并行调用 async/await 函数

Question

据我了解，在 ES7/ES2016 中，将多个 await 放入代码中的工作方式类似于将 .then() 与 Promise 链接在一起，这意味着它们将一个接一个地执行，而不是并行执行。因此，例如，我们有以下代码：

await someCall();
await anotherCall();

我是否正确理解 anotherCall() 只有在 someCall() 完成时才会被调用？并行调用它们的最优雅方式是什么？

我想在Node中使用它，所以也许有异步库的解决方案？

编辑：我对这个问题中提供的解决方案不满意：Slowdown due to non-parallel awaiting of promises in async generators，因为它使用了生成器，我问的是更一般的用例。

Answer 1

您可以调用多个异步函数而无需等待它们。这将并行执行它们。这样做时，将返回的 Promise 保存在变量中，并在某个时候单独等待它们或使用 Promise.all() 并处理结果。

您还可以使用 try...catch 包装函数调用，以处理单个异步操作的失败并提供回退逻辑。

这是一个例子： 观察日志，在各个异步函数执行开始时打印的日志会立即打印，即使第一个函数需要 5 秒才能解决。

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()

Answer 2

2021年需要前端新星方式的读者：

以下是有史以来最漂亮的前端 API 网关代码，它并行调用三个不同的服务，然后使用其中一个结果循环调用另一个服务，ProblemService。

请注意，我使用 await、async 和 Promise.all 的方式胜过整个 StackOverflow。

class ExamScoreboardService {
    getExamScoreboard(examId) {
        return Promise.all([
            examService.getExaminees(examId),
            examService.getExamOverview(examId),
            examService.getExamScores(examId),
        ])
            .then(async ([examinees, exam, examScores]) => {
                const out = {}
                await Promise.all(exam.questions.map(async q =>
                    problemService.getProblemById(q.problemId).then(problem => out[q.problemId] = problem.testcases.length)))
                return [examinees, exam, examScores, out]
            })
            .then(values => {
                const [examinees, exam, examScores, totalTestcasesOf] = values;
                return new ExamScoreboard({examinees, exam, examScores, totalTestcasesOf});
            })
    }
}

Answer 3

我创建了a gist 测试了一些解决承诺的不同方法，并得到了结果。查看有效的选项可能会有所帮助。

编辑：根据Jin Lee的评论提供内容

// 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 }
*/

Answer 4

您可以在Promise.all() 上等待：

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

存储结果：

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

请注意，Promise.all 快速失败，这意味着一旦提供给它的承诺之一被拒绝，那么整个事情都会被拒绝。

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'

相反，如果您想等待所有承诺履行或拒绝，那么您可以使用Promise.allSettled。请注意，Internet Explorer 本身并不支持此方法。

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" }]

注意：如果您使用Promise.all 在拒绝发生之前成功完成的操作不会回滚，因此您可能需要处理这种情况。例如 如果您有 5 个动作，4 个快速，1 个慢速和慢速拒绝。那4个 操作可能已经执行，因此您可能需要回滚。在这种情况下，请考虑使用Promise.allSettled，它会提供哪些操作失败和哪些操作失败的确切细节。

Answer 5

await Promise.all([someCall(), anotherCall()]);正如已经提到的，它将充当线程围栏（在并行代码中非常常见，如 CUDA），因此它将允许其中的所有 Promise 运行而不会相互阻塞，但会阻止执行继续，直到 ALL 解决。

另一个值得分享的方法是 Node.js 异步，如果任务直接链接到使用有限资源作为 API 调用，它还允许您轻松控制通常需要的并发量，I/ O操作等

// 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');
});

感谢 Medium 文章作者 (read more)

Answer 6

就我而言，我有几个要并行执行的任务，但我需要对这些任务的结果做一些不同的事情。

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');

然后输出：

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

Answer 7

    // 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
    */

虽然设置 p1、p2 和 p3 并不是严格地并行运行它们，但它们不会阻止任何执行，您可以使用 catch 捕获上下文错误。

Answer 8

还有另一种没有 Promise.all() 的方法可以并行执行：

首先，我们有两个打印数字的函数：

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);
   });
}

这是顺序的：

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

这是并行的：

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

Answer 9

我创建了一个辅助函数waitAll，也许它可以让它更甜。 它目前仅在 nodejs 中有效，在浏览器 chrome 中不。

    //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());

Answer 10

更新：

原始答案使正确处理承诺拒绝变得困难（并且在某些情况下是不可能的）。正确的解决方法是使用Promise.all:

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

原答案：

请确保在等待任一函数之前调用这两个函数：

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

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

Answer 11

TL;DR

使用Promise.all进行并行函数调用，错误发生时应答行为不正确。

---

首先，一次性执行所有个异步调用，获取所有Promise对象。其次，在Promise 对象上使用await。这样，当您等待第一个 Promise 解决其他异步调用时仍在进行中。总的来说，您只会等待最慢的异步调用。例如：

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

警告：await 调用是在同一行还是在不同行都没有关系，只要第一个 await 调用发生在之后 所有的异步调用。请参阅 JohnnyHK 的评论。

---

更新：根据@bergi's answer，此答案在错误处理中的时间不同，它确实不在错误发生时抛出错误，但毕竟承诺被执行。 我将结果与@jonny 的提示进行比较：[result1, result2] = Promise.all([async1(), async2()])，检查以下代码 sn-p

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();
})();

Answer 12

我投票给：

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

注意你调用函数的那一刻，它可能会导致意想不到的结果：

// 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
}

但是关注总是触发创建新用户的请求

// 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]
}