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Throttling agent

Agent that can be used for controlling the number of concurrently executing asynchronous workflows. The agent runs a specified number of operations concurrently and queues remaining pending requests. The queued work items are started as soon as one of the previous items completes.

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/// Message type used by the agent - contains queueing 
/// of work items and notification of completion 
type internal ThrottlingAgentMessage = 
  | Completed
  | Work 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
type ThrottlingAgent(limit) = 
  let agent = MailboxProcessor.Start(fun agent -> 

    /// Represents a state when the agent is blocked
    let rec waiting () = 
      // Use 'Scan' to wait for completion of some work
      agent.Scan(function
        | Completed -> Some(working (limit - 1))
        | _ -> None)

    /// Represents a state when the agent is working
    and working count = async { 
      // Receive any message 
      let! msg = agent.Receive()
      match msg with 
      | Completed -> 
          // Decrement the counter of work items
          return! working (count - 1)
      | Work work ->
          // Start the work item & continue in blocked/working state
          async { try do! work 
                  finally agent.Post(Completed) }
          |> Async.Start
          if count < limit - 1 then return! working (count + 1)
          else return! waiting () }

    // Start in working state with zero running work items
    working 0)      

  /// Queue the specified asynchronous workflow for processing
  member x.DoWork(work) = agent.Post(Work work)
union case ThrottlingAgentMessage.Completed: ThrottlingAgentMessage
union case ThrottlingAgentMessage.Work: Async<unit> -> ThrottlingAgentMessage
Multiple items
type Async
static member AsBeginEnd : computation:('Arg -> Async<'T>) -> ('Arg * AsyncCallback * obj -> IAsyncResult) * (IAsyncResult -> 'T) * (IAsyncResult -> unit)
static member AwaitEvent : event:IEvent<'Del,'T> * ?cancelAction:(unit -> unit) -> Async<'T> (requires delegate and 'Del :> Delegate)
static member AwaitIAsyncResult : iar:IAsyncResult * ?millisecondsTimeout:int -> Async<bool>
static member AwaitTask : task:Task<'T> -> Async<'T>
static member AwaitWaitHandle : waitHandle:WaitHandle * ?millisecondsTimeout:int -> Async<bool>
static member CancelDefaultToken : unit -> unit
static member Catch : computation:Async<'T> -> Async<Choice<'T,exn>>
static member FromBeginEnd : beginAction:(AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg:'Arg1 * beginAction:('Arg1 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg1:'Arg1 * arg2:'Arg2 * beginAction:('Arg1 * 'Arg2 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg1:'Arg1 * arg2:'Arg2 * arg3:'Arg3 * beginAction:('Arg1 * 'Arg2 * 'Arg3 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromContinuations : callback:(('T -> unit) * (exn -> unit) * (OperationCanceledException -> unit) -> unit) -> Async<'T>
static member Ignore : computation:Async<'T> -> Async<unit>
static member OnCancel : interruption:(unit -> unit) -> Async<IDisposable>
static member Parallel : computations:seq<Async<'T>> -> Async<'T []>
static member RunSynchronously : computation:Async<'T> * ?timeout:int * ?cancellationToken:CancellationToken -> 'T
static member Sleep : millisecondsDueTime:int -> Async<unit>
static member Start : computation:Async<unit> * ?cancellationToken:CancellationToken -> unit
static member StartAsTask : computation:Async<'T> * ?taskCreationOptions:TaskCreationOptions * ?cancellationToken:CancellationToken -> Task<'T>
static member StartChild : computation:Async<'T> * ?millisecondsTimeout:int -> Async<Async<'T>>
static member StartChildAsTask : computation:Async<'T> * ?taskCreationOptions:TaskCreationOptions -> Async<Task<'T>>
static member StartImmediate : computation:Async<unit> * ?cancellationToken:CancellationToken -> unit
static member StartWithContinuations : computation:Async<'T> * continuation:('T -> unit) * exceptionContinuation:(exn -> unit) * cancellationContinuation:(OperationCanceledException -> unit) * ?cancellationToken:CancellationToken -> unit
static member SwitchToContext : syncContext:SynchronizationContext -> Async<unit>
static member SwitchToNewThread : unit -> Async<unit>
static member SwitchToThreadPool : unit -> Async<unit>
static member TryCancelled : computation:Async<'T> * compensation:(OperationCanceledException -> unit) -> Async<'T>
static member CancellationToken : Async<CancellationToken>
static member DefaultCancellationToken : CancellationToken

Full name: Microsoft.FSharp.Control.Async

--------------------
type Async<'T>

Full name: Microsoft.FSharp.Control.Async<_>
type unit = Unit

Full name: Microsoft.FSharp.Core.unit
Multiple items
type ThrottlingAgent =
  new : limit:int -> ThrottlingAgent
  member DoWork : work:Async<unit> -> unit

Full name: Script.ThrottlingAgent


 Represents an agent that runs operations in concurrently. When the number
 of concurrent operations exceeds 'limit', they are queued and processed later


--------------------
new : limit:int -> ThrottlingAgent
val limit : int
val agent : MailboxProcessor<ThrottlingAgentMessage>
Multiple items
type MailboxProcessor<'Msg> =
  interface IDisposable
  new : body:(MailboxProcessor<'Msg> -> Async<unit>) * ?cancellationToken:CancellationToken -> MailboxProcessor<'Msg>
  member Post : message:'Msg -> unit
  member PostAndAsyncReply : buildMessage:(AsyncReplyChannel<'Reply> -> 'Msg) * ?timeout:int -> Async<'Reply>
  member PostAndReply : buildMessage:(AsyncReplyChannel<'Reply> -> 'Msg) * ?timeout:int -> 'Reply
  member PostAndTryAsyncReply : buildMessage:(AsyncReplyChannel<'Reply> -> 'Msg) * ?timeout:int -> Async<'Reply option>
  member Receive : ?timeout:int -> Async<'Msg>
  member Scan : scanner:('Msg -> Async<'T> option) * ?timeout:int -> Async<'T>
  member Start : unit -> unit
  member TryPostAndReply : buildMessage:(AsyncReplyChannel<'Reply> -> 'Msg) * ?timeout:int -> 'Reply option
  ...

Full name: Microsoft.FSharp.Control.MailboxProcessor<_>

--------------------
new : body:(MailboxProcessor<'Msg> -> Async<unit>) * ?cancellationToken:System.Threading.CancellationToken -> MailboxProcessor<'Msg>
static member MailboxProcessor.Start : body:(MailboxProcessor<'Msg> -> Async<unit>) * ?cancellationToken:System.Threading.CancellationToken -> MailboxProcessor<'Msg>
val waiting : (unit -> Async<'a>)


 Represents a state when the agent is blocked
member MailboxProcessor.Scan : scanner:('Msg -> Async<'T> option) * ?timeout:int -> Async<'T>
union case Option.Some: Value: 'T -> Option<'T>
val working : (int -> Async<'a>)


 Represents a state when the agent is working
union case Option.None: Option<'T>
val count : int
val async : AsyncBuilder

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.async
val msg : ThrottlingAgentMessage
member MailboxProcessor.Receive : ?timeout:int -> Async<'Msg>
val work : Async<unit>
member MailboxProcessor.Post : message:'Msg -> unit
static member Async.Start : computation:Async<unit> * ?cancellationToken:System.Threading.CancellationToken -> unit
val x : ThrottlingAgent
member ThrottlingAgent.DoWork : work:Async<unit> -> unit

Full name: Script.ThrottlingAgent.DoWork


 Queue the specified asynchronous workflow for processing

More information

Link:http://fssnip.net/5o
Posted:6 years ago
Author:Tomas Petricek
Tags: agent , message passing , mailboxprocessor , throttling