How to resume PowerShell script after system reboot?

Question

I am using below powershell script to delete SharePoint Alerts.

foreach ($site in Get-SPSite -Limit All) 
{ 
  "Site Collection $site" 
  foreach ($web in $site.allwebs)
  {
     "  Web $web"
     $c = $web.alerts.count
     "    Deleting $c alerts"
     for ($i=$c-1;$i -ge 0; $i--) { $web.alerts.delete($i) }
  }
}

There are around million alerts in each of Dev, Test and UAT environments. It takes many hours to delete all the alerts at one go and as the servers automatically get restarted periodically, the script doesn't get executed fully.

I am aware that to resume PowerShell scripts after reboot we can use PowerShell Workflow with Checkpoint-workflow but not sure where to place checkpoints and PSPersist .

Need help to resume deleting of alerts in the above script after system reboot.

Update: After trying to implement it, I realized that SharePoint PowerShell cmdlets cannot be coupled with PowerShell Workflow. It doesn't allow

Add-PSSnapin "Microsoft.SharePoint.PowerShell"

to be added to workflows

Workflow SPAlerts
{
//Below tweaks didn't work
 InlineScript{Add-PSSnapin "Microsoft.SharePoint.PowerShell"}
 Invoke-Command -ScriptBlock {Add-PSSnapin "Microsoft.SharePoint.PowerShell"}
 Invoke-Expression "......"
}

Answer 1

The MSDN documentation states:

You can place checkpoints anywhere in a workflow, including before and after each command or expression, or after each activity in a workflow. Windows PowerShell Workflow allows you to do so.

... Add a checkpoint after each significant part of the workflow completes; a part that you do not want to repeat if the workflow is interrupted.

... When the workflow definitions is complete, use the testing process to refine your checkpoints. Add and remove (comment out) checkpoints to achieve both a reasonable running time and a reasonable recovery time.

Looking at your code, I would place the checkpoint right after "Site Collection $site" if deleting alerts for the given website takes reasonable time on average. If there are just few sites each of them containing tons of alerts then I would place it on the start of the next foreach.

I would definitely not place it inside the worker foreach which deletes alerts.

I would also suggest you look at the foreach -Parallel capability of workflows to make the deleting parallel. That way even the last site/website should get it's turn, even if the server is restarted often.