c# - Throttle - 最多每 N 毫秒调用一个函数？

Question

如果过于频繁地调用以下函数中的函数 callWebServiceAsync 将被拒绝。完成可能需要几毫秒到一分钟。

F#

let doWorkAsync p = async { // doWorkAsync will be called many times
    ....
    let! ret = callWebServiceAsync p' // need to be throttled, high volumn of requestion in a short time will cause blocking
    ....
    let ret' = process ret
    ret'
}

C#

async Task<T> DoWorkAsync(S p) // DoWorkAsync will be called many times
{
    ....
    ret = await CallWebServiceAsync(...); // need to be throttled, high volumn of requestion in a short time will cause blocking
    ....
    return Process(ret);
}

如何限制调用频率？我不确定他们是如何检测调用的，所以最好均匀地调用函数(没有突发请求)。

Answer 1

我的第一 react 是使用 MailboxProcessor .这就是您通常强制所有调用通过单个网关的方式。

下面是一个 throttle 函数，每个时间跨度最多返回一次异步延续。高水平，它

1. 从消息队列中获取调用请求 (inbox.Receive)。此请求包含返回结果的 channel 。
2. 检查上次运行是否需要延迟，然后休眠
3. 记下当前调用的开始时间(请注意，如果您想根据调用的结束时间进行节流，则可以交换此步骤和第 4 步)
4. 触发调用者(chan.Reply)
5. 循环获取另一个请求

代码如下

let createThrottler (delay: TimeSpan) =
  MailboxProcessor.Start(fun inbox ->
    let rec loop (lastCallTime: DateTime option) =
      async {
        let! (chan: AsyncReplyChannel<_>) = inbox.Receive()
        let sleepTime =
          match lastCallTime with
          | None -> 0
          | Some time -> int((time - DateTime.Now + delay).TotalMilliseconds)
        if sleepTime > 0 then
          do! Async.Sleep sleepTime
        let lastCallTime = DateTime.Now
        chan.Reply()
        return! loop(Some lastCallTime)
      }
    loop None)

然后你可以像这样使用它:

[<EntryPoint>]
let main argv =
  // Dummy implementation of callWebServiceAsync
  let sw = Stopwatch.StartNew()
  let callWebServiceAsync i =
    async {
      printfn "Start %d %d" i sw.ElapsedMilliseconds
      do! Async.Sleep(100)
      printfn "End %d %d" i sw.ElapsedMilliseconds
      return i
    }

  // Create a throttler MailboxProcessor and then the throttled function from that.
  let webServiceThrottler = createThrottler (TimeSpan.FromSeconds 1.)
  let callWebServiceAsyncThrottled i =
    async {
       do! webServiceThrottler.PostAndAsyncReply(id)
       return! callWebServiceAsync i
    }

  // Some tests
  Async.Start(async { let! i = callWebServiceAsyncThrottled 0
                      printfn "0 returned %d" i
                      let! i = callWebServiceAsyncThrottled 1
                      printfn "1 returned %d" i
                      let! i = callWebServiceAsyncThrottled 2
                      printfn "2 returned %d" i })
  Async.Start(callWebServiceAsyncThrottled 3 |> Async.Ignore)
  Async.Start(callWebServiceAsyncThrottled 4 |> Async.Ignore)
  Async.Start(callWebServiceAsyncThrottled 5 |> Async.Ignore)
  Async.Start(callWebServiceAsyncThrottled 6 |> Async.Ignore)
  Console.ReadLine() |> ignore
  0

如果您运行此程序，您会发现它会根据需要限制对该服务的调用，无论您是并行运行还是串行运行或两者兼而有之。