Async/await

Incomputer programming,theasync/await patternis a syntactic feature of manyprogramming languagesthat allows anasynchronous,non-blockingfunctionto be structured in a way similar to an ordinary synchronous function. It is semantically related to the concept of acoroutineand is often implemented using similar techniques, and is primarily intended to provide opportunities for the program to execute other code while waiting for a long-running, asynchronous task to complete, usually represented bypromisesor similardata structures.The feature is found inC#,C++,Python,F#,Hack,Julia,Dart,Kotlin,Rust,Nim,JavaScript,andSwift.

History

F# added asynchronous workflows with await points in version 2.0 in 2007.This influenced the async/await mechanism added to C#.

Microsoft first released a version of C# with async/await in the Async CTP (2011). It was later officially released in C# 5 (2012).

Haskell lead developerSimon Marlowcreated the async package in 2012.

Python added support for async/await with version 3.5 in 2015adding 2 newkeywords,asyncandawait.

TypeScriptadded support for async/await with version 1.7 in 2015.

JavaScript added support for async/await in 2017 as part ofECMAScript2017 JavaScript edition.

Rust added support for async/await with version 1.39.0 in 2019 using theasynckeyword and the.awaitpostfix operator, both introduced in the 2018 edition of the language.

C++ added support for async/await withversion 20in 2020 with 3 new keywordsco_return,co_await,co_yield.

Swift added support for async/await withversion 5.5in 2021, adding 2 new keywordsasyncandawait.This was released alongside a concrete implementation of theActor modelwith theactorkeywordwhich uses async/await to mediate access to each actor from outside.

Example C#

TheC#function below, which downloads a resource from aURIand returns the resource's length, uses this async/await pattern:

publicasyncTask<int>FindSizeOfPageAsync(Uriuri)
{
varclient=newHttpClient();
byte[]data=awaitclient.GetByteArrayAsync(uri);
returndata.Length;
}

• First, theasynckeyword indicates to C# that the method is asynchronous, meaning that it may use an arbitrary number ofawaitexpressions and will bind the result to apromise.
• Thereturn type,Task<T>,is C#'s analogue to the concept of a promise, and here is indicated to have a result value of typeint.
• The first expression to execute when this method is called will benew HttpClient().GetByteArrayAsync(uri),which is another asynchronous method returning aTask<byte[]>.Because this method is asynchronous, it will not download the entire batch of data before returning. Instead, it will begin the download process using a non-blocking mechanism (such as abackground thread), and immediately return an unresolved, unrejectedTask<byte[]>to this function.
• With theawaitkeyword attached to theTask,this function will immediately proceed to return aTask<int>to its caller, who may then continue on with other processing as needed.
• OnceGetByteArrayAsync()finishes its download, it will resolve theTaskit returned with the downloaded data. This will trigger acallbackand causeFindPageSizeAsync()to continue execution by assigning that value todata.
• Finally, the method returnsdata.Length,a simple integer indicating the length of the array. The compiler re-interprets this as resolving theTaskit returned earlier, triggering a callback in the method's caller to do something with that length value.

A function using async/await can use as manyawaitexpressions as it wants, and each will be handled in the same way (though a promise will only be returned to the caller for the first await, while every other await will utilize internal callbacks). A function can also hold a promise object directly and do other processing first (including starting other asynchronous tasks), delaying awaiting the promise until its result is needed. Functions with promises also have promise aggregation methods that allow the program to await multiple promises at once or in some special pattern (such as C#'sTask.WhenAll(),which returns a valuelessTaskthat resolves when all of the tasks in the arguments have resolved). Many promise types also have additional features beyond what the async/await pattern normally uses, such as being able to set up more than one result callback or inspect the progress of an especially long-running task.

In the particular case of C#, and in many other languages with this language feature, the async/await pattern is not a core part of the language's runtime, but is instead implemented withlambdasorcontinuationsat compile time. For instance, the C# compiler would likely translate the above code to something like the following before translating it to its ILbytecodeformat:

publicTask<int>FindSizeOfPageAsync(Uriuri)
{
varclient=newHttpClient();
Task<byte[]>dataTask=client.GetByteArrayAsync(uri);
Task<int>afterDataTask=dataTask.ContinueWith((originalTask)=>{
returnoriginalTask.Result.Length;
});
returnafterDataTask;
}

Because of this, if an interface method needs to return a promise object, but itself does not requireawaitin the body to wait on any asynchronous tasks, it does not need theasyncmodifier either and can instead return a promise object directly. For instance, a function might be able to provide a promise that immediately resolves to some result value (such as C#'sTask.FromResult()), or it may simply return another method's promise that happens to be the exact promise needed (such as when deferring to anoverload).

One important caveat of this functionality, however, is that while the code resembles traditional blocking code, the code is actually non-blocking and potentially multithreaded, meaning that many intervening events may occur while waiting for the promise targeted by anawaitto resolve. For instance, the following code, while always succeeding in a blocking model withoutawait,may experience intervening events during theawaitand may thus find shared state changed out from under it:

vara=state.a;
varclient=newHttpClient();
vardata=awaitclient.GetByteArrayAsync(uri);
Debug.Assert(a==state.a);// Potential failure, as value of state.a may have been changed
// by the handler of potentially intervening event.
returndata.Length;

Implementations

In F#

In 2007, F#added asynchronous workflows with version 2.0.The asynchronous workflows are implemented as CE (computation expressions). They can be defined without specifying any special context (likeasyncin C#).F#asynchronous workflows append a bang (!) to keywords to start asynchronous tasks.

The following async function downloads data from an URL using an asynchronous workflow:

letasyncSumPageSizes(uris:#seq<Uri>):Async<int>=async{
usehttpClient=newHttpClient()
let!pages=
uris
|>Seq.map(httpClient.GetStringAsync>>Async.AwaitTask)
|>Async.Parallel
returnpages|>Seq.fold(funaccumulatorcurrent->current.Length+accumulator)0
}

In C#

In 2012, C# added the async/await pattern in C# with version 5.0, which Microsoft refers to as the task-based asynchronous pattern (TAP).Async methods usually return eithervoid,Task,Task<T>,ValueTaskorValueTask<T>.User code can define custom types that async methods can return through customasync method buildersbut this is an advanced and rare scenario.Async methods that returnvoidare intended forevent handlers;in most cases where a synchronous method would returnvoid,returningTaskinstead is recommended, as it allows for more intuitive exception handling.

Methods that make use ofawaitmust be declared with theasynckeyword. In methods that have a return value of typeTask<T>,methods declared withasyncmust have a return statement of type assignable toTinstead ofTask<T>;the compiler wraps the value in theTask<T>generic. It is also possible toawaitmethods that have a return type ofTaskorTask<T>that are declared withoutasync.

The following async method downloads data from a URL usingawait.Because this method issues a task for each uri before requiring completion with theawaitkeyword, the resources can load at the same time instead of waiting for the last resource to finish before starting to load the next.

publicasyncTask<int>SumPageSizesAsync(IEnumerable<Uri>uris)
{
varclient=newHttpClient();
inttotal=0;
varloadUriTasks=newList<Task<byte[]>>();

foreach(varuriinuris)
{
varloadUriTask=client.GetByteArrayAsync(uri);
loadUriTasks.Add(loadUriTask);
}

foreach(varloadUriTaskinloadUriTasks)
{
statusText.Text=$ "Found {total} bytes...";
varresourceAsBytes=awaitloadUriTask;
total+=resourceAsBytes.Length;
}

statusText.Text=$ "Found {total} bytes total";

returntotal;
}

In Python

Python 3.5 (2015)has added support for async/await as described in PEP 492 (written and implemented byYury Selivanov).

importasyncio

asyncdefmain():
print("hello")
awaitasyncio.sleep(1)
print("world")

asyncio.run(main())

In JavaScript

The await operator in JavaScript can only be used from inside an async function or at the top level of amodule.If the parameter is apromise,execution of the async function will resume when the promise is resolved (unless the promise is rejected, in which case an error will be thrown that can be handled with normal JavaScriptexception handling). If the parameter is not a promise, the parameter itself will be returned immediately.

Many libraries provide promise objects that can also be used with await, as long as they match the specification for native JavaScript promises. However, promises from thejQuerylibrary were not Promises/A+ compatible until jQuery 3.0.

Here's an example (modified from thisarticle):

asyncfunctioncreateNewDoc(){
letresponse=awaitdb.post({});// post a new doc
returndb.get(response.id);// find by id
}

asyncfunctionmain(){
try{
letdoc=awaitcreateNewDoc();
console.log(doc);
}catch(err){
console.log(err);
}
}
main();

Node.jsversion 8 includes a utility that enables using the standard library callback-based methods as promises.

In C++

In C++, await (named co_await in C++) has been officially merged intoversion 20.Support for it, coroutines, and the keywords such asco_awaitare available inGCCandMSVCcompilers whileClanghas partial support.

It is worth noting that std::promise and std::future, although it would seem that they would be awaitable objects, implement none of the machinery required to be returned from coroutines and be awaited using co_await. Programmers must implement a number of public member functions, such asawait_ready,await_suspend,andawait_resumeon the return type in order for the type to be awaited on. Details can be found on cppreference.

#include<iostream>
#include"CustomAwaitableTask.h"

usingnamespacestd;

CustomAwaitableTask<int>add(inta,intb)
{
intc=a+b;
co_returnc;
}

CustomAwaitableTask<int>test()
{
intret=co_awaitadd(1,2);
cout<<"return"<<ret<<endl;
co_returnret;
}

intmain()
{
autotask=test();

return0;
}

In C

TheC languagedoes not support await/async. Some coroutine libraries such as s_tasksimulate the keywords await/async with macros.

#include<stdio.h>
#include"s_task.h"

// define stack memory for tasks
intg_stack_main[64*1024/sizeof(int)];
intg_stack0[64*1024/sizeof(int)];
intg_stack1[64*1024/sizeof(int)];

voidsub_task(__async__,void*arg){
inti;
intn=(int)(size_t)arg;
for(i=0;i<5;++i){
printf("task %d, delay seconds = %d, i = %d\n",n,n,i);
s_task_msleep(__await__,n*1000);
//s_task_yield(__await__);
}
}

voidmain_task(__async__,void*arg){
inti;

// create two sub-tasks
s_task_create(g_stack0,sizeof(g_stack0),sub_task,(void*)1);
s_task_create(g_stack1,sizeof(g_stack1),sub_task,(void*)2);

for(i=0;i<4;++i){
printf("task_main arg = %p, i = %d\n",arg,i);
s_task_yield(__await__);
}

// wait for the sub-tasks for exit
s_task_join(__await__,g_stack0);
s_task_join(__await__,g_stack1);
}

intmain(intargc,char*argv){

s_task_init_system();

//create the main task
s_task_create(g_stack_main,sizeof(g_stack_main),main_task,(void*)(size_t)argc);
s_task_join(__await__,g_stack_main);
printf("all task is over\n");
return0;
}

In Perl 5

The Future::AsyncAwaitmodule was the subject of a Perl Foundation grant in September 2018.

In Rust

On November 7, 2019, async/await was released on the stable version of Rust.Async functions in Rustdesugarto plain functions that return values that implement the Future trait. Currently they are implemented with afinite state machine.

// In the crate's Cargo.toml, we need `futures = "0.3.0" ` in the dependencies section,
// so we can use the futures crate

externcratefutures;// There is no executor currently in the `std` library.

// This desugars to something like
// `fn async_add_one(num: u32) -> impl Future<Output = u32>`
asyncfnasync_add_one(num:u32)->u32{
num+1
}

asyncfnexample_task(){
letnumber=async_add_one(5).await;
println!("5 + 1 = {}",number);
}

fnmain(){
// Creating the Future does not start the execution.
letfuture=example_task();

// The `Future` only executes when we actually poll it, unlike Javascript.
futures::executor::block_on(future);
}

In Swift

Swift 5.5 (2021)added support for async/await as described in SE-0296.

funcgetNumber()asyncthrows->Int{
tryawaitTask.sleep(nanoseconds:1_000_000_000)
return42
}

Task{
letfirst=tryawaitgetNumber()
letsecond=tryawaitgetNumber()
print(first+second)
}

Benefits and criticisms

The async/await pattern is especially attractive to language designers of languages that do not have or control their own runtime, as async/await can be implemented solely as a transformation to astate machinein the compiler.

Supporters claim that asynchronous, non-blocking code can be written with async/await that looks almost like traditional synchronous, blocking code. In particular, it has been argued that await is the best way of writing asynchronous code inmessage-passingprograms; in particular, being close to blocking code, readability and the minimal amount ofboilerplate codewere cited as await benefits.As a result, async/await makes it easier for most programmers to reason about their programs, and await tends to promote better, more robust non-blocking code in applications that require it.

Critics of async/await note that the pattern tends to cause surrounding code to be asynchronous too; and that its contagious nature splits languages' library ecosystems between synchronous and asynchronous libraries and APIs, an issue often referred to as "function coloring".Alternatives to async/await that do not suffer from this issue are called "colorless". Examples of colorless designs include Go'sgoroutinesand Java'svirtual threads.

See also

• Coroutines
• Continuation-passing style
• Direct style
• Cooperative multitasking