powershell start process within for each loop - powershell

Question

I have a for each loop I wish to run in parallel, however I'm not getting a uniform time as to when each iteration kicks off, which is leading to some edgecase timing issues.
As such, I've changed my loop just to be sequential. However, I don't want to wait for the iteration commands to complete.
Each iteration is essentially doing:
Invoke-Pester #{Path= "tests.ps1"; Parameters = #{...}} -Tag 'value' -OutputFile $xmlpath -OutputFormat NUnitXML -EnableExit
I want each iteration to run sequentially (tests expect to be run sequentially) however I don't wish to wait for tests to complete.
What is the best way to ensure the iteration does not wait for the Invoke-Pester command to complete, such that the next iteration kicks off after the previous iteration has initiated? I've tried using Start-Process Invoke-Pester which I think invalidated further code structure.
Thank you

Answer 1

One way for async processing are PowerShell jobs. A very basic example:
$jobs = foreach ($xmlPath in (...)) {
Start-job { Invoke-Pester -OutputFile $Args[0] ... } -ArgumentList $xmlPath
}
# get the job results later:
$jobs | Receive-Job
However, jobs are very slow. A more performant but also a little more complex way is using background runspaces:
$jobs = foreach ($xmlPath in (...)) {
$ps = [Powershell]::Create()
[void]$ps.AddScript({ Invoke-Pester -OutputFile $Args[0] ... })
[void]$ps.AddArgument($xmlPath)
#{
PowerShell = $ps
AsyncResult = $ps.BeginInvoke()
}
}
# get results:
foreach ($job in $jobs) {
$job.PowerShell.EndInvoke($_.AsyncResult)
$job.PowerShell.Dispose()
}