Related
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.
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.
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();
})();
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;
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
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 }
*/
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');
})();
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)
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()
// 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.
This can be accomplished with Promise.allSettled(), which is similar to Promise.all() but without the fail-fast behavior.
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!"}
Note: This is a bleeding edge feature with limited browser support, so I strongly recommend including a polyfill for this function.
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());
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]
}
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.
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.
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();
})();
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;
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
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 }
*/
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');
})();
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)
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()
// 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.
This can be accomplished with Promise.allSettled(), which is similar to Promise.all() but without the fail-fast behavior.
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!"}
Note: This is a bleeding edge feature with limited browser support, so I strongly recommend including a polyfill for this function.
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());
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]
}
I have several db mutations that I would like to execute all at once, instead of synchronously. The problem that I'm running into, is that when I try to push these promises into an array, they execute.
What am I doing wrong here? I've also tried pushing anonymous functions, like this,
promises.push(
async () => await someDbMutation1({ someForeignKey: "10" }),
)
but they aren't execute during Promise.all.
import * as React from "react";
import "./styles.css";
const someDbMutation1 = async ({ someForeignKey }) => {
return await new Promise((resolve) => {
console.log("should not enter");
return setTimeout(() => {
resolve("aa");
}, 2000);
});
};
const someDbMutation2 = async ({ someParameter }) =>
await new Promise((resolve) =>
setTimeout(() => {
resolve();
}, 2000)
);
export default function App() {
const [loaded, setLoaded] = React.useState(false);
React.useEffect(() => {
init();
}, []);
const init = React.useCallback(async () => {
const promises = [
someDbMutation1({ someForeignKey: "10" }),
someDbMutation2({ someParameter: "abc" })
];
// await Promise.all(promises);
setLoaded(true);
}, []);
return <div className="App">{loaded && <div>done</div>}</div>;
}
I would expect these promises in the promises array to be executed during a call to Promise.all, but clearly that's not the case here. I've noticed this only recently, when I passed null as a value to a foreign key, at which point the key constraint in my db picked it up and threw an error.
Now I'm worried, because I frequently use a promises array and loop over db objects and push mutation queries into promises -- this means, that each request is executed twice! I'm not sure what I'm missing here.
For the first part of your question where you say that it's not executing:
promises.push(
async () => await someDbMutation1({ someForeignKey: "10" }),
)
it's because you are pushing an anonymous function not a promise - They are two different things. Based on your array name, I think expected behavior would be for you to do this instead:
promises.push(
someDbMutation1({ someForeignKey: "10" })
)
If you want all promises to be executed at a single point in time then you could do this instead:
queries.push(
async () => await someDbMutation1({ someForeignKey: "10" }),
)
/ ** -- some point later -- ** /
const promises = queries.map(q => q()) // Execute queries
const results = await Promise.all(promises) // Wait for queries to finish
In addition, you have a misunderstanding on how Promise.all works here:
I would expect these promises in the promises array to be executed during a call to Promise.all
Promise.all doesn't execute the promises, it waits for the promises to resolve. There is a reference here.
So in this part:
const promises = [
someDbMutation1({ someForeignKey: "10" }),
someDbMutation2({ someParameter: "abc" })
];
You are actually executing the functions so that if you were to console.log the promises array it would look something like this:
[
Promise (unresolved),
Promise (unresolved)
];
And then after await Promise.all(), the promises array would look like this:
[
Promise (resolved: value),
Promise (resolved: value)
];
Issue 1: Promises must be awaited in the block that actually awaits for them.
const someDbMutation1 = async ({ someForeignKey }) => {
return new Promise((resolve) => {
console.log("should not enter");
return setTimeout(() => {
resolve("aa");
}, 2000);
});
};
const someDbMutation2 = async ({ someParameter }) =>
await new Promise((resolve) =>
setTimeout(() => {
resolve();
}, 2000)
);
The problem is you are executing the promises. You should instead add them into an array as anonymous functions that call your function with the parameters you want. So this should look like this. :
const init = React.useCallback(async () => {
const promises = [
async () => someDbMutation1({ someForeignKey: "10" }),
async () => someDbMutation2({ someParameter: "abc" })
];
await Promise.all(promises);
setLoaded(true);
}, []);
I hope this is the answer you are looking for.
I have an API that returns some values and depend on that values I want to create an array and pass that array in resolve. It works when set a timeout for resolve, for example, I set 5000 sec within 5 sec it fetch the data, push array and resolve. But I don't want to set a timeout. Please see the comments for bertter understand. its an nodejs function.
const calculateTotal = (arr) => {
let clientHotspotsTotal = [];
return new Promise((resolve, reject) => {
arr.forEach((data) => {
axios.get(`enpoint/data?.address`)
.then((response) => {
if (response?.data) {
console.log('API SUCCESS');
const val = (response?.data?.data?.total * data?.percentage) / 100;
clientHotspotsTotal.push({
total: val,
});
} else {
reject('API failed!');
}
})
.catch((error) => {
throw new Error(error);
});
});
resolve(clientHotspotsTotal); /// it return empty [], cause no time set for wait
setTimeout(() => {
resolve(clientHotspotsTotal) // it doesn't return empty because in that time api fetch data
an push into array
}, 5000)
});
};
So my question is how can I wait for that API response for multiple requests as I run a for loop for multiple users.
You can simplify this a lot by using async / await and Promise.all. We can create an array of promises from the axios calls, then get the response data using Promise.all.
We then iterate through the responses, returning the result.
const calculateTotal = async (arr) => {
const responses = await Promise.all(arr.map(data => axios.get(`enpoint/${data?.address}`)));
const clientHotspotsTotal = responses.map((response, i) => {
return({ total: (response?.data?.data?.total * arr[i]?.percentage) / 100});
});
return clientHotspotsTotal;
};
async function testIt() {
let result = await calculateTotal([
{ address: 'someaddress', percentage: 0.50 },
{ address: 'someaddress', percentage: 0.75 }
]);
console.log(result);
}
testIt()
I'm using browser's native fetch API for network requests. Also I am using the whatwg-fetch polyfill for unsupported browsers.
However I need to retry in case the request fails. Now there is this npm package whatwg-fetch-retry I found, but they haven't explained how to use it in their docs. Can somebody help me with this or suggest me an alternative?
From the fetch docs :
fetch('/users')
.then(checkStatus)
.then(parseJSON)
.then(function(data) {
console.log('succeeded', data)
}).catch(function(error) {
console.log('request failed', error)
})
See that catch? Will trigger when fetch fails, you can fetch again there.
Have a look at the Promise API.
Implementation example:
function wait(delay){
return new Promise((resolve) => setTimeout(resolve, delay));
}
function fetchRetry(url, delay, tries, fetchOptions = {}) {
function onError(err){
triesLeft = tries - 1;
if(!triesLeft){
throw err;
}
return wait(delay).then(() => fetchRetry(url, delay, triesLeft, fetchOptions));
}
return fetch(url,fetchOptions).catch(onError);
}
Edit 1: as suggested by golopot, p-retry is a nice option.
Edit 2: simplified example code.
I recommend using some library for promise retry, for example p-retry.
Example:
const pRetry = require('p-retry')
const fetch = require('node-fetch')
async function fetchPage () {
const response = await fetch('https://stackoverflow.com')
// Abort retrying if the resource doesn't exist
if (response.status === 404) {
throw new pRetry.AbortError(response.statusText)
}
return response.blob()
}
;(async () => {
console.log(await pRetry(fetchPage, {retries: 5}))
})()
I don't like recursion unless is really necessary. And managing an exploding number of dependencies is also an issue. Here is another alternative in typescript. Which is easy to translate to javascript.
interface retryPromiseOptions<T> {
retryCatchIf?:(response:T) => boolean,
retryIf?:(response:T) => boolean,
retries?:number
}
function retryPromise<T>(promise:() => Promise<T>, options:retryPromiseOptions<T>) {
const { retryIf = (_:T) => false, retryCatchIf= (_:T) => true, retries = 1} = options
let _promise = promise();
for (var i = 1; i < retries; i++)
_promise = _promise.catch((value) => retryCatchIf(value) ? promise() : Promise.reject(value))
.then((value) => retryIf(value) ? promise() : Promise.reject(value));
return _promise;
}
And use it this way...
retryPromise(() => fetch(url),{
retryIf: (response:Response) => true, // you could check before trying again
retries: 5
}).then( ... my favorite things ... )
I wrote this for the fetch API on the browser. Which does not issue a reject on a 500. And did I did not implement a wait. But, more importantly, the code shows how to use composition with promises to avoid recursion.
Javascript version:
function retryPromise(promise, options) {
const { retryIf, retryCatchIf, retries } = { retryIf: () => false, retryCatchIf: () => true, retries: 1, ...options};
let _promise = promise();
for (var i = 1; i < retries; i++)
_promise = _promise.catch((value) => retryCatchIf(value) ? promise() : Promise.reject(value))
.then((value) => retryIf(value) ? promise() : Promise.reject(value));
return _promise;
}
Javascript usage:
retryPromise(() => fetch(url),{
retryIf: (response) => true, // you could check before trying again
retries: 5
}).then( ... my favorite things ... )
EDITS: Added js version, added retryCatchIf, fixed the loop start.
One can easily wrap fetch(...) in a loop and catch potential errors (fetch only rejects the returning promise on network errors and the alike):
const RETRY_COUNT = 5;
async function fetchRetry(...args) {
let count = RETRY_COUNT;
while(count > 0) {
try {
return await fetch(...args);
} catch(error) {
// logging ?
}
// logging / waiting?
count -= 1;
}
throw new Error(`Too many retries`);
}