ABOUT_FOREACH-PARALLEL


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}
           }
       }
    }


See Also

-   about_ForEach
-   about_Language_Keywords
-   about_Parallel
-   about_Workflows
-   Writing a Script Workflow