true parallelism in node

migrate.js

import { cpus } from 'os';
import cluster from 'cluster';

if (cluster.isMaster) {
  getAllRows()
  .then(rows => {
    const CPUS = cpus().length;
    const chunked =chunks(rows, CPUS); // n rows into an array of length CPUS containing n/CPUS-ish rows (tries to balance)
    
    const statuses = chunked
      .filter(x => x.length)
      .map(chunk => new Promise((resolve, reject) => {
          const worker = cluster.fork({
              __chunk__: JSON.stringify(chunk)
          });

          worker.on('exit', resolve);
      }));

    Promise.all(statuses)
        .then(codes => {
            process.exit(codes.every(x => x === 0) ? 0 : 1);
        });
  })
} 
else {
  const rows = JSON.parse(process.env.__chunk__);
  // do stuff
  // ...
}