Async.SwitchSynchronizationContext allows an Async action to switch to running within a given SynchronizationContext. I would like to synchronously begin an Async computation within a SynchronizationContext, rather than switching inside the Async.
This would ensure that Async actions are run in the desired order, and that they are not run concurrently.
Is this possible? The documentation for Async.SwitchSynchronizationContext mentions using SynchronizationContext.Post, but no such functionality is exposed for Async.
I have not tested this, but I think the easiest way to achieve what you want is to combine the SynchronizationContext.Send method (mentioned in the comments) with the Async.StartImmediate operation. The former lets you start some work synchronously in the synchronization context. The latter lets you start an async workflow in the current context.
If you combine the two, you can define a helper that starts a function in a given synchronization context and, in this function, immediately starts an async workflow:
let startInContext (sync:SynchronizationContext) work =
SynchronizationContext.Current.Send((fun _ ->
Async.StartImmediate(work)), null)
For example:
async {
printfn "Running on the given sync context"
do! Async.Sleep(1000)
printfn "Should be back on the original sync context" }
|> startInContext SynchronizationContext.Current
Related
In F#, I'm implementing an interface that returns Async<'T>, but my implementation does not require any async method calls. Here's an example of my current implementation:
type CrappyIdGeneratorImpl() =
interface IdGenerator with
member this.Generate(): Async<Id> = async {
return System.Guid.NewGuid().ToString()
}
Is this the "best" way to wrap a non-Async<> value in Async<>, or is there a better way?
Put another way, I'm looking for the Async equivalent of System.Threading.Tasks.Task.FromResult().
Yes, that's the best way.
Note that, as stated in the comment, another way would be to do async.Return but it's not exactly the same thing, and in your use case it won't work, due to eager evaluation of the argument.
In your case you're interested in calling the .NewGuid() method each time the async is run, but if you use async.Return alone it will be evaluated once and the async computation will be created with that result as a fixed value.
In fact the equivalent of the CE is async.Delay (fun () -> async.Return (.. your expression ..))
UPDATE
As Tarmil noted, this can be interpreted as intended, given that the code resides in a method.
It might be the case that you created a method because that's the way you do with tasks, but Asyncs can be created and called independently (hot vs cold async models).
The question of whether the intention is delaying the async or creating an async at each method call is not entirely clear to me, but whichever is the case, the explanation above about each approach is still valid.
I just started learning asynchronous Rust, so this is propably not a difficult question to answer, however, I am scratching my head here.
I am not trying to run tasks in parallel yet, only trying to get them to run concurrently.
According to the guide at https://rust-lang.github.io/async-book/,
The futures::join macro makes it possible to wait for multiple different futures to complete while executing them all concurrently.
So when I create 2 Futures, I should be able to "await" both of them at once. It also states that
Whereas calling a blocking function in a synchronous method would block the whole thread, blocked Futures will yield control of the thread, allowing other Futures to run.
From what I understand here, if I await multiple Futures with join!, should the first one be blocked, the second one will start running.
So I made a very simple example where I created 2 async fns and tried to join! both, making sure the first one gets blocked. I used a mpsc::channel for the blocking, since the docs stated that thread::sleep() should not be used in async fns and that recv()
will always block the current thread if there is no data available
However, the behavior is not what I expected, as calling the blocking function will not yield control of the thread, allowing the other Future to run, like I would expect from the second quote I provided. Instead, it will just wait untill it is no longer blocked, finish the first Future and only then start the second. Pretty much as if they were synchronous and I would have just called one after the other.
My complete example code:
use std::{thread::{self}, sync::{mpsc::{self, Sender, Receiver}}, time::Duration};
use futures::{executor}; //added futures = "0.3" in cargo.toml dependencies
fn main(){
let fut = main_async();
executor::block_on(fut);
}
async fn main_async(){
let (sender, receiver) = mpsc::channel();
let thread_handle = std::thread::spawn(move || { //this thread is just here so the f1 function gets blocked by something and can later resume
wait_send_function(sender);
});
let f1 = f1(receiver);
let f2 = f2();
futures::join!(f1, f2);
thread_handle.join().unwrap();
}
fn wait_send_function(sender: Sender<i32>){
thread::sleep(Duration::from_millis(5000));
sender.send(1234).unwrap();
}
async fn f1(receiver: Receiver<i32>){
println!("starting f1");
let new_nmbr = receiver.recv().unwrap(); //I would expect f2 to start now, since this is blocking
println!("Received nmbr is: {}", new_nmbr);
}
async fn f2(){
println!("starting f2");
}
And the output is simply:
starting f1
Received nmbr is: 1234
starting f2
My question is what am I missing here, why does f2 only start after f1 is completed and what would I need to do to get the behavior I want (completing f2 first if f1 is blocked and then waiting for f1)?
Maybe the book is a little misleading, but when it refers to "a blocked future", it does not mean in the sense of blocking synchronous code (if that was the case, there would be no problem to use std::thread::sleep()), but rather, it means that the future is waiting to be polled by the executor.
Thus, std::mpsc that blocks the thread will not have the desired effect (definitely not on a single-threaded executor like future's, but it's a bad idea on multi-threaded executors too). Use futures::channel::mpsc and everything will work.
Boost's asio library allows the serialisation of asynchronous code in the following way. Handlers to asynchronous functions such as those which read from a stream, may be associated to a strand. A strand is associated with an "IO context". An IO context owns a thread pool. However many threads in the pool, it is guaranteed that no two handlers associated with the same strand are run concurrently. This makes it possible, for instance, to implement a state machine as if it were single-threaded, where all handlers for that machine serialise over a private strand.
I have been trying to figure out how this might be done with F#'s Async. I could not find any way to make sure that chosen sets of Async processes never run concurrently. Can anyone suggest how to do this?
It would be useful to know what is the use case that you are trying to implement. I don't think F# async has anything that would directly map to strands and you would likely use different techniques for implementing different things that might all be implemented using strands.
For example, if you are concerend with reading data from a stream, F# async block lets you write code that is asynchronous but sequential. The following runs a single logical process (which might be moved between threads of a thread pool when you wait using let!):
let readTest () = async {
let fs = File.OpenRead(#"C:\Temp\test.fs")
let buffer = Array.zeroCreate 10
let mutable read = 1
while read <> 0 do
let! r = fs.AsyncRead(buffer, 0, 10)
printfn "Read: %A" buffer.[0 .. r-1]
read <- r }
readTest() |> Async.Start
If you wanted to deal with events that occur without any control (i.e. push based rather than pull based), for example, when you cannot ask the system to read next buffer of data, you could serialize the events using a MailboxProcessor. The following sends two messages to the agent almost at the same time, but they are processed sequentially, with 1 second delay:
let agent = MailboxProcessor.Start(fun inbox -> async {
while true do
let! msg = inbox.Receive()
printfn "Got: %s" msg
do! Async.Sleep(1000)
})
agent.Post("hello")
agent.Post("world")
I am struggling to understand Async.[StartChild|Start] API design.
What I would like is to start an async process which does some tcp stream reading and calling a callback according to commands arriving on tcp.
As this async process does not really return any single value, it seems like I should use Async.Start. At some point I want to "close" my tcp client and `Async.Start takes CancellationToken, which gives me ability to implement 'close". So far so good.
The problem is, I would like to know when tcp client is done with cancellation. There is some buffer flushing work done, once Cancel is requested, so I do not want to terminate application before tcp client is done cleanup. But Async.Start returns unit, which means I have no way of knowing when such async process is complete. So, looks like Async.StartChild should help. I should be able to invoke cancellation, and when cleanup is done, this async will invoke next contiuation in chain (or throw an exception?). But... Async.StartChild does not take CancellationToken, only timeout.
Why Async.StartChild implements just single case of cancellation strategy (timeout) instead of exposing more generic way (accept CancellationToken)?
To answer the first part of the question - if you need to do some cleanup work, you can just put it in finally and it will be called when the workflow is cancelled. For example:
let work =
async {
try
printfn "first work"
do! Async.Sleep 1000
printfn "second work"
finally
printfn "cleanup" }
Say you run this using Async.Start, wait for 500ms and then cancel the computation:
let cts = new System.Threading.CancellationTokenSource()
Async.Start(work, cts.Token)
System.Threading.Thread.Sleep(500)
cts.Cancel()
The output will be "first work, cleanup". As you can see, cancelling the computation will run all the finally clauses.
To answer the second part of the question - if you need to wait until the work completes, you can use RunSynchronously (but then, perhaps you do not actually need asynchronous workflows, if you are blocking anyway...).
The following starts a background process that cancels the main work after 500ms and then starts the main work synchronously:
let cts = new System.Threading.CancellationTokenSource()
async {
do! Async.Sleep(500)
cts.Cancel() } |> Async.Start
try Async.RunSynchronously(work, cancellationToken=cts.Token)
with :? System.OperationCanceledException -> ()
printfn "completed"
This prints "first work, cleanup, completed" - as you can see, the RunSynchronously call was blocked until the work was cancelled.
To execute operations on a background thread and avoid blocking the UI in a WPF application, I often find myself writing this pattern:
async {
// some code on the UI thread
let uiThread = SynchronizationContext.Current
do! Async.SwitchToThreadPool()
let! result = // some Async<'t>
do! Async.SwitchToContext uiThread
// do things with the result if it wasn't () all along
}
Am I doing this right at all? Is this idiomatic? Should it be done differently?
If this is correct, of course I would prefer not to have to do it like that all the time - is there a built-in shorter way to achieve the same thing? None of the existing Async functions appears to do something like that.
If not, does it make sense to just turn the above code into a function?
let onThreadPool operation =
async {
let context = SynchronizationContext.Current
do! Async.SwitchToThreadPool()
let! result = operation
do! Async.SwitchToContext context
return result
}
That adds another level of async { } nesting - can this cause issues at "some" point?
What you're doing here definitely makes sense. One useful operation here is Async.StartImmediate, which starts the async workflow on the current thread. If you call this from the UI thread, this guarantees that the workflow will also start on the UI thread and so you can capture the synchronization context inside the workflow.
The other trick is that many built-in asynchronous F# operations automatically jump back to the original synchronization context (those that are created using Async.FromContinuations, including e.g. AsyncDownloadString), so when you're calling one of those, you do not even need to explicitly jump back to the original synchronization context.
But for other asynchronous operations (and for non-async operations that you want to run in the background), your onThreadPool function looks like a great way of doing this.
#random-dev is right capturing the context must happen outside the workflow
let onThreadPool operation =
let context = SynchronizationContext.Current
async {
do! Async.SwitchToThreadPool()
let! result = operation
do! Async.SwitchToContext context
return result
}