Async.Start vs Async.StartChild

Question

Assuming asyncSendMsg doesn't return anything and I want to start it inside another async block, but not wait for it to finish, is there any difference between this:

async {
    //(...async stuff...)
    for msg in msgs do 
        asyncSendMsg msg |> Async.Start
    //(...more async stuff...)
}

and

async {
    //(...async stuff...)
    for msg in msgs do 
        let! child = asyncSendMsg msg |> Async.StartChild
        ()
    //(...more async stuff...)
}

Answer 1

The key difference is that when you start a workflow with Async.StartChild, it will share the cancellation token with the parent. If you cancel the parent, all children will be cancelled too. If you start the child using Async.Start, then it is a completely independent workflow.

Here is a minimal example that demonstrates the difference:

// Wait 2 seconds and then print 'finished'
let work i = async {
  do! Async.Sleep(2000)
  printfn "work finished %d" i }

let main = async { 
    for i in 0 .. 5 do
      // (1) Start an independent async workflow:
      work i |> Async.Start
      // (2) Start the workflow as a child computation:
      do! work i |> Async.StartChild |> Async.Ignore 
  }

// Start the computation, wait 1 second and than cancel it
let cts = new System.Threading.CancellationTokenSource()
Async.Start(main, cts.Token)
System.Threading.Thread.Sleep(1000)    
cts.Cancel()

In this example, if you start the computation using (1), then all work items will finish and print after 2 seconds. If you use (2) they will all be cancelled when the main workflow is cancelled.