ChatGPT解决这个技术问题 Extra ChatGPT

Call async/await functions in parallel

As far as I understand, in ES7/ES2016 putting multiple await's in code will work similar to chaining .then() with promises, meaning that they will execute one after the other rather than in parallel. So, for example, we have this code:

await someCall();
await anotherCall();

Do I understand it correctly that anotherCall() will be called only when someCall() is completed? What is the most elegant way of calling them in parallel?

I want to use it in Node, so maybe there's a solution with async library?

EDIT: I'm not satisfied with the solution provided in this question: Slowdown due to non-parallel awaiting of promises in async generators, because it uses generators and I'm asking about a more general use case.

@Blindman67 - it does, at least the way the OP means, where two async operations are running simultaneously, but not in this case, what I meant to write was that they run in serial, the first await would wait for the first function to complete entirely before executing the second.
@Blindman67 - it's single threaded, but that limitation doesn't apply to async methods, they can run simultaneously, and return the response when they are done, i.e. what the OP means by "parallell".
@Blindman67 - I think it's pretty clear what the OP is asking, using the async/await pattern will make the functions run in serial, even if they are async, so the first would completely finish before the second is called etc. The OP is asking how to call both function in parallell, and as they are clearly async, the aim is to run them simultaneously, i.e. in parallell, for instance doing two ajax requests simultaneously, which is not a problem at all in javascript, as most async methods, as you've noted, runs native code, and uses more threads.
@Bergi this is not a duplicate of the linked question — this is specifically about async/await syntax and native Promises. The linked question is regarding the bluebird library with generators & yield. Conceptually similar perhaps, but not in implementation.
@Bergi The syntax very much does matter. To a person that has never used generators or bluebird the linked question is completely unhelpful.

L
Lukas Liesis

You can await on Promise.all():

await Promise.all([someCall(), anotherCall()]);

To store the results:

let [someResult, anotherResult] = await Promise.all([someCall(), anotherCall()]);

Note that Promise.all fails fast, which means that as soon as one of the promises supplied to it rejects, then the entire thing rejects.

const happy = (v, ms) => new Promise((resolve) => setTimeout(() => resolve(v), ms)) const sad = (v, ms) => new Promise((_, reject) => setTimeout(() => reject(v), ms)) Promise.all([happy('happy', 100), sad('sad', 50)]) .then(console.log).catch(console.log) // 'sad'

If, instead, you want to wait for all the promises to either fulfill or reject, then you can use Promise.allSettled. Note that Internet Explorer does not natively support this method.

const happy = (v, ms) => new Promise((resolve) => setTimeout(() => resolve(v), ms)) const sad = (v, ms) => new Promise((_, reject) => setTimeout(() => reject(v), ms)) Promise.allSettled([happy('happy', 100), sad('sad', 50)]) .then(console.log) // [{ "status":"fulfilled", "value":"happy" }, { "status":"rejected", "reason":"sad" }]

Note: If you use Promise.all actions that managed to finish before rejection happen are not rolled back, so you may need to take care of such situation. For example if you have 5 actions, 4 quick, 1 slow and slow rejects. Those 4 actions may be already executed so you may need to roll back. In such situation consider using Promise.allSettled while it will provide exact detail which action failed and which not.


Clean but be aware the fast-fail behaviour of Promise.all. If any of the functions throws an error, Promise.all will reject
You can handle partial results nicely with async/await, see stackoverflow.com/a/42158854/2019689
Pro tip: use array destructuring in order to initialise an arbitrary number of results from Promise.all(), like: [result1, result2] = Promise.all([async1(), async2()]);
@jonny Is this subject to fail-fast? Also, does one still need to = await Promise.all?
@theUtherSide You're absolutely right - I neglected to include the await.
H
Haven

TL;DR

Use Promise.all for the parallel function calls, the answer behaviors not correctly when the error occurs.

First, execute all the asynchronous calls at once and obtain all the Promise objects. Second, use await on the Promise objects. This way, while you wait for the first Promise to resolve the other asynchronous calls are still progressing. Overall, you will only wait for as long as the slowest asynchronous call. For example:

// Begin first call and store promise without waiting
const someResult = someCall();

// Begin second call and store promise without waiting
const anotherResult = anotherCall();

// Now we await for both results, whose async processes have already been started
const finalResult = [await someResult, await anotherResult];

// At this point all calls have been resolved
// Now when accessing someResult| anotherResult,
// you will have a value instead of a promise

JSbin example: http://jsbin.com/xerifanima/edit?js,console

Caveat: It doesn't matter if the await calls are on the same line or on different lines, so long as the first await call happens after all of the asynchronous calls. See JohnnyHK's comment.

Update: this answer has a different timing in error handling according to the @bergi's answer, it does NOT throw out the error as the error occurs but after all the promises are executed. I compare the result with @jonny's tip: [result1, result2] = Promise.all([async1(), async2()]), check the following code snippet

const correctAsync500ms = () => { return new Promise(resolve => { setTimeout(resolve, 500, 'correct500msResult'); }); }; const correctAsync100ms = () => { return new Promise(resolve => { setTimeout(resolve, 100, 'correct100msResult'); }); }; const rejectAsync100ms = () => { return new Promise((resolve, reject) => { setTimeout(reject, 100, 'reject100msError'); }); }; const asyncInArray = async (fun1, fun2) => { const label = 'test async functions in array'; try { console.time(label); const p1 = fun1(); const p2 = fun2(); const result = [await p1, await p2]; console.timeEnd(label); } catch (e) { console.error('error is', e); console.timeEnd(label); } }; const asyncInPromiseAll = async (fun1, fun2) => { const label = 'test async functions with Promise.all'; try { console.time(label); let [value1, value2] = await Promise.all([fun1(), fun2()]); console.timeEnd(label); } catch (e) { console.error('error is', e); console.timeEnd(label); } }; (async () => { console.group('async functions without error'); console.log('async functions without error: start') await asyncInArray(correctAsync500ms, correctAsync100ms); await asyncInPromiseAll(correctAsync500ms, correctAsync100ms); console.groupEnd(); console.group('async functions with error'); console.log('async functions with error: start') await asyncInArray(correctAsync500ms, rejectAsync100ms); await asyncInPromiseAll(correctAsync500ms, rejectAsync100ms); console.groupEnd(); })();


This looks like a much nicer option to me than Promise.all — and with destructuring assignment you can even do [someResult, anotherResult] = [await someResult, await anotherResult] if you change const to let.
But this still executes the await statements serially, right? That is, execution pauses until the first await resolves, then moves onto the second. Promise.all executes in parallel.
This answer is misleading as the fact that both awaits are done in the same line is irrelevant. What matters is that the two async calls are made before either is awaited.
@Haven this solution is not the same as Promise.all. If each request is a network call, await someResult will need to be resolved before await anotherResult is even started. Conversely, in Promise.all the two await calls can be started before either one is resolved.
The answer is misleading. The jsbin code appears to be executing promises in parallel, but they are not. When you create promise with new operator, the constructor is called in sync. That's the reason we see start call starts & second call starts immediately.
J
Jonathan Potter

Update:

The original answer makes it difficult (and in some cases impossible) to correctly handle promise rejections. The correct solution is to use Promise.all:

const [someResult, anotherResult] = await Promise.all([someCall(), anotherCall()]);

Original answer:

Just make sure you call both functions before you await either one:

// Call both functions
const somePromise = someCall();
const anotherPromise = anotherCall();

// Await both promises    
const someResult = await somePromise;
const anotherResult = await anotherPromise;

I would add a comments to the code because there's nothing saying that the next dev is going to understand what you're doing OOB.
I feel like this is certainly the most pure answer
This answer is much more clear than Haven's. It's clear that the function calls will return promise objects, and await will then resolve them into actual values.
This appears to work at a cursory glance, but has horrible problems with unhandled rejections. Do not use this!
@Bergi You're right, thanks for pointing that out! I've updated the answer with a better solution.
M
Marc_L

There is another way without Promise.all() to do it in parallel:

First, we have 2 functions to print numbers:

function printNumber1() {
   return new Promise((resolve,reject) => {
      setTimeout(() => {
      console.log("Number1 is done");
      resolve(10);
      },1000);
   });
}

function printNumber2() {
   return new Promise((resolve,reject) => {
      setTimeout(() => {
      console.log("Number2 is done");
      resolve(20);
      },500);
   });
}

This is sequential:

async function oneByOne() {
   const number1 = await printNumber1();
   const number2 = await printNumber2();
} 
//Output: Number1 is done, Number2 is done

This is parallel:

async function inParallel() {
   const promise1 = printNumber1();
   const promise2 = printNumber2();
   const number1 = await promise1;
   const number2 = await promise2;
}
//Output: Number2 is done, Number1 is done

This is dangerous, promise2 might reject before promise1 is resolved. If that happens you cannot catch the error from promise1. Either use the sequential pattern in this answer, or use Promise.all([printNumber1(), printNumber2()])
couldn't you handle the error where the async functions are called? To me, this appears to make it easier to add a .catch to each thing individually then the Promise.all answers above
S
SkarXa

I've created a gist testing some different ways of resolving promises, with results. It may be helpful to see the options that work.

Edit: Gist content as per Jin Lee's comment

// Simple gist to test parallel promise resolution when using async / await

function promiseWait(time) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve(true);
    }, time);
});
}


async function test() {
    return [
    await promiseWait(1000),
    await promiseWait(5000),
    await promiseWait(9000),
    await promiseWait(3000),
    ]
}

async function test2() {
    return {
        'aa': await promiseWait(1000),
        'bb': await promiseWait(5000),
        'cc': await promiseWait(9000),
        'dd': await promiseWait(3000),
    }
}

async function test3() {
    return await {
        'aa': promiseWait(1000),
        'bb': promiseWait(5000),
        'cc': promiseWait(9000),
        'dd': promiseWait(3000),
    }
}

async function test4() {
    const p1 =  promiseWait(1000);
    const p2 =  promiseWait(5000);
    const p3 =  promiseWait(9000);
    const p4 =  promiseWait(3000);
    return {
        'aa': await p1,
        'bb': await p2,
        'cc': await p3,
        'dd': await p4,
    };
}

async function test5() {
    return await Promise.all([
                             await promiseWait(1000),
                             await promiseWait(5000),
                             await promiseWait(9000),
                             await promiseWait(3000),
                             ]);
}

async function test6() {
    return await Promise.all([
                             promiseWait(1000),
                             promiseWait(5000),
                             promiseWait(9000),
                             promiseWait(3000),
                             ]);
}

async function test7() {
    const p1 =  promiseWait(1000);
    const p2 =  promiseWait(5000);
    const p3 =  promiseWait(9000);
    return {
        'aa': await p1,
        'bb': await p2,
        'cc': await p3,
        'dd': await promiseWait(3000),
    };
}

let start = Date.now();

test().then((res) => {
    console.log('Test Done, elapsed', (Date.now() - start) / 1000, res);

    start = Date.now();
    test2().then((res) => {
        console.log('Test2 Done, elapsed', (Date.now() - start) / 1000, res);

        start = Date.now();
        test3().then((res) => {
            console.log('Test3 Done, elapsed', (Date.now() - start) / 1000, res);

            start = Date.now();
            test4().then((res) => {
                console.log('Test4 Done, elapsed', (Date.now() - start) / 1000, res);

                start = Date.now();
                test5().then((res) => {
                    console.log('Test5 Done, elapsed', (Date.now() - start) / 1000, res);

                    start = Date.now();
                    test6().then((res) => {
                        console.log('Test6 Done, elapsed', (Date.now() - start) / 1000, res);
                    });

                    start = Date.now();
                    test7().then((res) => {
                        console.log('Test7 Done, elapsed', (Date.now() - start) / 1000, res);
                    });
                });
            });

        });
    });

});
/*
Test Done, elapsed 18.006 [ true, true, true, true ]
Test2 Done, elapsed 18.009 { aa: true, bb: true, cc: true, dd: true }
Test3 Done, elapsed 0 { aa: Promise { <pending> },
  bb: Promise { <pending> },
  cc: Promise { <pending> },
  dd: Promise { <pending> } }
Test4 Done, elapsed 9 { aa: true, bb: true, cc: true, dd: true }
Test5 Done, elapsed 18.008 [ true, true, true, true ]
Test6 Done, elapsed 9.003 [ true, true, true, true ]
Test7 Done, elapsed 12.007 { aa: true, bb: true, cc: true, dd: true }
*/

Tests 4 and 6 in the gist returned the expected results. See stackoverflow.com/a/42158854/5683904 by NoNameProvided who explains the difference between the options.
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review
@JinLee Per your suggestion I've added the content, didn't add it originally because it looked like a lot of content
@SkarXa SO will like your answer even more now. :) And your code is not that long. Don't worry. Thank you!
S
Scott Weaver

In my case, I have several tasks I want to execute in parallel, but I need to do something different with the result of those tasks.

function wait(ms, data) {
    console.log('Starting task:', data, ms);
    return new Promise(resolve => setTimeout(resolve, ms, data));
}

var tasks = [
    async () => {
        var result = await wait(1000, 'moose');
        // do something with result
        console.log(result);
    },
    async () => {
        var result = await wait(500, 'taco');
        // do something with result
        console.log(result);
    },
    async () => {
        var result = await wait(5000, 'burp');
        // do something with result
        console.log(result);
    }
]

await Promise.all(tasks.map(p => p()));
console.log('done');

And the output:

Starting task: moose 1000
Starting task: taco 500
Starting task: burp 5000
taco
moose
burp
done

(async function(){ function wait(ms, data) { console.log('Starting task:', data, ms); return new Promise(resolve => setTimeout(resolve, ms, data)); } var tasks = [ async () => { var result = await wait(1000, 'moose'); // do something with result console.log(result); }, async () => { var result = await wait(500, 'taco'); // do something with result console.log(result); }, async () => { var result = await wait(5000, 'burp'); // do something with result console.log(result); } ] await Promise.all(tasks.map(p => p())); console.log('done'); })();


T
Thiago Conrado

await Promise.all([someCall(), anotherCall()]); as already mention will act as a thread fence (very common in parallel code as CUDA), hence it will allow all the promises in it to run without blocking each other, but will prevent the execution to continue until ALL are resolved.

another approach that is worth to share is the Node.js async that will also allow you to easily control the amount of concurrency that is usually desirable if the task is directly linked to the use of limited resources as API call, I/O operations, etc.

// create a queue object with concurrency 2
var q = async.queue(function(task, callback) {
  console.log('Hello ' + task.name);
  callback();
}, 2);

// assign a callback
q.drain = function() {
  console.log('All items have been processed');
};

// add some items to the queue
q.push({name: 'foo'}, function(err) {
  console.log('Finished processing foo');
});

q.push({name: 'bar'}, function (err) {
  console.log('Finished processing bar');
});

// add some items to the queue (batch-wise)
q.push([{name: 'baz'},{name: 'bay'},{name: 'bax'}], function(err) {
  console.log('Finished processing item');
});

// add some items to the front of the queue
q.unshift({name: 'bar'}, function (err) {
  console.log('Finished processing bar');
});

Credits to the Medium article autor (read more)


bluebirds .map, .filter and .mapSeries also help with concurrency if you don't want to introduce the async modules callback api
T
Thrunobulax
    // A generic test function that can be configured 
    // with an arbitrary delay and to either resolve or reject
    const test = (delay, resolveSuccessfully) => new Promise((resolve, reject) => setTimeout(() => {
        console.log(`Done ${ delay }`);
        resolveSuccessfully ? resolve(`Resolved ${ delay }`) : reject(`Reject ${ delay }`)
    }, delay));

    // Our async handler function
    const handler = async () => {
        // Promise 1 runs first, but resolves last
        const p1 = test(10000, true);
        // Promise 2 run second, and also resolves
        const p2 = test(5000, true);
        // Promise 3 runs last, but completes first (with a rejection) 
        // Note the catch to trap the error immediately
        const p3 = test(1000, false).catch(e => console.log(e));
        // Await all in parallel
        const r = await Promise.all([p1, p2, p3]);
        // Display the results
        console.log(r);
    };

    // Run the handler
    handler();
    /*
    Done 1000
    Reject 1000
    Done 5000
    Done 10000
    */

Whilst setting p1, p2 and p3 is not strictly running them in parallel, they do not hold up any execution and you can trap contextual errors with a catch.


Welcome to Stack Overflow. While your code may provide the answer to the question, please add context around it so others will have some idea what it does and why it is there.
t
thisispraveenk

You can call multiple asynchronous functions without awaiting them. This will execute them in parallel. While doing so, save the returned promises in variables, and await them at some point either individually or using Promise.all() and process the results.

You can also wrap the function calls with try...catch to handle failures of individual asynchronous actions and provide fallback logic.

Here's an example: Observe the logs, the logs printed at the beginning of execution of the individual asynchronous functions get printed immediately even though the first function takes 5 seconds to resolve.

function someLongFunc () { return new Promise((resolve, reject)=> { console.log('Executing function 1') setTimeout(resolve, 5000) }) } function anotherLongFunc () { return new Promise((resolve, reject)=> { console.log('Executing function 2') setTimeout(resolve, 5000) }) } async function main () { let someLongFuncPromise, anotherLongFuncPromise const start = Date.now() try { someLongFuncPromise = someLongFunc() } catch (ex) { console.error('something went wrong during func 1') } try { anotherLongFuncPromise = anotherLongFunc() } catch (ex) { console.error('something went wrong during func 2') } await someLongFuncPromise await anotherLongFuncPromise const totalTime = Date.now() - start console.log('Execution completed in ', totalTime) } main()


F
Fred Yang

I create a helper function waitAll, may be it can make it sweeter. It only works in nodejs for now, not in browser chrome.

    //const parallel = async (...items) => {
    const waitAll = async (...items) => {
        //this function does start execution the functions
        //the execution has been started before running this code here
        //instead it collects of the result of execution of the functions

        const temp = [];
        for (const item of items) {
            //this is not
            //temp.push(await item())
            //it does wait for the result in series (not in parallel), but
            //it doesn't affect the parallel execution of those functions
            //because they haven started earlier
            temp.push(await item);
        }
        return temp;
    };

    //the async functions are executed in parallel before passed
    //in the waitAll function

    //const finalResult = await waitAll(someResult(), anotherResult());
    //const finalResult = await parallel(someResult(), anotherResult());
    //or
    const [result1, result2] = await waitAll(someResult(), anotherResult());
    //const [result1, result2] = await parallel(someResult(), anotherResult());


Nope, parallelization isn't happening at all here. The for loop sequentially awaits each promise and adds the result to the array.
I understand this seems not working for people. So I tested in node.js and browser. The test is passed in node.js (v10, v11), firefox, it it does not work in browser chrome. The test case is in gist.github.com/fredyang/ea736a7b8293edf7a1a25c39c7d2fbbf
I refuse to believe this. There is nothing in the standard that says different iterations of a for loop can be automagically parallelized; this is not how javascript works. The way the loop code is written, it means this: "await one item (the await expr), THEN push result to temp, THEN take next item (next iteration of the for loop). The "awaiting" for each item is completely confined to a single iteration of the loop. If tests show that there is parallelization, it must be because the transpiler is doing something nonstandard or is flat out buggy.
@SzczepanHołyszewski Your confidence of disbieving without running the test case inspire me to do some rename refactory and extra comments. All code are plain old ES6, no transpiling is required.
Not sure why this is downvoted so heavily. It's essentially the same answer that @user2883596 gave.
H
Hoang Le Anh Tu

I vote for:

await Promise.all([someCall(), anotherCall()]);

Be aware of the moment you call functions, it may cause unexpected result:

// Supposing anotherCall() will trigger a request to create a new User

if (callFirst) {
  await someCall();
} else {
  await Promise.all([someCall(), anotherCall()]); // --> create new User here
}

But following always triggers request to create new User

// Supposing anotherCall() will trigger a request to create a new User

const someResult = someCall();
const anotherResult = anotherCall(); // ->> This always creates new User

if (callFirst) {
  await someCall();
} else {
  const finalResult = [await someResult, await anotherResult]
}

Since you declare the function outside/before the condition test, and called them. Try wrapping them in else block.
@Haven: I mean when you separate the moments you call functions vs await can lead to unexpected results, for example: async HTTP requests.