Read a large file without blocking the event loop

README.MD

To test:

Set the path to a large file in worker.js

Run: node index.js

Then you can run this bash: while true; do date && curl -m 5 http://localhost:3000/ && echo; sleep 1; done

and open a browser http://127.0.0.1:3000/read

Result

You can see that reading a large file does not block incoming requests.

index.js

const http = require('http')
const { Worker } = require('worker_threads')

function startWorker (cb) {
  const w = new Worker(`${__dirname}/worker.js`)
  w.on('message', (msg) => {
    cb(msg)
  })
  w.on('error', (e) => console.error(e))
  w.on('exit', (code) => {
    if (code !== 0) {
      console.error(new Error(`Worker stopped with exit code ${code}`))
    }
  })
  return w
}

let counterReadFile = 1
let counterRequestHome = 1

const server = http.createServer()
server.on('request', (req, res) => {
    res.statusCode = 200
    res.setHeader('Content-Type', 'text/plain')

    if (req.url === '/read') {
      startWorker((m) => res.end(`Reading the file has been ${m}. Counter: ${counterReadFile++}`))
    } else if (req.url === '/') {
      res.end(`Home page. Counter ${counterRequestHome++}`)
    } else {
      res.end(`Page not found`)
    }
  }
)

server.listen(3000, '127.0.0.1', () => {
  console.log(`Server running at http://127.0.0.1:3000/`)
})

worker.js

const { parentPort } = require('worker_threads')
const fs = require('fs')

fs.readFile(
  `${__dirname}/bigfile.tgz`, // the path to a very large file
  { encoding: 'utf8' }, (err, data) => {

    //... some actions

    parentPort.postMessage('completed')
  })