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Node.js - Async.js: how does parallel execution work?

I want to know how parallel execution works in async.js

async = require('async')

async.parallel([
    function(callback){
        for (var i = 0; i < 1000000000; i++) /* Do nothing */;
        console.log("function: 1")
    },
    function(callback){
        console.log("function: 2")
    }
]);

In the above example, I expect obtain the output:

function: 2

function: 1

but, the console throws the inverse, what is happening? thanks.

like image 728
Danilo Aburto Vivians Avatar asked Jun 16 '13 19:06

Danilo Aburto Vivians


1 Answers

You get the answer you don't expect because async launches function: 1 first and it doesn't release control back to the event loop. You have no async functions in function: 1.

Node.js is a single-threaded asynchronous server. If you block the event loop with a long running CPU task then no other functions can be called until your long running CPU task finishes.

Instead for a big for loop, try making http requests. For example...

async = require('async')
request = require('request')

async.parallel([
    function(callback){
      request("http://google.jp", function(err, response, body) {
        if(err) { console.log(err); callback(true); return; }
        console.log("function: 1")
        callback(false);
      });
    },
    function(callback){
      request("http://google.com", function(err, response, body) {
        if(err) { console.log(err); callback(true); return; }
        console.log("function: 2")
        callback(false);
      });
    }
]);
like image 192
Daniel Avatar answered Oct 12 '22 01:10

Daniel