Issue with Async.Sequential, in F#

Question

I have the following:

do!
    [
        [
            TradesDatabase.deleteTradesAsync    credentials time (time.Add oneDay)
            TradesDatabase.insertTradesAsync    credentials trades
        ] |> Async.Sequential
        
        [
            KVWAPDatabase.deleteKVWAPAsync      credentials time (time.Add oneDay)
            KVWAPDatabase.insertKVWAPAsync      credentials output.KVWAP
        ] |> Async.Sequential
        
        [
            CandlesDatabase.deleteCandlesAsync  credentials time (time.Add oneDay)
            CandlesDatabase.insertCandlesAsync  credentials output.Candles
        ] |> Async.Sequential
    ]
    |> Async.Parallel
    |> Async.Ignore

The inner code is probably not very useful, these are async db calls.

So for 3 data types I want to process in parallel, I want to do a sequential delete following by an insert.

But the debug's printout is:

delete kvwap              <- called by deleteKVWAPAsync
inserting kvwap           <- called by insertKVWAPAsync
done deleting kvwap       <- called by deleteKVWAPAsync

so this block:

[
    KVWAPDatabase.deleteKVWAPAsync      credentials time (time.Add oneDay)
    KVWAPDatabase.insertKVWAPAsync      credentials output.KVWAP
] |> Async.Sequential

was supposed to be executed in order, but it's not.

What am I missing regarding Async.Sequential?

I'm not able to reproduce this, even if I put a long Async.Sleep in the delete step. I suspect that maybe you've accidentally copy/pasted the kvwap debug output into one of the other parallel branches (either Trades or Candles) without changing the output text? — Brian Berns
– Brian Berns, Commented Apr 18, 2022 at 17:45
It's separate pieces of code and the DB complains about it as well, so the delete is not done in time. When I do the deletes with let!/and!/and! and then the inserts with let!/and!/and! it works properly though. Thanks for looking into it, I'll keep investigating! — Thomas
– Thomas, Commented Apr 18, 2022 at 17:56
Why don't you use an async CE and FusionTasks NuGet and call your async code inside the CE? — XperiAndri
– XperiAndri, Commented Apr 27, 2022 at 11:38
@Andrii if something can run with the base libs, it's better than adding anther package — Thomas
– Thomas, Commented Apr 27, 2022 at 12:09
If something is already implemented is better to reuse it than invent the wheel 😉 — XperiAndri
– XperiAndri, Commented Apr 28, 2022 at 18:42

Roland Andrag · Accepted Answer · 2022-04-20 07:58:36Z

2

I have experienced a similar issue.

It may be the case that your database functions are being run as "hot" tasks. For example, if I remember correctly the following code starts the computation immediately, even though it is returning an Async<int>:

  type DbContext =
   Microsoft.Data.SqlClient.SqlConnection * Microsoft.Data.SqlClient.SqlTransaction

  let doQuery ((connection, transaction) : DbContext) query =
      (connection.ExecuteAsync(query, transaction))
      |> Async.AwaitTask

While this code does not:

  let doQuery2 ((connection, transaction) : DbContext) query =
    async {
      return! 
        (connection.ExecuteAsync(query, transaction))
        |> Async.AwaitTask
    }

answered Apr 20, 2022 at 7:58

Roland Andrag

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