simple node js cluster with mysql connections

app.js

const express = require('express');
const cluster = require('cluster');
const mysql = require('mysql');

const numCPUs = require('os').cpus().length;

const app = express();
const PORT = 3000;

const pool = mysql.createPool({
  host: process.env.MYSQL_HOST || 'localhost',
  port: process.env.MYSQL_PORT || 3306,
  user: process.env.MYSQL_USER,
  password: process.env.MYSQL_PASS,
  database: process.env.MYSQL_DB,
  connectionLimit: process.env.MYSQL_CONN_LIMIT || numCPUs,
});

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

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

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

} else {
  app.listen(PORT, err => {
    err ?
      console.log("Error in server setup") :
      console.log(`Worker ${process.pid} started`);
  });

  app.get('/', (req, res) => {
    console.log(`Worker process id is ${process.pid}`);

    pool.query('SELECT 1 + 1 AS solution', function (err, rows, fields) {
      if (err) throw err;

      res
        .status(200)
        .send(`The solution is: ${rows[0].solution}`);
    });
  })
}