Throttling

Is it possible to throttle a Stream by an “elements by second” values? Something like this:

seq { ... }
|> Stream.ofSeq
|> Stream.throttle (perSecond 20)
|> ...

No elements must be lost (i.e. it should do back pressure if incoming stream rate is bigger that needed).

[polytypic]

I added Stream.duringEach for the kind of throttling where you actually do want to roughly aim at a given average time per element. Here is an example:

  let inline (^) x = x

  let randomTimeOutMillis min max = Alt.random <| fun r ->
    let t = int (r % (uint64 max - uint64 min) + uint64 min)
    timeOutMillis t ^->. t

  let test n =
    let start = Stopwatch.StartNew ()
    let source () = Stream.indefinitely ^ randomTimeOutMillis 0 1000

    source ()
    |> Stream.take n
    |> Stream.consumeFun ^ fun t ->
         printfn "%A plain  %3dms" start.Elapsed t

    source ()
    |> Stream.afterEach ^ timeOutMillis 1000
    |> Stream.take n
    |> Stream.consumeFun ^ fun t ->
         printfn "%A after  %3dms" start.Elapsed t

    source ()
    |> Stream.duringEach ^ timeOutMillis 1000
    |> Stream.take n
    |> Stream.consumeFun ^ fun t ->
         printfn "%A during %3dms" start.Elapsed t

Running test 20L gives the following output:

00:00:00.1576488 plain  153ms
00:00:00.2253215 during 221ms
00:00:00.2468896 after  244ms
00:00:01.0783881 during  71ms
00:00:01.0783523 plain  916ms
00:00:01.3960721 plain  313ms
00:00:02.0017792 plain  601ms
00:00:02.0422724 during  38ms
00:00:02.0588948 plain   57ms
00:00:02.1386503 after  884ms
00:00:02.1786046 plain  115ms
00:00:02.9503481 plain  768ms
00:00:03.0014354 plain   51ms
00:00:03.2534785 plain  248ms
00:00:03.7342758 after  590ms
00:00:03.7351870 during 731ms
00:00:04.1003132 plain  845ms
00:00:04.3714115 plain  267ms
00:00:04.5802716 plain  204ms
00:00:04.7805773 after   38ms
00:00:04.7805773 plain  198ms
00:00:04.9655979 during 957ms
00:00:05.5510391 plain  766ms
00:00:05.5575898 during 550ms
00:00:05.8140434 plain  258ms
00:00:05.8196720 plain    5ms
00:00:06.0293120 during  18ms
00:00:06.3057120 plain  485ms
00:00:06.4705700 after  682ms
00:00:06.9917063 plain  682ms
00:00:07.2592276 during 244ms
00:00:07.4995643 plain  504ms
00:00:07.9431675 after  464ms
00:00:08.3516039 plain  852ms
00:00:08.6102287 during 595ms
00:00:09.4811373 during 461ms
00:00:09.6259404 after  675ms
00:00:10.1214586 during  97ms
00:00:11.2936779 during 265ms
00:00:11.4575013 after  823ms
00:00:12.9393301 during 907ms
00:00:12.9787406 after  512ms
00:00:13.8574682 during 811ms
00:00:14.2032069 after  220ms
00:00:14.7311408 during 681ms
00:00:15.5234377 after  311ms
00:00:15.8989877 during 845ms
00:00:16.3184764 during 260ms
00:00:16.8427622 after  311ms
00:00:17.2441344 during 182ms
00:00:18.6631350 during 597ms
00:00:18.7536870 after  903ms
00:00:19.7451558 during 675ms
00:00:20.2443917 after  486ms
00:00:21.2817035 after   28ms
00:00:23.1290225 after  839ms
00:00:24.4493359 after  313ms
00:00:26.4266176 after  969ms
00:00:27.8035456 after  369ms
00:00:29.3323187 after  522ms

As you can see, plain completes in (very) roughly 10 seconds, during in roughly 20 seconds and after in roughly 30 seconds. Elements take a random amount of time between 0 and 1 seconds to generate.

[vasily-kirichenko]

Thanks, very cool! 😃