Running next.js in cluster mode (dead simple)

#javascript #nextjs #tutorial #webdev

Honestly, I feel like an idiot - I've been maintaining Next.js apps last few years, and thought Next.js is automatically running clustered in production.

Moreover, I did not find any nice solutions for running Next.js clusterized - everything is over-complicated with ton of code.

Well, turned out (at least for Next 12) it is really simple - and Google did not give me any nice answer, so I had to research on it by myself today.

So, here's the solution - mostly simply based on node cluster docs:

const cluster = require('node:cluster');
const process = require('node:process');
const { cpus } = require('node:os');

const numCPUs = cpus().length;

if (cluster.isPrimary) {
  console.log(`Primary ${process.pid} is running`);
  // all the magic goes here. 
  // `setupPrimary` is altering cluster behaviour
  // see docs: https://nodejs.org/api/cluster.html#clustersetupprimarysettings
  cluster.setupPrimary({
    // node_modules/.bin is directory for all runtime scripts,
    // including `next`.
    // we can work with it like with a common node package,
    // so require.resolve simply gives us path for `next` CLI 
    exec: require.resolve('.bin/next'),
    // args should skip first 2 arguments, 
    // "node" path and file to execute path
    args: ['start', ...process.argv.slice(2)],
    // just directly passing all the IO to not handle pipes
    stdio: 'inherit',
    // making stdout and stderr colored
    shell: true,
  });

  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`, { code, signal });
  });
}

// we do not need to do "else" here as cluster will run different file