about_Sequence

Article
09/19/2022

Short description

Describes the Sequence keyword that runs selected activities sequentially.

Long description

The Sequence keyword runs selected workflow activities sequentially. Workflow activities run in the order that they appear and do not run concurrently. The Sequence keyword is only valid in a PowerShell Workflow.

The Sequence keyword is used in a Parallel script block to run selected commands sequentially.

Because workflow activities run sequentially by default, the Sequence keyword is only effective in a Parallel script block. If the Sequence keyword isn't included in a Parallel script block, it's valid but ineffective.

The Sequence script block lets you run more commands in parallel by allowing you to run dependent commands sequentially.

Syntax

Workflow using Sequence

workflow <Verb-Noun>
{
    Sequence
    {
        [<Activity>]
        [<Activity>]
        # ...
    }
}

Workflow using Parallel and Sequence

workflow <Verb-Noun>
{
    Parallel
    {
        [<Activity>]
        Sequence
        {
            [<Activity>]
            [<Activity>]
            # ...
        }
    }
}

Detailed description

The commands in a Parallel script block can run concurrently. The order in which they run is not determined. This feature improves the performance of a script workflow.

You can use a Sequence script block to run selected activities sequentially, even though the activities appear in a Parallel script block.

The activities in a Sequence script block run consecutively in the order that they are listed. An activity in a Sequence script block starts only after the previous activity completes.

However, when the Sequence script block appears in a Parallel script block, the order in which the Sequence script block runs isn't determined. It might run before, after, or concurrent with other activities in the Parallel script block.

For example, the following workflow includes a Parallel script block that runs activities that get processes and services on the computer. The Parallel script block contains a Sequence script block that gets information from a file and uses the information as input to a script.

The Get-Process, Get-Service, and hotfix-related commands are independent of each other. The commands can run concurrently or in any order. But, the command that gets the hotfix information must run before the command that uses it.

workflow Test-Workflow
{
    Parallel
    {
    Get-Process
    Get-Service

    Sequence
    {
        $Hotfix = Get-Content 'D:\HotFixes\Required.txt'
        Foreach ($h in $Hotfix) {'D:\Scripts\Verify-Hotfix' -Hotfix $h}
        }
    }
}

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