I'm trying to write a lambda function which accepts an image file via web form, and writes it as a new commit to a repository using code commit. For some reason, my lambda function seems to be exiting before the call to createCommit, even though I'm using await in a similar way to my previous calls in the function.
I've tried rewriting the function that wraps createCommit to just use promises, but that doesn't seem to work either. I'm wondering if there is some quirk of lambda that I don't know about or if I'm using async/await incorrectly (I just recently learned how to use them)
this is my main lambda event handler:
exports.handler = async (event) => {
const [file, lastCommitId] = await Promise.all([getFile(event), getLastCommitId()]);
return await createCommit(file, lastCommitId)
.then(result => returnResponse(result, 200))
.catch(err => returnResponse(err, 500));
};
this is my wrapper function for createCommit
async function createCommit(file, parentCommitId) {
const fileContent = await file.content.toString('base64');
const params = {
"authorName": "admin",
branchName,
"commitMessage": "add image",
"email": "n/a",
"parentCommitId": parentCommitId,
"putFiles": [
{
fileContent,
"fileMode": "NORMAL",
"filePath": `src/images/${file.filename}`
}
],
repositoryName
};
console.log("creating commit against last commit id " + parentCommitId);
const result = await codecommit.createCommit(params).promise();
console.log(JSON.stringify(result));
return result;
}
I expect the lambda function to wait until the call to createCommit finished, but it simply prints out the console.log starting with "creating commit against last commit..." and exits.
You should not use await and .then together.
change your code to and trycatch if you want to catch the exception or failed case.
exports.handler = async (event) => {
const [file, lastCommitId] = await Promise.all([getFile(event), getLastCommitId()]);
return await createCommit(file, lastCommitId);
};
See below example to understand the result in a better way.
function resolveAfter2Seconds() {
return new Promise(resolve => {
setTimeout(() => {
resolve('resolved');
}, 2000);
});
}
async function asyncCall() {
console.log('calling');
var result = await resolveAfter2Seconds().then(x=>console.log('inside then ', x));
console.log('after await ',result);
}
asyncCall();
and with out then
function resolveAfter2Seconds() {
return new Promise(resolve => {
setTimeout(() => {
resolve('resolved');
}, 2000);
});
}
async function asyncCall() {
console.log('calling');
var result = await resolveAfter2Seconds();
console.log('after await ',result);
}
asyncCall();
So it turns out I was using async/await correctly, I just had a 3 second timeout on the lambda function, so it was exiting before it was able to get a response from the createCommit call.
I'm creating an API using Express JS (Express 4) framework. I'm pretty new to NodeJS so I would like to know the best way to perform multiple async calls inside the same function.
For this example, I've got a function called Login in my Login Controller. Inside this function, let's say I gotta make quite a few async calls for authenticating a user, saving login info and similar functionalities like that.
But for some async calls, data to be processed should be fetched from the previous async call (they are dependent functions) and some functions are not like that.
Right now, this is how I'm doing the calls.
exports.login = function (req, res) {
var user = {
email: 'fakeemail#id.com',
password: 'password'
};
//Async call 1
Login.authUser(user, function (err1, rows1) {
var user_id = rows1[0].id;
//Async call 2 (depends on async call 1 for user_id)
Login.fetchUserDetails(user_id, function (err2, rows2) {
//Async call 3
Login.updateLoginInfo(param3, function (err3, rows3) {
//Some functionality occurs here then async call 4 happens
//Async call 4
Login.someOtherFunctionality(param4, function (err4, rows4) {
//return response to user
res.json({
success: true
});
});
});
});
});
};
Now all these async calls are nested. Is there any other way I can do this?
P.S: I didnt add error handling in this example
you can use promise as well. It will make you syntax more pretty. Your code will look like
Login.authUser(user).
then(fetchUser).
then(updateLoginInfo).
then(someOtherFunctionality).
catch(function(error){
//log your error or whatever
});
You can use Promise as suggested by Shahzeb.
Promises are objects that get resolved in future (asynchronous). Once a Promise is completed it either resolves or rejects. All promises resolved are then ed and those get rejected are catch ed
pseudo code
let myPromise = function() {
return new Promise(function(resolve, reject) {
resolve('foo');
});
};
myPromise().then( (v) => {
console.log(v);
})
Use Promise Chaining
new Promise(function(resolve, reject) {
setTimeout(() => resolve(1), 1000); // (*)
}).then(function(result) { // (**)
alert(result); // 1
return result * 2;
}).then(function(result) { // (***)
alert(result); // 2
return result * 2;
}).then(function(result) {
alert(result); // 4
return result * 2;
});
More details refer Promise Chaining and chain promises in javascript
Using async/await (Requires Node.js v7.6)
This strategy also uses promises. In my opinion it improves readability since you are refactoring out each individual promise call into separate methods.
Codepen
// mock async calls
const authUser = user => Promise.resolve([{ id: 1 }]); // returns users id
const fetchUserDetails = user_id => Promise.resolve({ name: 'Fred', age: '10000' }); // returns users details
const updateLoginInfo = param3 => Promise.resolve({ status: 'success' }); // returns success?
const someOtherFunctionality = param3 => Promise.resolve({ field: 'value' }); // returns something
// all async functions return a promise
const login = async (/*req, res*/) => {
// User object
const user = {
email: 'fakeemail#id.com',
password: 'password'
};
// Async call 1
console.log(`Authorizing user...`);
const rows1 = await authUser(user);
const user_id = rows1[0].id;
console.log(`User ${user_id} authorized.`);
// Async call 2 (depends on async call 1 for user_id)
console.log(`Fetching user detail...`);
const rows2 = await fetchUserDetails(user_id);
console.log(`User Detail was fetched: ${JSON.stringify(rows2)}`);
// Async call 3
console.log(`Updating login info...`);
const param3 = `something`;
const rows3 = await updateLoginInfo(param3);
console.log(`Login info was successful: ${JSON.stringify(rows3)}`);
// Some functionality occurs here then async call 4 happens
console.log(`\nDoing stuff after async call 3, but before async call 4....\n`);
// Async call 4
console.log(`Async call 4...`);
const param4 = `something`;
const rows4 = await someOtherFunctionality(param4);
console.log(`END OF LOGIN FUNCTION`);
return 'returned value';
}
// run the async function
login()
.then(result => {
// respond
// res.json({ success: true });
console.log(`Promise value: ${result}`);
console.log(`Response: { success: true }`);
})
.catch(err => {
console.log(err);
})
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]
}