cluster-test.js

const cluster = require('cluster');
const http = require('http');
const pEachSeries = require('p-each-series');

/*
# start polling via:
while
do
  curl http://127.0.0.1:8080
  sleep 0.5
done

now kill the main cluster pid
kill <pid>

you can also kill the worker PID and it will restart
kill <workerpid>
*/

const shutdown = 'shutdown';

const startWorker = async () => new Promise((resolve) => {
  let resolved = false;
  const newWorker = cluster.fork();

  newWorker.once('listening', () => resolve());
  setTimeout(() => (!resolved && resolve(), resolved = true), 4000);
});

const startWorkers = (amount) => pEachSeries(new Array(amount).fill(), startWorker);

const restartWorkers = () => pEachSeries(Object.keys(cluster.workers), async (id) => {
  // bring a new worker up
  await startWorker();

  // stop the current worker
  return new Promise((resolve) => {
    let resolved = false;
    const worker = cluster.workers[id];
    worker.send('shutdown');
    worker.disconnect();

    worker.once('exit', () => (!resolved && resolve(), resolved = true));
    setTimeout(() => (!resolved && resolve(), worker.kill('SIGTERM'), resolved = true), 4000);
  });
});

const main = async () => {
  if (cluster.isMaster) {
    console.log('INFO: Master PID:', process.pid);

    await startWorkers(1);

    cluster.on('exit', async (worker, code) => {
      if (code === 0) {
        return;
      }
      // unexpected failure, start a new worker
      console.log('DEBUG: Worker', worker.id, 'died, starting new worker');
      await startWorker();
    });

    // SIGTERM restarts workers
    process.on('SIGTERM', async () => await restartWorkers());
  } else {
    const server = await http.createServer((request, response) => {
      if (request.url ==='/crash') {
        throw new Error('boom!');
      }

      response.writeHead(200, {
        'Content-Type': 'application/json',
      });
      response.end(JSON.stringify({id: cluster.worker.id}) + '\n');
    }).listen(8080);

    process.on('message', (message) => {
      if (message === shutdown) {
        console.log('DEBUG: worker', cluster.worker.id, 'shutting down....');
        server.close(() => {
          console.log('DEBUG: worker', cluster.worker.id, 'shut down');
          process.exit(0);
        });
      }
    });

    console.log(`DEBUG: Worker ${cluster.worker.id} started`);
  }
};

main();