Concurrent in Nodejs

Question

I have node server like below. And i push 2 request that almost simultaneously(with the same url = "localhost:8080/"). And my question is: "Why the server wait for 1st request handle done, then 2st request will be handle"?

Output in console of my test:

Home..
Home..

(Notes: 2nd line will be display after 12second) - server.js:

    var express = require('express')
    var app = express()
    app.use(express.json())

    app.get('/', async(request, response) => {
        try {
            console.log('Home ...')
            await sleep(12000)
            response.send('End')
        } catch (error) {
            response.end('Error')
        }
    })


    function sleep(ms) {
      return new Promise(resolve => setTimeout(resolve, ms));
    }

    var server = app.listen(8080, '127.0.0.1', async () => {
        var host = server.address().address
        var port = server.address().port
        console.log('====================  START SERVER  ===================')
        console.log('* Running on http://%s:%s (Press CTRL+C to quit)', host, port)
        console.log('* Create time of ' + new Date() + '\n')
    })

How did you push those 2 request? Cannot reproduce your result, the 2 Home... appears without sleeping for me, I am using 2 bash instance with curl — William Chong
– William Chong, Commented Jul 31, 2018 at 17:10

akaphenom · Accepted Answer · 2018-08-03 20:17:57Z

1

Agreed with @unclexo - understand what the blocking/non-blocking calls are and optimize your code around that. If you really want to add capacity for parallel requests, you could consider leveraging the cluster module.

https://nodejs.org/api/cluster.html

This will kick off children processes and proxy the HTTP requests to those children. Each child can block as long as it wants and not affect the other processes (unless there is some race condition between them).

edited Aug 3, 2018 at 20:17

answered Jul 31, 2018 at 18:40

akaphenom

6,89611 gold badges62 silver badges112 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

unclexo · Accepted Answer · 2018-07-31 18:29:04Z

0

"Why the server wait for 1st request handle done, then 2st request will be handle"?

This is not right for Node.Js. You are making a synchronous and asynchronous call in your code. The asynchronous call to the sleep() method waits for 12 seconds to be run.

Now this does not mean the asynchronous call waits until the synchronous call finishes. This is synchronous behavior. Note the following examples

asyncFunction1()
asyncFunction2()
asyncFunction3()

Here whenever the 1st function starts running, then node.js releases thread/block, then 2nd function starts running, again node releases thread, so the call to 3rd function starts. Each function returns response whenever it finishes and does not wait for other's return.

NodeJs is single-threaded or has non-blocking i/o architecture. Asynchronous feature is its beauty.

answered Jul 31, 2018 at 18:29

unclexo

3,9812 gold badges22 silver badges29 bronze badges

1 Comment

rootExplorr Over a year ago

Just to add on the above answer by @unclexo , nodejs engine itself is single threaded , so if you fire 2 requests simultaneously then node handles them one by one. Now since you have written "await" before the sleep call , node engine would be stalled at that point and all new request would then naturally have to wait.

hutabalian · Accepted Answer · 2018-12-06 18:20:43Z

You are blocking the other request by calling await at app.get('/'.... If you want the sleep method to run asynchronously between requests, wrap it in a new async function. The code will be like this:

app.get('/', (request, response) => {
    try {
        print()
        response.send('End')
    } catch (error) {
        response.end('Error')
    }
})

async function print() {
    await sleep(12000)
    console.log('Home ...')
}

function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

This way, if you send a request and then send another request, the second request's print() function will be executed without waiting for 12 seconds from the first request. Therefore, there will be 2 Home ... printed at around 12 seconds after the request sent. To have more understanding about this, you should learn about NodeJS Event Loop.

Collectives™ on Stack Overflow

Concurrent in Nodejs

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related