I'm trying to execute several sqlx queries in parallel given by a iterator.
This is probably the closest I've got so far.
let mut futures = HahshMap::new() // placeholder, filled HashMap in reality
.iter()
.map(async move |(_, item)| -> Result<(), sqlx::Error> {
let result = sqlx::query_file_as!(
// omitted
)
.fetch_one(&pool)
.await?;
channel.send(Enum::Event(result)).ignore();
Ok(())
})
.clollect();
futures::future::join_all(futures);
All queries and sends are independent from each other, so if one of them fails, the others should still get processed.
Futthermore the current async closure is not possible like this.
Rust doesn't yet have async closures. You instead need to have the closure return an async block:
move |(_, item)| async move { ... }
Additionally, make sure you .await the future returned by join_all in order to ensure the individual tasks are actually polled.
In JavaScript, async code is written with Promises and async/await syntax similar to that of Rust. It is generally considered redundant (and therefore discouraged) to return and await a Promise when it can simply be returned (i.e., when an async function is executed as the last thing in another function):
async function myFn() { /* ... */ }
async function myFn2() {
// do setup work
return await myFn()
// ^ this is not necessary when we can just return the Promise
}
I am wondering whether a similar pattern applies in Rust. Should I prefer this:
pub async fn my_function(
&mut self,
) -> Result<()> {
// do synchronous setup work
self.exec_command(
/* ... */
)
.await
}
Or this:
pub fn my_function(
&mut self,
) -> impl Future<Output = Result<()>> {
// do synchronous setup work
self.exec_command(
/* ... */
)
}
The former feels more ergonomic to me, but I suspect that the latter might be more performant. Is this the case?
One semantic difference between the two variants is that in the first variant the synchronous setup code will run only when the returned future is awaited, while in the second variant it will run as soon as the function is called:
let fut = x.my_function();
// in the second variant, the synchronous setup has finished by now
...
let val = fut.await; // in the first variant, it runs here
For the difference to be noticeable, the synchronous setup code must have side effects, and there needs to be a delay between calling the async function and awaiting the future it returns.
Unless you have specific reason to execute the preamble immediately, go with the async function, i.e. the first variant. It makes the function slightly more predictable, and makes it easier to add more awaits later as the function is refactored.
There is no real difference between the two since async just resolves down to impl Future<Output=Result<T, E>>. I don't believe there is any meaningful performance difference between the two, at least in my empirical usage of both.
If you are asking for preference in style then in my opinion the first one is preferred as the types are clearer to me and I agree it is more ergonomic.
I have an async method that uses tokio::fs to explore a directory:
use failure::Error;
use futures::Future;
use std::path::PathBuf;
use tokio::prelude::*;
fn visit_async(path: PathBuf) -> Box<Future<Item = (), Error = Error> + Send> {
let task = tokio::fs::read_dir(path)
.flatten_stream()
.for_each(move |entry| {
let path = entry.path();
if path.is_dir() {
let task = visit_async(entry.path());
tokio::spawn(task.map_err(drop));
} else {
println!("File: {:?}", path);
}
future::ok(())
})
.map_err(Error::from);
Box::new(task)
}
I need to execute another future after all the the future returned by this method ends as well as all the tasks spawned by it. Is there a better way that just starting another runtime?
let t = visit_async(PathBuf::from(".")).map_err(drop);
tokio::run(t);
tokio::run(future::ok(()));
I'd strive to avoid using tokio::spawn() here, and try to wrap it all into a single future (in general, I think you only do tokio::spawn when you don't care about the result or execution, which we do here). That should make it easy to wait for completion. I haven't tested this, but something along these lines might do the trick:
let task = tokio::fs::read_dir(path)
.flatten_stream()
.for_each(move |entry| {
let path = entry.path();
if path.is_dir() {
let task = visit_async(entry.path());
future::Either::A(task)
} else {
println!("File: {:?}", path);
future::Either::B(future::ok(()))
}
})
.map_err(Error::from)
.and_then(|_| {
// Do some work once all tasks complete
});
Box::new(task)
This will cause the asynchronous tasks to execute in sequence. You could use and_then instead of for_each to execute them in parallel and then into_future().and_then(|_| { ... }) to tuck on some action to execute afterwards.
There's another issue with parallel descent in the FS: you may run out of file descriptors.
There is a way to solve both issues by creating a tokio::sync::Semaphore to limit the concurrent number of these tasks. After you are done spawning all of them, you can use Semaphore::acquire_many with the same value you used at creation, to block until all other tasks are finished.
For correctness, you should acquire the semaphore before spawning the task, and then pass the SemaphorePermit to the task (and make sure it doesn't get dropped before you are done). If you acquire the semaphore inside the tasks, there is a risk the main task might acquire all the permits before the first sub-task has a chance to run.
Since you can only move a SemaphorePermit<'static> inside the task, you will need to have a &'static Semaphore, for instance using lazy_static! or Box::leak.
I'm facing a problem with async methods and Future in Dart.
I think I did/understood something wrong but I don't know what.
I'm trying to figure out the difference between Future and async and understand how the event loop works. I read the documentation and a lot of articles about that. I thought I understood so I tried to write some code that create a Future object with a sleep() call in it.
First, I tried with Future and I think it's behaving like it should:
main(List<String> arguments) {
print('before future');
test_future();
print('after future');
}
test_future() {
Future (() {
print('Future active before 5 seconds call');
sleep(Duration(seconds: 5));
print('Future active after 5 seconds call');
}).then((_) => print("Future completed"));
}
So this returns:
print before future
create a future object, put it in the event queue and return immediately
print after future
call the code of the future from the event queue
print before 5 seconds
wait 5 seconds
print after 5 seconds*
print future completed
I think all of this is normal.
Now, i'm trying to do the same with async. From the documentation, adding the async keyword to a function make it return a Future immediately.
So I wrote this:
main(List<String> arguments) {
print('before future 2');
test().then((_) => print("Future completed 2"));
print('after future 2');
}
test() async {
print('Future active before 5 seconds call');
sleep(Duration(seconds: 5));
print('Future active after 5 seconds call');
}
Normally, when calling test().then(), it should put the content of test() in the event queue and return a Future immediately but no. The behavior is this one:
print before future 2
call test() function (should return a future I think, but the code is executed right now)
print before 5 seconds
wait for 5 seconds
print after 5 seconds
print future completed 2
print after future 2
Can someone explain if I did not use async properly or if there is something wrong ?
Best
You should be aware that sleep() just blocks the whole program. sleep() is not related in any way to the event loop or async execution. Perhaps you want to use instead:
await Future.delayed(const Duration(seconds: 5), (){});
async system calls do not block the isolate. The event queue is still processed, (continues immediately after invoking the system call). If you make sync system calls, they block like sleep.
There are often sync and async variants in dart:io of system calls like api.dartlang.org/stable/2.2.0/dart-io/File/readAsLinesSync.html. Even though sleep does not have the sync suffix, it's sync and no way to work around. You can use Future.delayed() as shown above to get the effect in an async way.
In the kotlinx.coroutines library you can start new coroutine using either launch (with join) or async (with await). What is the difference between them?
launch is used to fire and forget coroutine. It is like starting a new thread. If the code inside the launch terminates with exception, then it is treated like uncaught exception in a thread -- usually printed to stderr in backend JVM applications and crashes Android applications. join is used to wait for completion of the launched coroutine and it does not propagate its exception. However, a crashed child coroutine cancels its parent with the corresponding exception, too.
async is used to start a coroutine that computes some result. The result is represented by an instance of Deferred and you must use await on it. An uncaught exception inside the async code is stored inside the resulting Deferred and is not delivered anywhere else, it will get silently dropped unless processed. You MUST NOT forget about the coroutine you’ve started with async.
I find this guide to be useful. I will quote the essential parts.
🦄 Coroutines
Essentially, coroutines are light-weight threads.
So you can think of a coroutine as something that manages thread in a very efficient way.
🐤 launch
fun main(args: Array<String>) {
launch { // launch new coroutine in background and continue
delay(1000L) // non-blocking delay for 1 second (default time unit is ms)
println("World!") // print after delay
}
println("Hello,") // main thread continues while coroutine is delayed
Thread.sleep(2000L) // block main thread for 2 seconds to keep JVM alive
}
So launch starts a coroutine, does something, and returns a token immediately as Job. You can call join on this Job to block until this launch coroutine completes.
fun main(args: Array<String>) = runBlocking<Unit> {
val job = launch { // launch new coroutine and keep a reference to its Job
delay(1000L)
println("World!")
}
println("Hello,")
job.join() // wait until child coroutine completes
}
🦆 async
Conceptually, async is just like launch. It starts a separate coroutine which is a light-weight thread that works concurrently with all the other coroutines. The difference is that launch returns a Job and does not carry any resulting value, while async returns a Deferred -- a light-weight non-blocking future that represents a promise to provide a result later.
So async starts a background thread, does something, and returns a token immediately as Deferred.
fun main(args: Array<String>) = runBlocking<Unit> {
val time = measureTimeMillis {
val one = async { doSomethingUsefulOne() }
val two = async { doSomethingUsefulTwo() }
println("The answer is ${one.await() + two.await()}")
}
println("Completed in $time ms")
}
You can use .await() on a deferred value to get its eventual result, but Deferred is also a Job, so you can cancel it if needed.
So Deferred is actually a Job. Read this for more details.
interface Deferred<out T> : Job (source)
🦋 async is eager by default
There is a laziness option to async using an optional start parameter with a value of CoroutineStart.LAZY. It starts coroutine only when its result is needed by some await or if a start function is invoked.
launch and async are used to start new coroutines. But, they execute them in different manner.
I would like to show very basic example which will help you understand difference very easily
launch
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
btnCount.setOnClickListener {
pgBar.visibility = View.VISIBLE
CoroutineScope(Dispatchers.Main).launch {
val currentMillis = System.currentTimeMillis()
val retVal1 = downloadTask1()
val retVal2 = downloadTask2()
val retVal3 = downloadTask3()
Toast.makeText(applicationContext, "All tasks downloaded! ${retVal1}, ${retVal2}, ${retVal3} in ${(System.currentTimeMillis() - currentMillis)/1000} seconds", Toast.LENGTH_LONG).show();
pgBar.visibility = View.GONE
}
}
// Task 1 will take 5 seconds to complete download
private suspend fun downloadTask1() : String {
kotlinx.coroutines.delay(5000);
return "Complete";
}
// Task 1 will take 8 seconds to complete download
private suspend fun downloadTask2() : Int {
kotlinx.coroutines.delay(8000);
return 100;
}
// Task 1 will take 5 seconds to complete download
private suspend fun downloadTask3() : Float {
kotlinx.coroutines.delay(5000);
return 4.0f;
}
}
In this example, my code is downloading 3 data on click of btnCount button and showing pgBar progress bar until all download gets completed. There are 3 suspend functions downloadTask1(), downloadTask2() and downloadTask3() which downloads data. To simulate it, I've used delay() in these functions. These functions waits for 5 seconds, 8 seconds and 5 seconds respectively.
As we've used launch for starting these suspend functions, launch will execute them sequentially (one-by-one). This means that, downloadTask2() would start after downloadTask1() gets completed and downloadTask3() would start only after downloadTask2() gets completed.
As in output screenshot Toast, total execution time to complete all 3 downloads would lead to 5 seconds + 8 seconds + 5 seconds = 18 seconds with launch
async
As we saw that launch makes execution sequentially for all 3 tasks. The time to complete all tasks was 18 seconds.
If those tasks are independent and if they do not need other task's computation result, we can make them run concurrently. They would start at same time and run concurrently in background. This can be done with async.
async returns an instance of Deffered<T> type, where T is type of data our suspend function returns. For example,
downloadTask1() would return Deferred<String> as String is return type of function
downloadTask2() would return Deferred<Int> as Int is return type of function
downloadTask3() would return Deferred<Float> as Float is return type of function
We can use the return object from async of type Deferred<T> to get the returned value in T type. That can be done with await() call. Check below code for example
btnCount.setOnClickListener {
pgBar.visibility = View.VISIBLE
CoroutineScope(Dispatchers.Main).launch {
val currentMillis = System.currentTimeMillis()
val retVal1 = async(Dispatchers.IO) { downloadTask1() }
val retVal2 = async(Dispatchers.IO) { downloadTask2() }
val retVal3 = async(Dispatchers.IO) { downloadTask3() }
Toast.makeText(applicationContext, "All tasks downloaded! ${retVal1.await()}, ${retVal2.await()}, ${retVal3.await()} in ${(System.currentTimeMillis() - currentMillis)/1000} seconds", Toast.LENGTH_LONG).show();
pgBar.visibility = View.GONE
}
This way, we've launched all 3 tasks concurrently. So, my total execution time to complete would be only 8 seconds which is time for downloadTask2() as it is largest of all of 3 tasks. You can see this in following screenshot in Toast message
both coroutine builders namely launch and async are basically lambdas with receiver of type CoroutineScope which means their inner block is compiled as a suspend function, hence they both run in an asynchronous mode AND they both will execute their block sequentially.
The difference between launch and async is that they enable two different possibilities. The launch builder returns a Job however the async function will return a Deferred object. You can use launch to execute a block that you don't expect any returned value from it i.e writing to a database or saving a file or processing something basically just called for its side effect. On the other hand async which return a Deferred as I stated previously returns a useful value from the execution of its block, an object that wraps your data, so you can use it for mainly its result but possibly for its side effect as well. N.B: you can strip the deferred and get its value using the function await, which will block the execution of your statements until a value is returned or an exceptions is thrown! You could achieve the same thing with launch by using the function join()
both coroutine builder (launch and async) are cancelable.
anything more?: yep with launch if an exception is thrown within its block, the coroutine is automatically canceled and the exceptions is delivered. On the other hand, if that happens with async the exception is not propagated further and should be caught/handled within the returned Deferred object.
more on coroutines https://kotlinlang.org/docs/tutorials/coroutines/coroutines-basic-jvm.html
https://www.codementor.io/blog/kotlin-coroutines-6n53p8cbn1
Async and Launch, both are used to create coroutines that run in the background. In almost every situation one can use either of them.
tl;dr version:
When you dont care about the task's return value, and just want to run it, you may use Launch. If you need the return type from the task/coroutine you should use async.
Alternate:
However, I feel the above difference/approach is a consequence of thinking in terms of Java/one thread per request model. Coroutines are so inexpensive, that if you want to do something from the return value of some task/coroutine(lets say a service call) you are better off creating a new coroutine from that one. If you want a coroutine to wait for another coroutine to transfer some data, I would recommend using channels and not the return value from Deferred object. Using channels and creating as much number of coroutines as required, is the better way IMO
Detailed answer:
The only difference is in the return type and what functionality it provides.
Launch returns Job while Async returns Deferred. Interestingly enough, Deferred extends Job. Which implies it must be providing additional functionality on top of Job. Deferred is type parameterised over where T is the return type. Thus, Deferred object can return some response from the block of code executed by async method.
p.s. I only wrote this answer because I saw some factually incorrect answers on this question and wanted to clarify the concept for everyone. Also, while working on a pet project myself I faced similar problem because of previous Java background.
launch returns a job
async returns a result (deferred job)
launch with join is used to wait until the job gets finished. It simply suspends the coroutine calling join(), leaving the current thread free to do other work (like executing another coroutine) in the meantime.
async is used to compute some results. It creates a coroutine and returns its future result as an implementation of Deferred. The running coroutine is cancelled when the resulting deferred is cancelled.
Consider an async method that returns a string value. If the async method is used without await it will return a Deferred string but if await is used you will get a string as the result
The key difference between async and launch:
Deferred returns a particular value of type T after your Coroutine finishes executing, whereas Job doesn’t.
launch / async no result
Use when don't need the result,
Don't block the code where is called,
Run in sequential
async for result
When you need to wait for the result and can run in parallel for
efficiency,
Block the code where is called,
Run in concurrent
Alongside the other great answers, for the people familiar with Rx and getting into coroutines, async returns a Deferred which is akin to Single while launch returns a Job that is more akin to Completable. You can .await() to block and get the value of the first one, and .join() to block until the Job is completed.