SIGTERM", function() {
  console.log(`stopping`);
  process.exit();
});

SIGHUB can be used for less serious signals, for example to get the app to dump some statistics, or reload a configuration file:

let count = 1;
process.on( "SIGHUP", function() {
  console.log(`count = ${count}`);
});

kill –HUP 20877

7 Scaling Node with cluster.js

V8 has a default memory limit of ~1.5GB If your server has more memory this might be unsatisfactory! You can increase this by starting node with the option --max-old-space-size.

The Eventloop uses 1 core If your server has 64 cores this might be unsatisfactory! To get around this limitation use cluster.js

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;
if (cluster.isMaster) {
  for (var i = 0; i < numCPUs; i++) {
    cluster.fork();
  }
} else {
  console.log('working!');
}

sockets are shared between slave processes:

if (cluster.isMaster) {
  // ...
} else {
  // Workers can share any 
  // TCP connections
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8000);             
}

the master process is in charge:

if (cluster.isMaster) {
  for (var i = 0; i < numCPUs; i++) {
    cluster.fork();
  }
  cluster.on('exit', (worker, code, signal) => {
    console.log(`${worker.process.pid} died`);
  });
}

they do not share the same memory space = objects are local to each instance of the app. You cannot maintain state in the application code! You can send messages:

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;
if (cluster.isMaster) {
  for (var i = 0; i < numCPUs; i++) {
    let worker = cluster.fork();
    worker.send('do something!'); 
  }
} else {
 process.on('message', (msg) => { 
   process.send('nope.'); 
 }); 
}


Where do we keep the state of the app? The answers are the same as for PHP, Rails, etc:

  • in a Database (full featured)
  • in a key value store like Memcached, Redis (faster, minimal features)
  • by sending messages

8 See Also