Background.
I am trying to figure out MailboxProcessor. The idea is to use it as a some kind of state machine and pass arguments around between the states and then quit. Some parts are going to have async communication so I made a Sleep there.
It's a console application, making a Post does nothing because main thread quits and kills everything behind it. I am making a PostAndReply in main.
Also, I have tried without
let sleepWorkflow = async
, doesn't make any difference.
Questions.
(I am probably doing something wrong)
Go24 is not async. Changing RunSynchronously to StartImmediate makes no visible difference. The end should be somewhere below GetMe instead. At the same time Done is printed after Fetch. Isn't the control supposed t be returned to the main thread on sleep?
Go24, wait
go24 1, end
Fetch 1
Done
GetMe
...
Run time is terrible slow. Without delay in Fetch it's about 10s (stopwatch). I thought F# threads are lightweight and should use threadpool.
According to debugger it takes appr 1s to create every and it looks like real threads.
Also, changing to [1..100] will "pause" the program for 100s, according to ProcessExplorer 100 threads are created during that time and only then everything is printed. I would actually prefer fewer threads and slow increase.
Code.
Program.fs
[<EntryPoint>]
let main argv =
let a = Mailbox.MessageBasedCounter.DoGo24 1
let a = Mailbox.MessageBasedCounter.DoFetch 1
let b = Mailbox.MessageBasedCounter.GetMe
let task i = async {
//Mailbox.MessageBasedCounter.DoGo24 1
let a = Mailbox.MessageBasedCounter.DoFetch i
return a
}
let stopWatch = System.Diagnostics.Stopwatch.StartNew()
let x =
[1..10]
|> Seq.map task
|> Async.Parallel
|> Async.RunSynchronously
stopWatch.Stop()
printfn "%f" stopWatch.Elapsed.TotalMilliseconds
printfn "a: %A" a
printfn "b: %A" b
printfn "x: %A" x
0 // return an integer exit code
Mailbox.fs
module Mailbox
#nowarn "40"
type parserMsg =
| Go24 of int
| Done
| Fetch of int * AsyncReplyChannel<string>
| GetMe of AsyncReplyChannel<string>
type MessageBasedCounter () =
/// Create the agent
static let agent = MailboxProcessor.Start(fun inbox ->
// the message processing function
let rec messageLoop() = async{
let! msg = inbox.Receive()
match msg with
| Go24 n ->
let sleepWorkflow = async{
printfn "Go24, wait"
do! Async.Sleep 4000
MessageBasedCounter.DoDone() // POST Done.
printfn "go24 %d, end" n
return! messageLoop()
}
Async.RunSynchronously sleepWorkflow
| Fetch (i, repl) ->
let sync = async{
printfn "Fetch %d" i
do! Async.Sleep 1000
repl.Reply( "Reply Fetch " + i.ToString() ) // Reply to the caller
return! messageLoop()
}
Async.RunSynchronously sync
| GetMe (repl) ->
let sync = async{
printfn "GetMe"
repl.Reply( "GetMe" ) // Reply to the caller
return! messageLoop()
}
Async.RunSynchronously sync
| Done ->
let sync = async{
printfn "Done"
return! messageLoop()
}
Async.RunSynchronously sync
}
// start the loop
messageLoop()
)
// public interface to hide the implementation
static member DoDone () = agent.Post( Done )
static member DoGo24 (i:int) = agent.Post( Go24(i) )
static member DoFetch (i:int) = agent.PostAndReply( fun reply -> Fetch(i, reply) )
static member GetMe = agent.PostAndReply( GetMe )
I'm not necessarily sure that this is the main problem, but the nested asyncs and Async.RunSynchrously in the agent code look suspicious.
You do not need to create a nested async - you can just call asynchronous operations in the body of the match clauses directly:
// the message processing function
let rec messageLoop() = async{
let! msg = inbox.Receive()
match msg with
| Go24 n ->
printfn "Go24, wait"
do! Async.Sleep 4000
MessageBasedCounter.DoDone()
printfn "go24 %d, end" n
return! messageLoop()
| Fetch (i, repl) ->
(...)
Aside from that, it is important to understand that the agent has exactly one instance of the body computation running. So, if you block the body of the agent, all other operations will be queued.
If you want to start some task (like the synchronous operations) in the background and resume the agent immediately, you can use Async.Start inside the body (but be sure to call the main loop recursively in the main part of the body):
| Go24 n ->
// Create work item that will run in the background
let work = async {
printfn "Go24, wait"
do! Async.Sleep 4000
MessageBasedCounter.DoDone()
printfn "go24 %d, end" n }
// Queue the work in a thread pool to be processed
Async.Start(work)
// Continue the message loop, waiting for other messages
return! messageLoop()
Related
Building on a snippet and answer, would it be possible to return results to the caller from the throttling queue? I've tried PostAndAsyncReply to receive reply on a channel but it's throwing an error if I pipe it with Enqueue. Here's the code.
Appreciate a F# core vanilla based solution around Queue or Mailbox design patterns.
Question
The question is to be able to call functions asynchronously based on the throttle (max 3 at a time), passing each item from the array, wait on the whole queue/array until it's finished while collecting all the results and then return the results to the caller. (Return the results to the caller is what's pending in here)
Callee Code
// Message type used by the agent - contains queueing
// of work items and notification of completion
type ThrottlingAgentMessage =
| Completed
| Enqueue of Async<unit>
/// Represents an agent that runs operations in concurrently. When the number
/// of concurrent operations exceeds 'limit', they are queued and processed later
let throttlingAgent limit =
MailboxProcessor.Start(fun inbox ->
async {
// The agent body is not executing in parallel,
// so we can safely use mutable queue & counter
let queue = System.Collections.Generic.Queue<Async<unit>>()
let running = ref 0
while true do
// Enqueue new work items or decrement the counter
// of how many tasks are running in the background
let! msg = inbox.Receive()
match msg with
| Completed -> decr running
| Enqueue w -> queue.Enqueue(w)
// If we have less than limit & there is some work to
// do, then start the work in the background!
while running.Value < limit && queue.Count > 0 do
let work = queue.Dequeue()
incr running
do! // When the work completes, send 'Completed'
// back to the agent to free a slot
async {
do! work
inbox.Post(Completed)
}
|> Async.StartChild
|> Async.Ignore
})
let requestDetailAsync (url: string) : Async<Result<string, Error>> =
async {
Console.WriteLine ("Simulating request " + url)
try
do! Async.Sleep(1000) // let's say each request takes about a second
return Ok (url + ":body...")
with :? WebException as e ->
return Error {Code = "500"; Message = "Internal Server Error"; Status = HttpStatusCode.InternalServerError}
}
let requestMasterAsync() : Async<Result<System.Collections.Concurrent.ConcurrentBag<_>, Error>> =
async {
let urls = [|
"http://www.example.com/1";
"http://www.example.com/2";
"http://www.example.com/3";
"http://www.example.com/4";
"http://www.example.com/5";
"http://www.example.com/6";
"http://www.example.com/7";
"http://www.example.com/8";
"http://www.example.com/9";
"http://www.example.com/10";
|]
let results = System.Collections.Concurrent.ConcurrentBag<_>()
let agent = throttlingAgent 3
for url in urls do
async {
let! res = requestDetailAsync url
results.Add res
}
|> Enqueue
|> agent.Post
return Ok results
}
Caller Code
[<TestMethod>]
member this.TestRequestMasterAsync() =
match Entity.requestMasterAsync() |> Async.RunSynchronously with
| Ok result -> Console.WriteLine result
| Error error -> Console.WriteLine error
You could use Hopac.Streams for that. With such tool it is pretty trivial:
open Hopac
open Hopac.Stream
open System
let requestDetailAsync url = async {
Console.WriteLine ("Simulating request " + url)
try
do! Async.Sleep(1000) // let's say each request takes about a second
return Ok (url + ":body...")
with :? Exception as e ->
return Error e
}
let requestMasterAsync() : Stream<Result<string,exn>> =
[| "http://www.example.com/1"
"http://www.example.com/2"
"http://www.example.com/3"
"http://www.example.com/4"
"http://www.example.com/5"
"http://www.example.com/6"
"http://www.example.com/7"
"http://www.example.com/8"
"http://www.example.com/9"
"http://www.example.com/10" |]
|> Stream.ofSeq
|> Stream.mapPipelinedJob 3 (requestDetailAsync >> Job.fromAsync)
requestMasterAsync()
|> Stream.iterFun (printfn "%A")
|> queue //prints all results asynchronously
let allResults : Result<string,exn> list =
requestMasterAsync()
|> Stream.foldFun (fun results cur -> cur::results ) []
|> run //fold stream into list synchronously
ADDED
In case you want to use only vanilla FSharp.Core with mailboxes only try this:
type ThrottlingAgentMessage =
| Completed
| Enqueue of Async<unit>
let inline (>>=) x f = async.Bind(x, f)
let inline (>>-) x f = async.Bind(x, f >> async.Return)
let throttlingAgent limit =
let agent = MailboxProcessor.Start(fun inbox ->
let queue = System.Collections.Generic.Queue<Async<unit>>()
let startWork work =
work
>>- fun _ -> inbox.Post Completed
|> Async.StartChild |> Async.Ignore
let rec loop curWorkers =
inbox.Receive()
>>= function
| Completed when queue.Count > 0 ->
queue.Dequeue() |> startWork
>>= fun _ -> loop curWorkers
| Completed ->
loop (curWorkers - 1)
| Enqueue w when curWorkers < limit ->
w |> startWork
>>= fun _ -> loop (curWorkers + 1)
| Enqueue w ->
queue.Enqueue w
loop curWorkers
loop 0)
Enqueue >> agent.Post
It is pretty much the same logic, but slightly optimized to not use queue if there is free worker capacity (just start job and don't bother with queue/dequeue).
throttlingAgent is a function int -> Async<unit> -> unit
Because we don't want client to bother with our internal ThrottlingAgentMessage type.
use like this:
let throttler = throttlingAgent 3
for url in urls do
async {
let! res = requestDetailAsync url
results.Add res
}
|> throttler
I'm trying to make an async workflow, where there's a main async loop, which executes an async sub-block in each loop. And I want this async sub-block to be cancellable, but when it cancels then I don't want the main loop to cancel. I want it to continue, at the line after the do! subBlock.
The only method I see in Async that even has an acceptable signature (takes CancellationToken, returns something that can be converted to async) is Async.StartAsTask, but that seems to hang when canceled; in the below, it prints "cancelled" and then nothing else.
open System
open System.Threading
open System.Threading.Tasks
// runs until cancelled
let subBlock =
async {
try
while true do
printfn "doing it"
do! Async.Sleep 1000
printfn "did it"
finally
printfn "cancelled!"
}
[<EntryPoint>]
let main argv =
let ctsRef = ref <| new CancellationTokenSource()
let mainBlock =
//calls subBlock in a loop
async {
while true do
ctsRef := new CancellationTokenSource()
do! Async.StartAsTask(subBlock, TaskCreationOptions.None, (!ctsRef).Token)
|> Async.AwaitTask
printfn "restarting"
}
Async.Start mainBlock
//loop to cancel CTS at each keypress
while true do
Console.ReadLine() |> ignore
(!ctsRef).Cancel()
0
Is there any way to do this?
Whether the caller that starts and cancels the worker is an async too doesn't really affect this problem, since the worker is managed via its explicitly specified cancellation token.
Asyncs have three continutations: the normal one, which can return a value, one for exceptions, and one for cancellation. There are multiple ways to add a cancellation continuation to an async, such as Async.OnCancel, Async.TryCancelled, or the general Async.FromContinuations, which includes the exception case. Here's a program that has the desired output:
let rec doBlocks () =
async { printfn "doing it"
do! Async.Sleep 1000
printfn "did it"
do! doBlocks () }
let rec runMain () =
use cts = new CancellationTokenSource()
let worker = Async.TryCancelled(doBlocks (), fun _ -> printfn "Cancelled")
Async.Start(worker, cts.Token)
let k = Console.ReadKey(true)
cts.Cancel()
if k.Key <> ConsoleKey.Q then runMain ()
This works just as well if runMain is an async. In this simple case, you could also just have it print the "cancelled" message itself.
I hope this helps. I don't think there is a general answer to how to structure the program; that depends on the concrete use case.
What happens here is that when your child task is cancelled, the OperationCanceledException brings down your mainBlock as well. I was able to get it to work by using this:
let rec mainBlock =
async {
ctsRef := new CancellationTokenSource()
let task = Async.StartAsTask(subBlock, TaskCreationOptions.None, (!ctsRef).Token) |> Async.AwaitTask
do! Async.TryCancelled(task, fun e ->
(!ctsRef).Dispose()
printfn "restarting"
Async.Start mainBlock)
}
When the task is cancelled, mainBlock is explicitly restarted in the cancelation handler. You need to add #nowarn "40" for it since mainBlock is used inside its definition. Also note the dispose on token source.
You can find more information on this problem (and perhaps a nicer solution in the form of StartCatchCancellation) in these two threads.
Experimenting some with F# async taught me that I can StartImmediate on the current running thread. This allows me, it seems, to run an async expression which can still pass out control, whenever getting inside of it to some async operation (e.g. do!), to the code outside of the async expression. Please see the simple example below:
open System.Threading
let workThenWait() = async {
printfn "async start"
do! Async.Sleep(1000)
printfn "async end"
}
let demo() =
workThenWait() |> Async.StartImmediate
printfn "main started"
// here I want to wait to the async expression in case it has passed control
printfn "main end"
demo()
The result we get is:
async start
main started
main end
async end
On the other hand, if I execute the same async expression (in this case workThenWait) using StartAsTask (inside demo), I can potentially wait on it at the end.
MY QUESTION is:
using the previous example using StartImmediate, can I run on the same thread, but ALSO wait at the end for the async expression in case some async operation (such as do!) is called and passes control forward?
I think You need Async.RunSynchronously (http://msdn.microsoft.com/en-us/library/ee370262.aspx)
update:
Ok, now I understand better what You want, and I was able to achieve this with Async.StartWithContinuations method.
Here's the code:
open System.Threading
let f() =
printfn "main thread: %A" Thread.CurrentThread.ManagedThreadId
let c1 =
async {
printfn "c1 async thread: %A" Thread.CurrentThread.ManagedThreadId
do! Async.Sleep(1000)
return "some result"
}
let continuation s =
printfn "continuation thread: %A" Thread.CurrentThread.ManagedThreadId
printfn "now the code You want after waiting and the result %s" s
Async.StartWithContinuations(
c1,
continuation,
(fun _ -> ()),
(fun _ -> ())
)
printfn "Code that runs during async computation"
Now this is definitely not very readable as the flow of the code is not obvious. I couldn't find any better solution.
You can do this with Hopac libary:
let workThenWait() = job {
printfn "async start"
do! Hopac.Timer.Global.sleep (TimeSpan.FromMilliseconds 1000.)
printfn "async end"
}
let demo() =
let promise = workThenWait() |> Promise.start |> run
printfn "main started"
// here I want to wait to the async expression in case it has passed control
let result = run promise
printfn "main end"
demo()
Hopac is both more performant and functional than async and is little known compared to how good it is. I highly recommend it.
I am using F# mailBoxProcessor to asynchronously process messages received from multiple network ends.
The code works as expected until I added function call getTreasuryYield after inbox.receive().
It gets stuck every time at inbox.receive() after running for a few seconds.
GetTreasuryYield is a quite slow method since it involves database and IO operations, but I
still do not understand how it gets stuck.
Any HELP will be appreciated.
let start rvSetting (agent:Agent<Message>) messageSelector=
try
TIBCO.Rendezvous.Environment.Open()
let _transport = new NetTransport(rvSetting.rvService, rvSetting.rvNetwork, rvSetting.rvDaemon)
let _listener = new Listener(TIBCO.Rendezvous.Queue.Default, _transport, rvSetting.rvSubject, null)
_listener.MessageReceived.Add(fun args->
printfn "before sent"
if messageSelector(args.Message) then
printfn "Message sent to agent: %A" args.Message
agent.Post(args.Message))
let rec dispatch() =
async{
try
TIBCO.Rendezvous.Queue.Default.Dispatch()
return! dispatch()
with
| e -> _log.Error(e.ToString())
}
Async.Start(dispatch())
with
|e -> printfn "%A" e.Message
_log.Error(e.Message)
let agent = new Agent<Message>(fun inbox ->
let rec loop() =
async{
let! (m : Message) = inbox.Receive()
// This line causes the problem
printfn "%A" (getTreasuryYieldFromMessage m)
Async.Start(treasuryAction m)
return! loop()
}
loop())
agent.Error.Add raise
[<EntryPoint>]
let main argv =
//start rvCorporate agent (fun x -> true)
agent.Start()
start rvTreasury agent treasurySelector
Console.ReadLine() |> ignore
0
I have problems with seemingly inconsistent behavior when cancelling different kinds of Asyncs.
To reproduce the problem, let's says there is a function that takes a list of "jobs" (Async<_> list), waits for them to complete and prints their results. The function also gets a cancellation token so it can be cancelled:
let processJobs jobs cancel =
Async.Start(async {
try
let! results = jobs |> Async.Parallel
printfn "%A" results
finally
printfn "stopped"
}, cancel)
The function is called like that:
let jobs = [job1(); job2(); job3(); job4(); job5()]
use cancel = new CancellationTokenSource()
processJobs jobs cancel.Token
And somewhat later it is cancelled:
Thread.Sleep(1000)
printfn "cancelling..."
cancel.Cancel()
When the cancellation token source is cancelled, the function should execute the finally-block and print "stopped".
That works fine for job1, 2 and 3, but doesn't work when there is a job4 or job5 in the list.
Job1 just Async.Sleeps:
let job1() = async {
do! Async.Sleep 1000000
return 10
}
Job2 starts some async childs and waits for them:
let job2() = async {
let! child1 = Async.StartChild(async {
do! Async.Sleep 1000000
return 10
})
let! child2 = Async.StartChild(async {
do! Async.Sleep 1000000
return 10
})
let! results = [child1; child2] |> Async.Parallel
return results |> Seq.sum
}
Job3 waits for some ugly wait handle that's set by some even uglier thread:
let job3() = async {
use doneevent = new ManualResetEvent(false)
let thread = Thread(fun () -> Thread.Sleep(1000000); doneevent.Set() |> ignore)
thread.Start()
do! Async.AwaitWaitHandle(doneevent :> WaitHandle) |> Async.Ignore
return 30
}
Job4 posts to and waits for a reply from a MailboxProcessor:
let job4() = async {
let worker = MailboxProcessor.Start(fun inbox -> async {
let! (msg:AsyncReplyChannel<int>) = inbox.Receive()
do! Async.Sleep 1000000
msg.Reply 40
})
return! worker.PostAndAsyncReply (fun reply -> reply) // <- cannot cancel this
}
Job5 waits for a Task (or TaskCompletionSource):
let job5() = async {
let tcs = TaskCompletionSource<int>()
Async.Start(async {
do! Async.Sleep 1000000
tcs.SetResult 50
})
return! (Async.AwaitTask tcs.Task) // <- cannot cancel this
}
Why can Job1, 2 and 3 be cancelled ("stopped" gets printed), while Job4 and 5 make the function hang "forever"?
So far I always relied on F# to handle cancellation behind the scenes - as long as I'm in async-blocks and use !s (let!, do!, return!,...) everything should be fine.. but that doesn't seem to be the case all the time.
Quote:
In F# asynchronous workflows, the CancellationToken object is passed
around automatically under the cover. This means that we don't have to
do anything special to support cancellation. When running asynchronous
workflow, we can give it cancellation token and everything will work
automatically.
Complete code is available here: http://codepad.org/euVO3xgP
EDIT
I noticed that piping an async through Async.StartAsTask followed by Async.AwaitTask makes it cancelable in all cases.
i.e. for Job4 that means changing the line:
return! worker.PostAndAsyncReply (fun reply -> reply)
to:
return! cancelable <| worker.PostAndAsyncReply (fun reply -> reply)
With cancelable being:
let cancelable (x:Async<_>) = async {
let! cancel = Async.CancellationToken
return! Async.StartAsTask(x, cancellationToken = cancel) |> Async.AwaitTask
}
The same works for making Job5 cancelable.
But.. that's just a workaround and I can hardly put that around each call to an unknown Async<_>.
Only the Async. methods handle using the default CancellationToken themselves.
In your MailboxProcessor example the cancel should go on the Start method
let! ct= Async.CancellationToken
use worker := MailboxProcessor.Start( theWork, ct)
In the TaskCompletionSource example, you are going to have to register a callback to cancel it.
let! ct = Async.CancellationToken
use canceler = ct.Register( fun () -> tcs.TrySetCanceled() |> ignore )