How to sleep the thread in node.js without affecting other threads?

As per Understanding the node.js event loop, node.js supports a single thread model. That means if I make multiple requests to a node.js server, it won't spawn a new thread for each request but will execute each request one by one. It means if I do the following for the first request in my node.js code, and meanwhile a new request comes in on node, the second request has to wait until the first request completes, including 5 second sleep time. Right?

var sleep = require('sleep');
sleep.sleep(5)//sleep for 5 seconds

Is there a way that node.js can spawn a new thread for each request so that the second request does not have to wait for the first request to complete, or can I call sleep on specific thread only?

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最佳答案

If you are referring to the npm module sleep, it notes in the readme that sleep will block execution. So you are right - it isn't what you want. Instead you want to use setTimeout which is non-blocking. Here is an example:

setTimeout(function() {
console.log('hello world!');
}, 5000);

For anyone looking to do this using es7 async/await, this example should help:

const snooze = ms => new Promise(resolve => setTimeout(resolve, ms));


const example = async () => {
console.log('About to snooze without halting the event loop...');
await snooze(1000);
console.log('done!');
};


example();
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In case you have a loop with an async request in each one and you want a certain time between each request you can use this code:

   var startTimeout = function(timeout, i){
setTimeout(function() {
myAsyncFunc(i).then(function(data){
console.log(data);
})
}, timeout);
}


var myFunc = function(){
timeout = 0;
i = 0;
while(i < 10){
// By calling a function, the i-value is going to be 1.. 10 and not always 10
startTimeout(timeout, i);
// Increase timeout by 1 sec after each call
timeout += 1000;
i++;
}
}

This examples waits 1 second after each request before sending the next one.

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Please consider the deasync module, personally I don't like the Promise way to make all functions async, and keyword async/await anythere. And I think the official node.js should consider to expose the event loop API, this will solve the callback hell simply. Node.js is a framework not a language.

var node = require("deasync");
node.loop = node.runLoopOnce;


var done = 0;
// async call here
db.query("select * from ticket", (error, results, fields)=>{
done = 1;
});


while (!done)
node.loop();


// Now, here you go
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When working with async functions or observables provided by 3rd party libraries, for example Cloud firestore, I've found functions the waitFor method shown below (TypeScript, but you get the idea...) to be helpful when you need to wait on some process to complete, but you don't want to have to embed callbacks within callbacks within callbacks nor risk an infinite loop.

This method is sort of similar to a while (!condition) sleep loop, but yields asynchronously and performs a test on the completion condition at regular intervals till true or timeout. 

export const sleep = (ms: number) => {
return new Promise(resolve => setTimeout(resolve, ms))
}
/**
* Wait until the condition tested in a function returns true, or until
* a timeout is exceeded.
* @param interval The frenequency with which the boolean function contained in condition is called.
* @param timeout  The maximum time to allow for booleanFunction to return true
* @param booleanFunction:  A completion function to evaluate after each interval. waitFor will return true as soon as the completion function returns true.
*/
export const waitFor = async function (interval: number, timeout: number,
booleanFunction: Function): Promise<boolean> {
let elapsed = 1;
if (booleanFunction()) return true;
while (elapsed < timeout) {
elapsed += interval;
await sleep(interval);
if (booleanFunction()) {
return true;
}
}
return false;
}

The say you have a long running process on your backend you want to complete before some other task is undertaken. For example if you have a function that totals a list of accounts, but you want to refresh the accounts from the backend before you calculate, you can do something like this:

async recalcAccountTotals() : number {
this.accountService.refresh();   //start the async process.
if (this.accounts.dirty) {
let updateResult = await waitFor(100,2000,()=> {return !(this.accounts.dirty)})
}
if(!updateResult) {
console.error("Account refresh timed out, recalc aborted");
return NaN;
}
return ... //calculate the account total.
}
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It is quite an old question, and though the accepted answer is still entirely correct, the timers/promises API added in v15 provides a simpler way.

import { setTimeout } from 'timers/promises';


// non blocking wait for 5 secs
await setTimeout(5 * 1000);

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