How to write non-blocking async function in Express request handler [duplicate] - node.js

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How node.js server serve next request, if current request have huge computation?
(2 answers)
NodeJs how to create a non-blocking computation
(5 answers)
Closed 5 years ago.
All:
I am pretty new to Node async programming, I wonder how can I write some Express request handler which can handle time consuming heavy calculation task without block Express handling following request?
I thought setTimeout can do that to put the job in a event loop, but it still block other requests:
var express = require('express');
var router = express.Router();
function heavy(callback){
setTimeout(callback, 1);
}
router.get('/', function(req, res, next) {
var callback = function(req, res){
var loop = +req.query.loop;
for(var i=0; i<loop; i++){
for(var j=0; j<loop; j++){}
}
res.send("finished task: "+Date.now());
}.bind(null, req, res);
heavy(callback)
});
I guess I did not understand how setTimeout works(my understanding about setTimeout is after that 1ms delay it will fire up the callback in a seperated thread/process without blocking other call of heavy), could any one show me how to do this without blocking other request to heavy()?
Thanks

Answer 1

Instead of setTimeout it's better to use process.nextTick or setImmediate (depending od when you want your callback to be run). But it is not enough to put a long running code into a function because it will still block your thread, just a millisecond later.
You need to break your code and run setImmediate or process.nextTick multiple times - like in every iteration and then schedule a new iteration from that. Otherwise you will not gain anything.
Example
Instead of a code like this:
var a = 0, b = 10000000;
function numbers() {
while (a < b) {
console.log("Number " + a++);
}
}
numbers();
you can use code like this:
var a = 0, b = 10000000;
function numbers() {
var i = 0;
while (a < b && i++ < 100) {
console.log("Number " + a++);
}
if (a < b) setImmediate(numbers);
}
numbers();
The first one will block your thread (and likely overflow your call stack) and the second one will not block (or, more precisely, it will block your thread 10000000 times for a very brief moment, letting other stuff to run in between those moments).
You can also consider spawning an external process or writing a native add on in C/C++ where you can use threads.
For more info see:
How node.js server serve next request, if current request have huge computation?
Maximum call stack size exceeded in nodejs
Node; Q Promise delay
How to avoid jimp blocking the code node.js
NodeJS, Promises and performance