Node cluster example

index.js

#!/usr/bin/env node

/**
 * External dependencies
 */
const cluster = require( 'cluster' )

const WORKERS = process.env.WORKERS || require( 'os' ).cpus().length
const SHUTDOWN_TIMEOUT = process.env.SHUTDOWN_TIMEOUT || 5000

if ( cluster.isMaster ) {
	for ( let i = 0; i < WORKERS; i++ ) {
		cluster.fork()
	}

	cluster.on( 'listening', ( worker, address ) => {
		console.log( 'Worker %d (pid %d) listening on http://%s:%d',
			worker.id,
			worker.process.pid,
			address.address || '127.0.0.1',
			address.port
		)
	} )

	cluster.on( 'exit', ( worker, code, signal ) => {
		if ( ! worker.exitedAfterDisconnect ) {
			console.log( 'Worker %d (pid %d) died with code %d and signal %s, restarting', worker.id, worker.process.pid, code, signal )
			cluster.fork()
		}
	} )

	const gracefulShutdown = worker => {
		worker.send( 'shutdown' )
		worker.disconnect()

		const shutdown = setTimeout( () => {
			worker.process.kill()
		}, SHUTDOWN_TIMEOUT )

		worker.on( 'exit', () => clearTimeout( shutdown ) )
	}

	process.on( 'SIGHUP', () => {
		console.log( 'Caught SIGHUP, reloading workers' )

		for ( const id in cluster.workers ) {
			cluster.fork().on( 'listening', () => {
				gracefulShutdown( cluster.workers[ id ] )
			} )
		}
	} )

	process.on( 'SIGINT', () => {
		console.log( 'Caught SIGINT, initiating graceful shutdown' )

		for ( const id in cluster.workers ) {
			gracefulShutdown( cluster.workers[ id ] )
		}
	} )
} else {
	const server = require( './server' )
	process.on( 'message', message => {
		switch( message ) {
			case 'shutdown':
				server.close( () => process.exit( 0 ) )
				return
		}
	} )

	process.on( 'SIGINT', () => {
		// Ignore first SIGINT from parent

		process.on( 'SIGINT', () => {
			process.exit( 1 )
		} )
	} )

	process.on( 'uncaughtException', error => {
		console.log( error.stack )

		// Stop accepting connections and exit
		server.close( () => process.exit( 1 ) )

		// Force shutdown after timeout
		setTimeout( () => {
			process.exit( 1 )
		}, SHUTDOWN_TIMEOUT )
	} )
}

server.js

/**
 * External dependencies
 */
const { createServer } = require( 'http' )

module.exports = createServer( ( req, res ) => {
	if ( Math.random() > 0.99999 ) {
		// Randomly throws an uncaught error 0.001% of the time
		throw Error( '0.001% error' )
	}

	res.end( 'Hello World!\n' )
} ).listen( process.env.port || 3000 )