server.js

require('dotenv').config()

const throng = require('throng')
const os = require('os')
const portUsed = require('port-used')
const prefixStreamWrite = require('./lib/prefix-stream-write')

// Intentionally require these for both cluster primary and workers
require('./lib/check-node-version')
require('./lib/handle-exceptions')
require('./lib/feature-flags')

const { PORT, NODE_ENV } = process.env
const port = Number(PORT) || 4000

// Start the server!
if (NODE_ENV === 'production') {
 clusteredMain()
} else {
 nonClusteredMain()
}

function clusteredMain () {
 // Spin up a cluster!
 throng({
 master: setupPrimary,
 worker: setupWorker,
 count: calculateWorkerCount()
 })
}

async function nonClusteredMain () {
 await checkPortAvailability()
 await startServer()
}

async function checkPortAvailability () {
 // Check that the development server is not already running
 const portInUse = await portUsed.check(port)
 if (portInUse) {
 console.log(`\n\n\nPort ${port} is not available. You may already have a server running.`)
 console.log('Try running `killall node` to shut down all your running node processes.\n\n\n')
 console.log('\x07') // system 'beep' sound
 process.exit(1)
 }
}

async function startServer () {
 const app = require('./lib/app')
 const warmServer = require('./lib/warm-server')

 // If in a deployed environment...
 if (NODE_ENV === 'production') {
 // If in a true production environment, wait for the cache to be fully warmed.
 if (process.env.HEROKU_PRODUCTION_APP || process.env.GITHUB_ACTIONS) {
 await warmServer()
 }
 }

 // Workaround for https://github.com/expressjs/express/issues/1101
 const server = require('http').createServer(app)
 server
 .listen(port, () => console.log(`app running on http://localhost:${port}`))
 .on('error', () => server.close())
}

// This function will only be run in the primary process
async function setupPrimary () {
 process.on('beforeExit', () => {
 console.log('Shutting down primary...')
 console.log('Exiting!')
 })

 console.log('Starting up primary...')

 await checkPortAvailability()
}

// IMPORTANT: This function will be run in a separate worker process!
async function setupWorker (id, disconnect) {
 let exited = false

 // Wrap stdout and stderr to include the worker ID as a static prefix
 // console.log('hi') => '[worker.1]: hi'
 const prefix = `[worker.${id}]: `
 prefixStreamWrite(process.stdout, prefix)
 prefixStreamWrite(process.stderr, prefix)

 process.on('beforeExit', () => {
 console.log('Exiting!')
 })

 process.on('SIGTERM', shutdown)
 process.on('SIGINT', shutdown)

 console.log('Starting up worker...')

 // Load the server in each worker process and share the port via sharding
 await startServer()

 function shutdown () {
 if (exited) return
 exited = true

 console.log('Shutting down worker...')
 disconnect()
 }
}

function calculateWorkerCount () {
 // Heroku's recommended WEB_CONCURRENCY count based on the WEB_MEMORY config,
 // or explicitly configured by us
 const { WEB_CONCURRENCY } = process.env

 const recommendedCount = parseInt(WEB_CONCURRENCY, 10)
 const cpuCount = os.cpus().length

 // Ensure the recommended count is AT LEAST 1 for safety
 let workerCount = Math.max(recommendedCount || 1, 1)

 // If WEB_CONCURRENCY value was configured to a valid number...
 if (recommendedCount > 0) {
 // Use the smaller value between the recommendation vs. the CPU count
 workerCount = Math.min(workerCount, cpuCount)
 } else if (NODE_ENV === 'production') {
 // Else if in a deployed environment, default to the CPU count
 workerCount = cpuCount
 }

 return workerCount
}