This is more of a conceptual question. I understand the Promise design pattern, but couldn't find a reliable source to answer my question about promise.all():
What is(are) the correct scenario(s) to use promise.all()
OR
Are there any best practices to use promise.all()? Should it be ideally used only if all of the promise objects are of the same or similar types?
The only one I could think of is:
Use promise.all() if you want to resolve the promise only if all of the promise objects resolve and reject if even one rejects.
I'm not sure anyone has really given the most general purpose explanation for when to use Promise.all() (and when not to use it):
What is(are) the correct scenario(s) to use promise.all()
Promise.all() is useful anytime you have more than one promise and your code wants to know when all the operations that those promises represent have finished successfully. It does not matter what the individual async operations are. If they are async, are represented by promises and your code wants to know when they have all completed successfully, then Promise.all() is built to do exactly that.
For example, suppose you need to gather information from three separate remote API calls and when you have the results from all three API calls, you then need to run some further code using all three results. That situation would be perfect for Promise.all(). You could so something like this:
Promise.all([apiRequest(...), apiRequest(...), apiRequest(...)]).then(function(results) {
// API results in the results array here
// processing can continue using the results of all three API requests
}, function(err) {
// an error occurred, process the error here
});
Promise.all() is probably most commonly used with similar types of requests (as in the above example), but there is no reason that it needs to be. If you had a different case where you needed to make a remote API request, read a local file and read a local temperature probe and then when you had data from all three async operations, you wanted to then do some processing with the data from all three, you would again use Promise.all():
Promise.all([apiRequest(...), fs.promises.readFile(...), readTemperature(...)]).then(function(results) {
// all results in the results array here
// processing can continue using the results of all three async operations
}, function(err) {
// an error occurred, process the error here
});
On the flip side, if you don't need to coordinate among them and can just handle each async operation individually, then you don't need Promise.all(). You can just fire each of your separate async operations with their own .then() handlers and no coordination between them is needed.
In addition Promise.all() has what is called a "fast fail" implementation. It returns a master promise that will reject as soon as the first promise you passed it rejects or it will resolve when all the promises have resolved. So, to use Promise.all() that type of implementation needs to work for your situation. There are other situations where you want to run multiple async operations and you need all the results, even if some of them failed. Promise.all() will not do that for you directly. Instead, you would likely use something like Promise.settle() for that situation. You can see an implementation of .settle() here which gives you access to all the results, even if some failed. This is particularly useful when you expect that some operations might fail and you have a useful task to pursue with the results from whatever operations succeeded or you want to examine the failure reasons for all the operations that failed to make decisions based on that.
Are there any best practices to use promise.all()? Should it be
ideally used only if all of the promise objects are of the same or
similar types?
As explained above, it does not matter what the individual async operations are or if they are the same type. It only matters whether your code needs to coordinate them and know when they all succeed.
It's also useful to list some situations when you would not use Promise.all():
When you only have one async operation. With only one operation, you can just use a .then() handler on the one promise and there is no reason for Promise.all().
When you don't need to coordinate among multiple async operations.
When a fast fail implementation is not appropriate. If you need all results, even if some fail, then Promise.all() will not do that by itself. You will probably want something like Promise.allSettled() instead.
If your async operations do not all return promises, Promise.all() cannot track an async operation that is not managed through a promise.
Promise.all is for waiting for several Promises to resolve in parallel (at the same time). It returns a Promise that resolves when all of the input Promises have resolved:
// p1, p2, p3 are Promises
Promise.all([p1, p2, p3])
.then(([p1Result, p2Result, p3Result]) => {
// This function is called when p1, p2 and p3 have all resolved.
// The arguments are the resolved values.
})
If any of the input Promises is rejected, the Promise returned by Promise.all is also rejected.
A common scenario is waiting for several API requests to finish so you can combine their results:
const contentPromise = requestUser();
const commentsPromise = requestComments();
const combinedContent = Promise.all([contentPromise, commentsPromise])
.then(([content, comments]) => {
// content and comments have both finished loading.
})
You can use Promise.all with Promise instance.
It's hard to answer these questions as they are the type that tend to answer themselves as one uses the available APIs of a language feature. Basically, it's fine to use Promises any way that suits your use case, so long as you avoid their anti-patterns.
What is(are) the correct scenario(s) to use promise.all()
Any situation in which an operation depends on the successful resolution of multiple promises.
Are there any best practices to use promise.all()? Should it be ideally used only if all of the promise objects are of the same or similar types?
Generally, no and no.
I use promise.all() when I have to do some requests to my API and I don't want to display something before the application loads all the data requested, so I delay the execution flow until I have all the data I need.
Example:
What I want to do I want to load the users of my app and their products (imagine that you have to do multiple requests) before displaying a table in my app with the user emails and the product names of each user.
What I do next I send the requests to my API creating the promises and using promise.all()
What I do when all the data has been loaded Once the data arrives to my app, I can execute the callback of promises.all() and then make visible the table with the users.
I hope it helps you to see in which scenario makes sense to use promises.all()
As #joews mentioned, probably one of the important features of Promise.all that should be explicitly indicated is that it makes your async code much faster.
This makes it ideal in any code that contains independent calls (that we want to return/finish before the rest of the code continues), but especially when we make frontend calls and want the user's experience to be as smooth as possible.
async function waitSecond() {
return new Promise((res, rej) => {
setTimeout(res, 1000);
});
}
function runSeries() {
console.time('series');
waitSecond().then(() => {
waitSecond().then(() => {
waitSecond().then(() => {
console.timeEnd('series');
});
});
});
}
function runParallel() {
console.time('parallel');
Promise.all([
waitSecond(),
waitSecond(),
waitSecond(),
]).then(() => {
console.timeEnd('parallel');
});
}
runSeries();
runParallel();
I tend to use promise all for something like this:
myService.getUsers()
.then(users => {
this.users = users;
var profileRequests = users.map(user => {
return myService.getProfile(user.Id); // returns a promise
});
return Promise.all(profileRequests);
})
.then(userProfilesRequest => {
// do something here with all the user profiles, like assign them back to the users.
this.users.forEach((user, index) => {
user.profile = userProfilesRequest[index];
});
});
Here, for each user we're going off and getting their profile. I don't want my promise chain to get out of hand now that i have x amount of promises to resolve.
So Promise.all() will basically aggregate all my promises back into one, and I can manage that through the next then. I can keep doing this for as long as a like, say for each profile I want to get related settings etc. etc. Each time I create tonnes more promises, I can aggregate them all back into one.
Promise.all-This method is useful for when you want to wait for more than one promise to complete or The Promise.all(iterable) method returns a promise that resolves when all of the promises in the iterable argument have resolved, or rejects with the reason of the first passed promise that rejects.
2.Just use Promise.all(files).catch(err => { })
This throws an error if ANY of the promises are rejected.
3.Use .reflect on the promises before .all if you want to wait for all
promises to reject or fulfill
Syntax -Promise.all(iterable);
Promise.all passes an array of values from all the promises in the iterable object that it was passed.
https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Promise/all
var isCallFailed = false;
function myEndpoint1() {
return isCallFailed ? Promise.reject("Bohoo!") :Promise.resolve({"a":"a"});
}
function myEndpoint2() {
return Promise.resolve({"b":"b"});
}
Promise.all([myEndpoint1(), myEndpoint2()])
.then(values => {
var data1 = values[0];
var data2 = values[1];
alert("SUCCESS... data1: " + JSON.stringify(data1) + "; data2: " + JSON.stringify(data2));
})
.catch(error => {
alert("ERROR... " + error);
});
you can try another case by making isCallFailed = true.
Use Promise.all only when you need to run a code according to the result of more than one asynchronous operations using promises.
For example:
You have a scenario like, You need to download 2000 mb file from server, and at the same time you are going to free the user storage to make sure it can save the downloaded file.
And you need to save only in case if the file is downloaded successfully and the storage space is created successfully.
you will do like this.
your first asynchronous operation
var p1 = new Promise(function(resolve, reject) {
// you need to download 2000mb file and return resolve if
// you successfully downloaded the file
})
and your second asynchronous operation
var p2 = new Promise(function(resolve, reject) {
// you need to clear the user storage for 2000 mb
// which can take some time
})
Now you want to save only when both of the promises resolved successfully, otherwise not.
You will use promise.all like this.
Promise.all([p1,p2]).then((result)=>{
// you will be here only if your both p1 and p2 are resolved successfully.
// you code to save the downloaded file here
})
.catch((error)=>{
// you will be here if at-least one promise in p1,p2 is rejected.
// show error to user
// take some other action
})
Promise.all can be used in a scenario when there is a routine which is validating multiplerules based on particular criteria and you have to execute them all in parallel and need to see the results of those rules at one point. Promise.all returns the results as an array which were resolved in your rule vaidator routine.
E.g.
const results = await Promise.all([validateRule1, validateRule2, validateRule3, ...]);
then results array may look like (depending upon the conditions) as for example: [true, false, false]
Now you can reject/accept the results you have based on return values. Using this way you won't have to apply multiple conditions with if-then-else.
If you are interested only Promise.all then read below Promise.all
Promise (usually they are called "Promise") - provide a convenient way to organize asynchronous code.
Promise - is a special object that contains your state. Initially, pending ( «waiting"), and then - one of: fulfilled ( «was successful") or rejected ( «done with error").
On the promise to hang callbacks can be of two types:
unFulfilled - triggered when the promise in a state of "completed
successfully."
Rejected - triggered when the promise in the "made in error."
The syntax for creating the Promise:
var promise = new Promise(function(resolve, reject) {
// This function will be called automatically
// It is possible to make any asynchronous operations,
// And when they will end - you need to call one of:
// resolve(result) on success
// reject(error) on error
})
Universal method for hanging handlers:
promise.then(onFulfilled, onRejected)
onFulfilled - a function that will be called with the result with
resolve.
onRejected - a function that will be called when an error reject.
With its help you can assign both the handler once, and only one:
// onFulfilled It works on success
promise.then(onFulfilled)
// onRejected It works on error
promise.then(null, onRejected)
Synchronous throw - the same that reject
'use strict';
let p = new Promise((resolve, reject) => {
// то же что reject(new Error("o_O"))
throw new Error("o_O");
});
p.catch(alert); // Error: o_O
Promisification
Promisification - When taking asynchronous functionality and make it a wrapper for returning PROMIS.
After Promisification functional use often becomes much more convenient.
As an example, make a wrapper for using XMLHttpRequest requests
httpGet function (url) will return PROMIS, which upon successful data loading with the url will go into fulfilled with these data, and in case of error - in rejected with an error information:
function httpGet(url) {
return new Promise(function(resolve, reject) {
var xhr = new XMLHttpRequest();
xhr.open('GET', url, true);
xhr.onload = function() {
if (this.status == 200) {
resolve(this.response);
} else {
var error = new Error(this.statusText);
error.code = this.status;
reject(error);
}
};
xhr.onerror = function() {
reject(new Error("Network Error"));
};
xhr.send();
});
}
As you can see, inside the function XMLHttpRequest object is created and sent as usual, when onload / onerror are called, respectively, resolve (at the status 200) or reject.
Using:
httpGet("/article/promise/user.json")
.then(
response => alert(`Fulfilled: ${response}`),
error => alert(`Rejected: ${error}`)
);
Parallel execution
What if we want to implement multiple asynchronous processes simultaneously and to process their results?
The Promise class has the following static methods.
Promise.all(iterable)
Call Promise.all (iterable) receives an array (or other iterable object) and returns PROMIS PROMIS, which waits until all transferred PROMIS completed, and changes to the state "done" with an array of results.
For example:
Promise.all([
httpGet('/article/promise/user.json'),
httpGet('/article/promise/guest.json')
]).then(results => {
alert(results);
});
Let's say we have an array of URL.
let urls = [
'/article/promise/user.json',
'/article/promise/guest.json'
];
To download them in parallel, you need to:
Create for each URL corresponding to PROMIS.
Wrap an array of PROMIS in Promise.all.
We obtain this:
'use strict';
let urls = [
'/article/promise/user.json',
'/article/promise/guest.json'
];
Promise.all( urls.map(httpGet) )
.then(results => {
alert(results);
});
Note that if any of Promise ended with an error, the result will
Promise.all this error.
At the same time the rest of PROMIS ignored.
For example:
Promise.all([
httpGet('/article/promise/user.json'),
httpGet('/article/promise/guest.json'),
httpGet('/article/promise/no-such-page.json') // (нет такой страницы)
]).then(
result => alert("не сработает"),
error => alert("Ошибка: " + error.message) // Ошибка: Not Found
)
In total:
Promise - is a special object that stores its state, the current
result (if any), and callbacks.
When you create a new Promise ((resolve, reject) => ...) function
argument starts automatically, which should call resolve (result) on
success, and reject (error) - error.
Argument resolve / reject (only the first, and the rest are ignored)
is passed to handlers on this Promise.
Handlers are appointed by calling .then / catch.
To transfer the results from one processor to another using Channing.
https://www.promisejs.org/patterns/
So, I'm having a problem. I need to make potentially hundreds of http calls and they must be done one after the other. I also need to be able to interrupt the whole process at any time.
The solution I'm using right now is this one, but it doesn't allow me to properly interrupt the "chain" (I use jQuery and Deferreds because I haven't learned to replace them with Promises yet):
function doAnHttpCall() {
return $.post('http://the.domain/theendpoint', { theParam: this });
}
var anArrayOfParams = ['param1', 'param2', 'param3'];
var p = $.Deferred();
setTimeout(p.resolve.bind(p), 0);
var promise = anArrayOfParams.reduce(function(prev, cur) {
return prev.then(doAnHttpCall.bind(cur));
}, p)
.done(function() {
console.log('all done.');
});
(I've found this solution here)
The problem here is that the only way to sort-of break out of the reduce function is by modifying the array you're looping through and check for a particular value and always return immediately when you find it, instead of executing the "doAnHttpCall" method (in this case). This would still make me loop over potentially hundreds of elements of the array instead of just interrupting the process, which is very ugly.
There must be a better way to do this. Or do I really need to use a function that calls itself with the next element to process when an http call has finished? It sounds "bad-practice-y".
Thanks for the help.
You would use async/await:
const request = param => $.post('http://the.domain/theendpoint', { param });
(async () => {
for(const param of ['param1', 'param2', 'param3']) {
let result = await request(param);
if (result === 'particular value') break;
}
console.log('all done!');
})();
I would like to do a synchronous loop in a part of my code.
The function saveInDatabase checks if the item title (string) already exists in the database. That's why it can't be resolved in parallel, otherwise the condition will never apply (and would create duplicates).
Promise.all(arr.map(item => {
saveInDatabase(item).then((myResult) => ... );
}));
I tried to encapsulate this function into separate promises, also tried with npm packages (synchronous.js, sync), but it seems that it does not fit with my code.
Maybe this solution is completely silly.
Do you think it's a better idea to replace promise.all by a synchronous loop (foreach for example) ? The problem is that I need the results of each iteration...
I'm using Node 6.11.2. Could you give me some tips to handle that ?
Thank you in advance.
Without using await (which is not in node.js v6.11.2, but would make this simpler), a classic pattern for serializing a bunch of async operations that return a promise is to use a reduce() loop like this:
arr.reduce(function(p, item) {
return p.then(function() {
return saveInDatabase(item).then((myResult) => ... );
});
}, Promise.resolve()).then(function() {
// all done here
}).catch(function(err) {
// error here
});
If you want to save all the results, you can use your .then(myResult => ...) handler to .push() the result into an array which you can access when done.
This will serialize all the calls to saveInDatabase(item) to it waits for the first one to be done before calling the second one, waits for the second one to be done before calling the third one, etc...
The default implementation here will stop if saveInDatabase(item) rejects. If you want to keep going (you don't say in your question), even when it gives an error, then you can add a .catch() to it to turn the rejected promise into a fulfilled promise.
In node.js v7+, you can use await in a regular for loop:
async function myFunc() {
let results = [];
for (let item of arr) {
let r = await saveInDatabase(item).then((myResult) => ... );
results.push(r);
}
return results;
}
myFunc().then(function(results) {
// all done here
}).catch(function(err) {
// error here
});
If you could run all the requests in parallel, then you could do that like this:
Promise.all(arr.map(item => {
return saveInDatabase(item).then((myResult) => ... );
})).then(function(results) {
// all done here
}).catch(function(err) {
// error here
});
In any of these, if you don't want it to stop upon a rejection, then add a .catch() to your saveInDatabase() promise chain to turn the rejection into a resolved promise with some known value or error value you can detect.
With node.js I want to http.get a number of remote urls in a way that only 10 (or n) runs at a time.
I also want to retry a request if an exception occures locally (m times), but when the status code returns an error (5XX, 4XX, etc) the request counts as valid.
This is really hard for me to wrap my head around.
Problems:
Cannot try-catch http.get as it is async.
Need a way to retry a request on failure.
I need some kind of semaphore that keeps track of the currently active request count.
When all requests finished I want to get the list of all request urls and response status codes in a list which I want to sort/group/manipulate, so I need to wait for all requests to finish.
Seems like for every async problem using promises are recommended, but I end up nesting too many promises and it quickly becomes uncypherable.
There are lots of ways to approach the 10 requests running at a time.
Async Library - Use the async library with the .parallelLimit() method where you can specify the number of requests you want running at one time.
Bluebird Promise Library - Use the Bluebird promise library and the request library to wrap your http.get() into something that can return a promise and then use Promise.map() with a concurrency option set to 10.
Manually coded - Code your requests manually to start up 10 and then each time one completes, start another one.
In all cases, you will have to manually write some retry code and as with all retry code, you will have to very carefully decide which types of errors you retry, how soon you retry them, how much you backoff between retry attempts and when you eventually give up (all things you have not specified).
Other related answers:
How to make millions of parallel http requests from nodejs app?
Million requests, 10 at a time - manually coded example
My preferred method is with Bluebird and promises. Including retry and result collection in order, that could look something like this:
const request = require('request');
const Promise = require('bluebird');
const get = Promise.promisify(request.get);
let remoteUrls = [...]; // large array of URLs
const maxRetryCnt = 3;
const retryDelay = 500;
Promise.map(remoteUrls, function(url) {
let retryCnt = 0;
function run() {
return get(url).then(function(result) {
// do whatever you want with the result here
return result;
}).catch(function(err) {
// decide what your retry strategy is here
// catch all errors here so other URLs continue to execute
if (err is of retry type && retryCnt < maxRetryCnt) {
++retryCnt;
// try again after a short delay
// chain onto previous promise so Promise.map() is still
// respecting our concurrency value
return Promise.delay(retryDelay).then(run);
}
// make value be null if no retries succeeded
return null;
});
}
return run();
}, {concurrency: 10}).then(function(allResults) {
// everything done here and allResults contains results with null for err URLs
});
The simple way is to use async library, it has a .parallelLimit method that does exactly what you need.
I'm trying to read JSON from a file, filter out stuff I don't want and possibly map and flatten the JSON elements, and then for each filtered JSON line in the file call a function that returns a promise. (Basically tail -f from a bunyan produced file). Additional Requirement: I want the pacing to be based on how fast the functions resolve the promise (output pacing, one element at a time, like Promise.each)
Sounds like a great use for RXJS and promises, right? Well, except that all the examples out there are for promises that produce observable streams, not streams that have subscribers who call functions that return promises.
The available schedulers for RXJS don't seem to have an output-based single concurrency pacing equivalent to Promise.each (I tried flatMapWithMaxConcurrent)
In this code, update returns a promise. The promises are resolved once I fixed a library bug, but the console output is out of order, and the promises are resolved in what appears to be a highly concurrent manner. The promises actually take 5 seconds to resolve, I would expect to a see a console message about every 5 seconds. Instead there's one delay of 5 seconds and a bunch of messages, meaning I think that many messages were run concurrently.
The library I created has Bluebird promises from the ground up, I can't rewrite it without removing promises from all my code. Kinda getting tempted, this is my second all nighter hunting down unresolved promises. I know how I'd write this in "C"...
function tailMyFile(filename, ocu, oce) {
var handle = new Tail(filename);
handle.bunyan().filter(function (x) {
if ("descr" in x && "units" in x && "value" in x && "time" in x) {
return true;
}
})
.subscribe (
function(x) {
if (x.descr == "enter") {
MyPromisifiedLib.update(oce, x.value, "").done(function() {
console.log("updated enter value");
})
} else if ("push" in x) {
MyPromisifiedLib.update(ocu, x.value, "push").done(function() {
console.log("updated with push");
})
} else {
MyPromisifiedLib.update(ocu, x.value, "").done(function() {
console.log("updated value");
})
}
},
function(err) {
console.error("err leaked through to subscriber");
log.error("err leaked through to subscriber");
},
function() { log.info("Completed") }
);
}