[英]Call async/await functions in parallel
據我了解,在 ES7/ES2016 中,在代碼中放置多個await
的工作方式類似於將.then()
與 Promise 鏈接起來,這意味着它們將一個接一個地執行而不是並行執行。 因此,例如,我們有以下代碼:
await someCall();
await anotherCall();
我是否正確理解anotherCall()
只有在someCall()
完成時才會被調用? 並行調用它們的最優雅方式是什么?
我想在 Node 中使用它,所以也許有異步庫的解決方案?
編輯:我對這個問題中提供的解決方案不滿意: Slowdown due to non-parallel waiting of promises in async generators ,因為它使用了生成器,我在問一個更一般的用例。
你可以在Promise.all()
上等待:
await Promise.all([someCall(), anotherCall()]);
存儲結果:
let [someResult, anotherResult] = await Promise.all([someCall(), anotherCall()]);
請注意, Promise.all
失敗很快,這意味着一旦提供給它的承諾之一被拒絕,整個事情就會被拒絕。
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諾履行或拒絕,那么您可以使用Promise.allSettled
。 請注意,Internet Explorer 本身不支持此方法。
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
在拒絕發生之前設法完成的操作不會回滾,因此您可能需要注意這種情況。 例如,如果您有 5 個操作,4 個快速、1 個慢速和慢速拒絕。 這 4 個操作可能已經執行,因此您可能需要回滾。 在這種情況下,請考慮使用Promise.allSettled
同時它會提供哪些操作失敗和哪些操作失敗的確切細節。
TL; 博士
使用Promise.all
進行並行函數調用,發生錯誤時應答行為不正確。
首先,一次性執行所有的異步調用,獲取所有的Promise
對象。 其次,在Promise
對象上使用await
。 這樣,當您等待第一個Promise
解決其他異步調用時,其他異步調用仍在進行中。 總的來說,您只會等待最慢的異步調用。 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
調用是在同一行還是在不同的行上都沒有關系,只要第一個await
調用發生在所有異步調用之后即可。 請參閱 JohnnyHK 的評論。
更新:這個回答有錯誤不同時間處理根據@ BERGI的答案,如發生錯誤時不會拋出了錯誤,但所有的承諾都執行之后。 我將結果與[result1, result2] = Promise.all([async1(), async2()])
的提示進行比較: [result1, result2] = Promise.all([async1(), async2()])
,檢查以下代碼片段
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(); })();
更新:
原始答案使得正確處理承諾拒絕變得困難(在某些情況下是不可能的)。 正確的解決方案是使用Promise.all
:
const [someResult, anotherResult] = await Promise.all([someCall(), anotherCall()]);
原答案:
只需確保在等待任一函數之前調用這兩個函數:
// Call both functions
const somePromise = someCall();
const anotherPromise = anotherCall();
// Await both promises
const someResult = await somePromise;
const anotherResult = await anotherPromise;
還有另一種沒有 Promise.all() 的方法可以並行執行:
首先,我們有兩個函數來打印數字:
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);
});
}
這是順序的:
async function oneByOne() {
const number1 = await printNumber1();
const number2 = await printNumber2();
}
//Output: Number1 is done, Number2 is done
這是平行的:
async function inParallel() {
const promise1 = printNumber1();
const promise2 = printNumber2();
const number1 = await promise1;
const number2 = await promise2;
}
//Output: Number2 is done, Number1 is done
我創建了一個 gist,測試了一些解決承諾的不同方法,並給出了結果。 查看有效的選項可能會有所幫助。
編輯:根據Jin Lee的評論要點內容
// 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 }
*/
就我而言,我有幾個要並行執行的任務,但我需要對這些任務的結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');
和輸出:
Starting task: moose 1000
Starting task: taco 500
Starting task: burp 5000
taco
moose
burp
done
function wait(ms, data) { console.log('Starting task:', data, ms); return new Promise( resolve => setTimeout(resolve, ms, data) ); } (async function parallel() { // step 1 - initiate all promises console.log('STARTING') let task1 = wait(2000, 'parallelTask1') // PS: see Exception handling below let task2 = wait(500, 'parallelTask2') let task3 = wait(1000, 'parallelTask3') // step 2 - await all promises console.log('WAITING') task1 = await task1 task2 = await task2 task3 = await task3 // step 3 - all results are 100% ready console.log('FINISHED') console.log('Result:', task1, task2, task3) })()
await
調用完成了代碼執行。 [task1, task2, task3] = [await task1, await task2, await task3]
let five = getAsyncFive() let ten = getAsyncTen() let result = await five * await ten
*請注意,它與let result = await getAsyncFive() * await getAsyncTen()
因為這不會並行運行異步任務!
在下面的代碼段中, .catch(e => e)
捕獲錯誤並允許鏈繼續進行,從而使promise得以解決而不是拒絕 。 如果沒有catch
,則代碼將引發未處理的異常,並且該函數將提前退出。
const wait = (ms, data) => log(ms,data) || new Promise( resolve => setTimeout(resolve, ms, data) ) const reject = (ms, data) => log(ms,data) || new Promise( (r, reject) => setTimeout(reject, ms, data) ) const e = e => 'err-' + e const l = l => (console.log('Done:', l), l) const log = (ms, data) => console.log('Started', data, ms) ;(async function parallel() { let task1 = reject(500, 'parallelTask1').catch(e).then(l) let task2 = wait(2000, 'parallelTask2').catch(e).then(l) let task3 = reject(1000, 'parallelTask3').catch(e).then(l) console.log('WAITING') task1 = await task1 task2 = await task2 task3 = await task3 console.log('FINISHED', task1, task2, task3) })()
第二個代碼段未處理,功能將失敗。
您也可以打開Devtools並在控制台輸出中查看錯誤。
const wait = (ms, data) => log(ms,data) || new Promise( resolve => setTimeout(resolve, ms, data) ) const reject = (ms, data) => log(ms,data) || new Promise( (r, reject) => setTimeout(reject, ms, data) ) const e = e => 'err-' + e const l = l => (console.log('Done:', l), l) const log = (ms, data) => console.log('Started', data, ms) console.log('here1') ;(async function parallel() { let task1 = reject(500, 'parallelTask1').then(l) // catch is removed let task2 = wait(2000, 'parallelTask2').then(l) let task3 = reject(1000, 'parallelTask3').then(l) console.log('WAITING') task1 = await task1 task2 = await task2 task3 = await task3 console.log('FINISHED', task1, task2, task3) })() console.log('here2') // Note: "FINISHED" will not run
等待 Promise.all([someCall(), anotherCall()]); 如前所述,它將充當線程柵欄(在作為 CUDA 的並行代碼中非常常見),因此它將允許其中的所有 promise 運行而不會相互阻塞,但會阻止執行繼續,直到解決所有問題。
另一種值得分享的方法是 Node.js 異步,如果任務直接鏈接到有限資源的使用,如 API 調用、I/O 操作,它也將允許您輕松控制通常需要的並發量,等等。
// 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');
});
歸功於 Medium 文章作者( 閱讀更多)
// 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
*/
雖然設置 p1、p2 和 p3 並不是嚴格並行運行它們,但它們不會阻止任何執行,您可以使用 catch 捕獲上下文錯誤。
您可以調用多個異步函數而無需等待它們。 這將並行執行它們。 這樣做時,將返回的承諾保存在變量中,並在某個時候單獨或使用 Promise.all() 等待它們並處理結果。
您還可以使用 try...catch 包裝函數調用以處理單個異步操作的失敗並提供回退邏輯。
這是一個例子:觀察日志,即使第一個函數需要 5 秒來解析,在單個異步函數執行開始時打印的日志也會立即打印。
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()
這可以通過Promise.allSettled()來實現,它類似於Promise.all()
但沒有快速失敗行為。
async function failure() {
throw "Failure!";
}
async function success() {
return "Success!";
}
const [failureResult, successResult] = await Promise.allSettled([failure(), success()]);
console.log(failureResult); // {status: "rejected", reason: "Failure!"}
console.log(successResult); // {status: "fulfilled", value: "Success!"}
注意 :這是一項最新功能,瀏覽器支持有限,因此,我強烈建議為此功能包括一個polyfill。
這可以通過Promise.allSettled()來完成,它類似於Promise.all()但沒有快速故障行為。
async function Promise1() {
throw "Failure!";
}
async function Promise2() {
return "Success!";
}
const [Promise1Result, Promise2Result] = await Promise.allSettled([Promise1(), Promise2()]);
console.log(Promise1Result); // {status: "rejected", reason: "Failure!"}
console.log(Promise2Result); // {status: "fulfilled", value: "Success!"}
注意:這是一個有限瀏覽器支持的前沿特性,所以我強烈建議為這個 function 包含一個 polyfill。
以下是有史以來最漂亮的前端 API 網關代碼,它並行調用三個不同的服務,然后使用結果之一循環調用另一個服務ProblemService
。
請注意,我如何使用await
、 async
和Promise.all
擊敗了整個 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});
})
}
}
我創建了一個輔助函數waitAll,也許它可以讓它更甜。 它目前僅適用於nodejs ,不適用於瀏覽器 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());
我投票贊成:
await Promise.all([someCall(), anotherCall()]);
注意調用函數的那一刻,可能會導致意想不到的結果:
// 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
}
但是以下總是觸發創建新用戶的請求
// 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]
}
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