multi-backend-server.js

// backend-cluster.js
const cluster = require('cluster');
const os = require('os');
const express = require('express');

const NUM_SERVERS = 7;      // how many backend instances you want
const BASE_PORT = 5001;     // first port; others increment automatically

// `cluster.isPrimary` in recent Node; fall back to isMaster for older versions
const isPrimary = cluster.isPrimary ?? cluster.isMaster;

if (isPrimary) {
  console.log(`Primary process PID: ${process.pid}`);
  for (let i = 0; i < NUM_SERVERS; i++) {
    cluster.fork({ PORT: BASE_PORT + i });
  }

  // (Optional) auto-respawn a crashed worker
  cluster.on('exit', (worker, code, signal) => {
    const oldPort = worker.process.env?.PORT;
    console.log(`Worker ${worker.process.pid} for port ${oldPort} died (${signal || code}). Restarting...`);
    // Re-spawn with same port if we know it; otherwise fall back to next available
    cluster.fork({ PORT: oldPort || (BASE_PORT + Math.floor(Math.random() * NUM_SERVERS)) });
  });

} else {
  const port = Number(process.env.PORT);
  const app = express();

  // identify this worker
  const workerId = cluster.worker?.id || 'unknown';

  app.get('/', async (req, res) => {
    console.log(`Received request on backend worker #${workerId} on port ${port}`);
    await new Promise(resolve => setTimeout(resolve, 1000));
    res.send(`Hello from backend worker #${workerId} on port ${port}`);
  });

  app.get('/health', (req, res) => {
    res.send('OK');
  });

  // helpful endpoint to simulate variable response time
  app.get('/work', async (req, res) => {
    const ms = Number(req.query.ms || 0);
    if (ms > 0) await new Promise(r => setTimeout(r, ms));
    res.json({ workerId, port, delayedMs: ms });
  });

  app.listen(port, () => {
    console.log(`Worker #${workerId} listening on port ${port}`);
  });
}