Hi guys i'm trying to improve performance of some computation in my system. Basically I want to generate a series of actions based on some data. This doesn't scale well and I want to try doing this in parallel and getting a result after (a bit like how futures work)
I have an interface with a series of implementations that get a collection of actions. And want to call all these in parallel and await the results at the end.
The issue is that, when I view the logs its clearly doing this sequentially and waiting on each action getter before going to the next one. I thought the async would do this asynchronously, but its not.
The method the runBlocking is in, is within a spring transaction. Maybe that has something to do with it.
runBlocking {
val actions = actionsReportGetters.map { actionReportGetter ->
async {
getActions(actionReportGetter, abstractUser)
}
}.awaitAll().flatten()
allActions.addAll(actions)
}
private suspend fun getActions(actionReportGetter: ActionReportGetter, traderUser: TraderUser): List<Action> {
return actionReportGetter.getActions(traderUser)
}
interface ActionReportGetter {
fun getActions(traderUser: TraderUser): List<Action>
}
Looks like you are doing some blocking operation in ActionReportGetter.getActions in a single threaded environment (probably in the main thread).
For such IO operations you should launch your coroutines in Dispatchers.IO which provides a thread pool with multiple threads.
Update your code to this:
async(Dispatchers.IO) { // Switch to IO dispatcher
getActions(actionReportGetter, abstractUser
}
Also getActions need not be a suspending function here. You can remove the suspend modifier from it.
I use a semaphore for two processes that share a resource (rest api endpoint), that can't be called concurrent. I do:
let tokenSemaphore = null;
class restApi {
async getAccessToken() {
let tokenResolve;
if (tokenSemaphore) {
await tokenSemaphore;
}
tokenSemaphore = new Promise((resolve) => tokenResolve = resolve);
return new Promise(async (resolve, reject) => {
// ...
resolve(accessToken);
tokenResolve();
tokenSemaphore = null;
});
}
}
But this looks too complicated. Is there a simpler way to achieve the same thing?
And how to do it for more concurrent processes.
This is not a server side Semaphore. You need interprocess communication for locking processes which are running independently in different threads. In that case the API must support something like that on the server side and this here is not for you.
As this was the first hit when googling for "JavaScript Promise Semaphore", here is what I came up with:
function Semaphore(max, fn, ...a1)
{
let run = 0;
const waits = [];
function next(x)
{
if (run<max && waits.length)
waits.shift()(++run);
return x;
}
return (...a2) => next(new Promise(ok => waits.push(ok)).then(() => fn(...a1,...a2)).finally(_ => run--).finally(next));
}
Example use (above is (nearly) copied from my code, following was typed in directly and hence is not tested):
// do not execute more than 20 fetches in parallel:
const fetch20 = Semaphore(20, fetch);
async function retry(...a)
{
for (let retries=0;; retries++)
{
if (retries)
await new Promise(ok => setTimeout(ok, 100*retries));
try {
return await fetch20(...a)
} catch (e) {
console.log('retry ${retries}', url, e);
}
}
}
and then
for (let i=0; ++i<10000000; ) retry(`https://example.com/?${i}`);
My Browser handles thousands of asynchronous parallel calls to retry very well. However when using fetch directly, the Tabs crash nearly instantly.
For your usage you probably need something like:
async function access_token_api_call()
{
// assume this takes 10s and must not be called in parallel for setting the Cookie
return fetch('https://api.example.com/nonce').then(r => r.json());
}
const get_access_token = Semaphore(1, access_token_api_call);
// both processes need to use the same(!) Semaphore, of course
async function process(...args)
{
const token = await get_access_token();
// processing args here
return //something;
}
proc1 = process(1);
proc2 = process(2);
Promise.all([proc1, proc2]).then( //etc.
YMMV.
Notes:
This assumes that your two processes are just asynchronous functions of the same single JS script (i.E. running in the same Tab).
A Browser usually does not open more than 5 concurrent connects to a backend and then pipelines excess requests. fetch20 is my workaround for a real-world problem when a JS-Frontend needs to queue, say, 5000 fetches in parallel, which crashes my Browser (for unknown reason). We have 2021 and that should not be any problem, right?
But this looks too complicated.
Not complicated enough, I'm afraid. Currently, if multiple code paths call getAccessToken when the semaphore is taken, they'll all block on the same tokenSemaphore instance, and when the semaphore is released, they'll all be released and resolve roughly at the same time, allowing concurrent access to the API.
In order to write an asynchronous lock (or semaphore), you'll need a collection of futures (tokenResolvers). When one is released, it should only remove and resolve a single future from that collection.
I played around with it a bit in TypeScript a few years ago, but never tested or used the code. My Gist is also C#-ish (using "dispoables" and whatnot); it needs some updating to use more natural JS patterns.
Currently I am reading a lot about coroutines.
While I think I only partly understand what they do (Allowing a function to be returned and continued at a certain point for instance), I really don't know why I should use them. I see no real benefit in using a coroutine. For me these things look to me like goto with extra steps. Could someone give me a good real world example where a coroutine might really improve a code base? Maybe that helps me to get the concept.
Coroutines allow us to model cooperative multitasking systems in a very simple and modular way. This kind of systems may suit many problems like multi-agent games, stock market simulators, operating systems, etc.
The simplicity comes from the fact that we may need only two function-like coroutines to express the concept in code. Classes, state structures, function pointers are no longer required. Below is a pseudo-code illustration which defines a scheduler and a task. Unlike normal functions, both instances of task pass the execution to each other after do_one_step call.
void scheduler(list<task> tasks) {
while(true) {
for(t : tasks)
await t;
}
}
void task(int arg) {
while(1) {
do_one_step(arg);
yield;
}
}
void main() { await scheduler( list( { task(0), task(1) } ); }
More importantly, the code for task may now be modular: tasks are allowed to include subtasks, which resemble its structure. Any subtask may be designed as a valid task itself:
void subtask(int arg) {
do_one_step(arg);
yield;
do_one_step(arg);
yield;
}
void task(int arg) {
while(1) {
await subtask(arg);
yield;
}
}
void main() { await scheduler( list({task(0), task(1), subtask(2)}); }
Coroutines allow us to stack subtasks into larger tasks, and provide us with a new tool to modularize our solutions.
I've been developing JavaScript for a few years and I don't understand the fuss about promises at all.
It seems like all I do is change:
api(function(result){
api2(function(result2){
api3(function(result3){
// do work
});
});
});
Which I could use a library like async for anyway, with something like:
api().then(function(result){
api2().then(function(result2){
api3().then(function(result3){
// do work
});
});
});
Which is more code and less readable. I didn't gain anything here, it's not suddenly magically 'flat' either. Not to mention having to convert things to promises.
So, what's the big fuss about promises here?
Promises are not callbacks. A promise represents the future result of an asynchronous operation. Of course, writing them the way you do, you get little benefit. But if you write them the way they are meant to be used, you can write asynchronous code in a way that resembles synchronous code and is much more easy to follow:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
});
Certainly, not much less code, but much more readable.
But this is not the end. Let's discover the true benefits: What if you wanted to check for any error in any of the steps? It would be hell to do it with callbacks, but with promises, is a piece of cake:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
}).catch(function(error) {
//handle any error that may occur before this point
});
Pretty much the same as a try { ... } catch block.
Even better:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
}).catch(function(error) {
//handle any error that may occur before this point
}).then(function() {
//do something whether there was an error or not
//like hiding an spinner if you were performing an AJAX request.
});
And even better: What if those 3 calls to api, api2, api3 could run simultaneously (e.g. if they were AJAX calls) but you needed to wait for the three? Without promises, you should have to create some sort of counter. With promises, using the ES6 notation, is another piece of cake and pretty neat:
Promise.all([api(), api2(), api3()]).then(function(result) {
//do work. result is an array contains the values of the three fulfilled promises.
}).catch(function(error) {
//handle the error. At least one of the promises rejected.
});
Hope you see Promises in a new light now.
Yes, Promises are asynchronous callbacks. They can't do anything that callbacks can't do, and you face the same problems with asynchrony as with plain callbacks.
However, Promises are more than just callbacks. They are a very mighty abstraction, allow cleaner and better, functional code with less error-prone boilerplate.
So what's the main idea?
Promises are objects representing the result of a single (asynchronous) computation. They resolve to that result only once. There's a few things what this means:
Promises implement an observer pattern:
You don't need to know the callbacks that will use the value before the task completes.
Instead of expecting callbacks as arguments to your functions, you can easily return a Promise object
The promise will store the value, and you can transparently add a callback whenever you want. It will be called when the result is available. "Transparency" implies that when you have a promise and add a callback to it, it doesn't make a difference to your code whether the result has arrived yet - the API and contracts are the same, simplifying caching/memoisation a lot.
You can add multiple callbacks easily
Promises are chainable (monadic, if you want):
If you need to transform the value that a promise represents, you map a transform function over the promise and get back a new promise that represents the transformed result. You cannot synchronously get the value to use it somehow, but you can easily lift the transformation in the promise context. No boilerplate callbacks.
If you want to chain two asynchronous tasks, you can use the .then() method. It will take a callback to be called with the first result, and returns a promise for the result of the promise that the callback returns.
Sounds complicated? Time for a code example.
var p1 = api1(); // returning a promise
var p3 = p1.then(function(api1Result) {
var p2 = api2(); // returning a promise
return p2; // The result of p2 …
}); // … becomes the result of p3
// So it does not make a difference whether you write
api1().then(function(api1Result) {
return api2().then(console.log)
})
// or the flattened version
api1().then(function(api1Result) {
return api2();
}).then(console.log)
Flattening does not come magically, but you can easily do it. For your heavily nested example, the (near) equivalent would be
api1().then(api2).then(api3).then(/* do-work-callback */);
If seeing the code of these methods helps understanding, here's a most basic promise lib in a few lines.
What's the big fuss about promises?
The Promise abstraction allows much better composability of functions. For example, next to then for chaining, the all function creates a promise for the combined result of multiple parallel-waiting promises.
Last but not least Promises come with integrated error handling. The result of the computation might be that either the promise is fulfilled with a value, or it is rejected with a reason. All the composition functions handle this automatically and propagate errors in promise chains, so that you don't need to care about it explicitly everywhere - in contrast to a plain-callback implementation. In the end, you can add a dedicated error callback for all occurred exceptions.
Not to mention having to convert things to promises.
That's quite trivial actually with good promise libraries, see How do I convert an existing callback API to promises?
In addition to the already established answers, with ES6 arrow functions Promises turn from a modestly shining small blue dwarf straight into a red giant. That is about to collapse into a supernova:
api().then(result => api2()).then(result2 => api3()).then(result3 => console.log(result3))
As oligofren pointed out, without arguments between api calls you don't need the anonymous wrapper functions at all:
api().then(api2).then(api3).then(r3 => console.log(r3))
And finally, if you want to reach a supermassive black hole level, Promises can be awaited:
async function callApis() {
let api1Result = await api();
let api2Result = await api2(api1Result);
let api3Result = await api3(api2Result);
return api3Result;
}
In addition to the awesome answers above, 2 more points may be added:
1. Semantic difference:
Promises may be already resolved upon creation. This means they guarantee conditions rather than events. If they are resolved already, the resolved function passed to it is still called.
Conversely, callbacks handle events. So, if the event you are interested in has happened before the callback has been registered, the callback is not called.
2. Inversion of control
Callbacks involve inversion of control. When you register a callback function with any API, the Javascript runtime stores the callback function and calls it from the event loop once it is ready to be run.
Refer The Javascript Event loop for an explanation.
With Promises, control resides with the calling program. The .then() method may be called at any time if we store the promise object.
In addition to the other answers, the ES2015 syntax blends seamlessly with promises, reducing even more boilerplate code:
// Sequentially:
api1()
.then(r1 => api2(r1))
.then(r2 => api3(r2))
.then(r3 => {
// Done
});
// Parallel:
Promise.all([
api1(),
api2(),
api3()
]).then(([r1, r2, r3]) => {
// Done
});
Promises are not callbacks, both are programming idioms that facilitate async programming. Using an async/await-style of programming using coroutines or generators that return promises could be considered a 3rd such idiom. A comparison of these idioms across different programming languages (including Javascript) is here: https://github.com/KjellSchubert/promise-future-task
No, Not at all.
Callbacks are simply Functions In JavaScript which are to be called and then executed after the execution of another function has finished. So how it happens?
Actually, In JavaScript, functions are itself considered as objects and hence as all other objects, even functions can be sent as arguments to other functions. The most common and generic use case one can think of is setTimeout() function in JavaScript.
Promises are nothing but a much more improvised approach of handling and structuring asynchronous code in comparison to doing the same with callbacks.
The Promise receives two Callbacks in constructor function: resolve and reject. These callbacks inside promises provide us with fine-grained control over error handling and success cases. The resolve callback is used when the execution of promise performed successfully and the reject callback is used to handle the error cases.
No promises are just wrapper on callbacks
example
You can use javascript native promises with node js
my cloud 9 code link : https://ide.c9.io/adx2803/native-promises-in-node
/**
* Created by dixit-lab on 20/6/16.
*/
var express = require('express');
var request = require('request'); //Simplified HTTP request client.
var app = express();
function promisify(url) {
return new Promise(function (resolve, reject) {
request.get(url, function (error, response, body) {
if (!error && response.statusCode == 200) {
resolve(body);
}
else {
reject(error);
}
})
});
}
//get all the albums of a user who have posted post 100
app.get('/listAlbums', function (req, res) {
//get the post with post id 100
promisify('http://jsonplaceholder.typicode.com/posts/100').then(function (result) {
var obj = JSON.parse(result);
return promisify('http://jsonplaceholder.typicode.com/users/' + obj.userId + '/albums')
})
.catch(function (e) {
console.log(e);
})
.then(function (result) {
res.end(result);
}
)
})
var server = app.listen(8081, function () {
var host = server.address().address
var port = server.address().port
console.log("Example app listening at http://%s:%s", host, port)
})
//run webservice on browser : http://localhost:8081/listAlbums
JavaScript Promises actually use callback functions to determine what to do after a Promise has been resolved or rejected, therefore both are not fundamentally different. The main idea behind Promises is to take callbacks - especially nested callbacks where you want to perform a sort of actions, but it would be more readable.
Promises overview:
In JS we can wrap asynchronous operations (e.g database calls, AJAX calls) in promises. Usually we want to run some additional logic on the retrieved data. JS promises have handler functions which process the result of the asynchronous operations. The handler functions can even have other asynchronous operations within them which could rely on the value of the previous asynchronous operations.
A promise always has of the 3 following states:
pending: starting state of every promise, neither fulfilled nor rejected.
fulfilled: The operation completed successfully.
rejected: The operation failed.
A pending promise can be resolved/fullfilled or rejected with a value. Then the following handler methods which take callbacks as arguments are called:
Promise.prototype.then() : When the promise is resolved the callback argument of this function will be called.
Promise.prototype.catch() : When the promise is rejected the callback argument of this function will be called.
Although the above methods skill get callback arguments they are far superior than using
only callbacks here is an example that will clarify a lot:
Example
function createProm(resolveVal, rejectVal) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (Math.random() > 0.5) {
console.log("Resolved");
resolve(resolveVal);
} else {
console.log("Rejected");
reject(rejectVal);
}
}, 1000);
});
}
createProm(1, 2)
.then((resVal) => {
console.log(resVal);
return resVal + 1;
})
.then((resVal) => {
console.log(resVal);
return resVal + 2;
})
.catch((rejectVal) => {
console.log(rejectVal);
return rejectVal + 1;
})
.then((resVal) => {
console.log(resVal);
})
.finally(() => {
console.log("Promise done");
});
The createProm function creates a promises which is resolved or rejected based on a random Nr after 1 second
If the promise is resolved the first then method is called and the resolved value is passed in as an argument of the callback
If the promise is rejected the first catch method is called and the rejected value is passed in as an argument
The catch and then methods return promises that's why we can chain them. They wrap any returned value in Promise.resolve and any thrown value (using the throw keyword) in Promise.reject. So any value returned is transformed into a promise and on this promise we can again call a handler function.
Promise chains give us more fine tuned control and better overview than nested callbacks. For example the catch method handles all the errors which have occurred before the catch handler.
Promises allows programmers to write simpler and far more readable code than by using callbacks.
In a program, there are steps want to do in series.
function f() {
step_a();
step_b();
step_c();
...
}
There's usually information carried between each step.
function f() {
const a = step_a( );
const b = step_b( a );
const c = step_c( b );
...
}
Some of these steps can take a (relatively) long time, so sometimes you want to do them in parallel with other things. One way to do that is using threads. Another is asynchronous programming. (Both approaches has pros and cons, which won't be discussed here.) Here, we're talking about asynchronous programming.
The simple way to achieve the above when using asynchronous programming would be to provide a callback which is called once a step is complete.
// step_* calls the provided function with the returned value once complete.
function f() {
step_a(
function( a )
step_b(
function( b )
step_c(
...
)
},
)
},
)
}
That's quite hard to read. Promises offer a way to flatten the code.
// step_* returns a promise.
function f() {
step_a()
.then( step_b )
.then( step_c )
...
}
The object returned is called a promise because it represents the future result (i.e. promised result) of the function (which could be a value or an exception).
As much as promises help, it's still a bit complicated to use promises. This is where async and await come in. In a function declared as async, await can be used in lieu of then.
// step_* returns a promise.
async function f()
const a = await step_a( );
const b = await step_b( a );
const c = await step_c( b );
...
}
This is undeniably much much more readable than using callbacks.
how to use dispatcher.BeginInvoke in for loop( httpwebrequest).With each dispatcher.BeginInvoke have complete before call another dispatcher.BeginInvoke. Because objects return by httpwerequest are wrong position.
No, BeginInvoke is asynchronous - you're basically adding delegates to a queue of items to be executed on the UI thread.
If you need to wait until the delegate has executed before you continue work in your background thread, you'll need to do a bit of work yourself, as Silverlight doesn't support the synchronous Dispatcher.Invoke method, or the DispatcherOperation.Wait() method. Silverlight tries to avoid synchronous approaches like this - if you can possibly redesign your code so that you don't need to wait, that would be preferable.
Being able to easily convert a synchronous sequence of operations into asynchrounous code has been a subject I've blogged about a fair bit. If you want to take up my approach you will need to add the following (relatively small) chunks of code:
The core AsyncOperationService
Code to create an AsyncOperation from the .NET Async Pattern
A couple of Extension methods for WebRequest
Here is some example code that has the flavour of what you describe in your question:-
IEnumerable<AsyncOperation> LoadSomeStuff(IList<string> urls)
{
for (string url in urls)
{
yield return AsyncOperationService.SwitchToBackgroundThread();
WebRequest req = WebRequest.Create(url);
WebResponse resp = null;
yield return req.GetResponseAsyncOp(r => resp = r);
using (resp)
{
// Do stuff with the Web Response such as construct model class instances from a stream.
}
// When ready to actually start touching the UI
yield return AsyncOperationService.SwitchToUIThread();
// Do stuff to the UI
}
}
usage:
List<string> urls = new List<string> {"pinkElephants.xml", "whileElephants.xml"}
LoadSomeStuff(urls).Run(err =>
{
if (err == null)
{
// Cool, it all worked and I probably don't need to do anything
}
else
{
// Something bad happened, lets tell the user about it in the UI somehow.
}
});
Note that this isn't the most efficient code possible. However in many cases the time it takes HTTP response to be delivered massively out-weighs the time the rest of the code uses up so the inefficiency can be quite small and well worth the reduced complexity of code.