about_Foreach-Parallel

Article
03/18/2022
2 minutes to read

SHORT DESCRIPTION

Describes the ForEach -Parallel language construct in Windows PowerShell Workflow.

LONG DESCRIPTION

The Parallel parameter of the ForEach keyword runs the commands in a ForEach script block once for each item in a specified collection.

The items in the collection, such as a disk in a collection of disks, are processed in parallel. The commands in the script block run sequentially on each item in the collection.

ForEach -Parallel is valid only in a Windows PowerShell Workflow.

SYNTAX

ForEach -Parallel ($<item> in $<collection>)
{
    [<Activity1>]
    [<Activity2>]
    ...
}

DETAILED DESCRIPTION

Like the ForEach statement in Windows PowerShell, the variable that contains collection $<collection> must be defined before the ForEach -Parallel statement, but the variable that represents the current item $<item> is defined in the ForEach -Parallel statement.

The ForEach -Parallel construct is different from the ForEach keyword and the Parallel parameter. The ForEach keyword processes the items in the collection in sequence. The Parallel parameter runs commands in a script block in parallel. You can enclose a Parallel script block in a ForEach -Parallel script block.

The target computers in a workflow, such as those specified by the PSComputerName workflow common parameter, are always processed in parallel. You do not need to specify the ForEach -Parallel keyword for this purpose.

EXAMPLES

The following workflow contains a ForEach -Parallel statement that processes the disks that the Get-Disk activity gets. The commands in the ForEach -Parallel script block run sequentially, but they run on the disks in parallel. The disks might be processed concurrently and in any order.

workflow Test-Workflow
{
    $Disks = Get-Disk

    # The disks are processed in parallel.
    ForEach -Parallel ($Disk in $Disks)
    {
        # The commands run sequentially on each disk.
        $DiskPath = $Disk.Path
        $Disk | Initialize-Disk
        Set-Disk -Path $DiskPath
    }
}

In this version of the workflow, the Get-Process and Get-Service commands are run in parallel. The workflow function continues to the ForEach -Parallel loop where the commands are run sequentially, but they run on the disks in parallel. The parallel commands and the ForEach -Parallel loop run concurrently.

workflow Test-Workflow
{
    #Run commands in parallel.
    Parallel
    {
        Get-Process
        Get-Service
    }

   $Disks = Get-Disk

   # The disks are processed in parallel.
   ForEach -Parallel ($Disk in $Disks)
   {
       # The commands run in parallel on each disk.
       Parallel
       {
           Initialize-Disk
           InlineScript {.\Get-DiskInventory}
       }
   }
}