Throttle-AzRunbooks.ps1

# Get the Job ID from PSPrivateMetadata. That's the only thing it contains!
$automationJobId = $PSPrivateMetadata.JobId.Guid

# Maximum number of instances of this Runbook I want running at any given time
$numberOfInstances = 1

# Connect to Azure. With a Managed Identity in this case as that's what I use. 
# Must connect to Azure before running Get-AzAutomationJob or Get-AzResource
try {
    # From https://docs.microsoft.com/en-us/azure/automation/enable-managed-identity-for-automation#authenticate-access-with-system-assigned-managed-identity
    # Ensures you do not inherit an AzContext in your runbook
    Disable-AzContextAutosave -Scope Process | Out-Null

    # Connect to Azure with system-assigned managed identity
    Connect-AzAccount -Identity | Out-Null

} catch {
    Write-Error "Runbook could not connect to Azure: $($_.Exception.Message)"
    exit
}

# Get all Runbooks in the current subscription
$allAutomationAccounts = Get-AzResource -ResourceType Microsoft.Automation/automationAccounts

$automationAccountName = $null
$resourceGroupName = $null
$runbookName = $null

foreach ($automationAccount in $allAutomationAccounts) {
	$runbookJob = Get-AzAutomationJob -AutomationAccountName $automationAccount.Name `
                                -ResourceGroupName $automationAccount.ResourceGroupName `
                                -Id $automationJobId `
                                -ErrorAction SilentlyContinue

    if (!([string]::IsNullOrEmpty($runbookJob))) {
        $automationAccountName = $runbookJob.AutomationAccountName
        $resourceGroupName = $runbookJob.ResourceGroupName
        $runbookName = $runbookJob.RunbookName
    }
}

# At this point I'll have the Automation Account Name, Runbook Name, Job ID and Resource Group Name, 
# Find all other active jobs of this Runbook.


$allActiveJobs = Get-AzAutomationJob -AutomationAccountName $automationAccountName `
                                -ResourceGroupName $resourceGroupName `
                                -RunbookName $runbookName | 
                Where-Object { ($_.Status -eq "Running") -or ($_.Status -eq "Starting") -or ($_.Status -eq "Queued")}  

$oldestJob = $AllActiveJobs | Sort-Object -Property CreationTime  | Select-Object -First 1

# If this job is not the oldest created job we will wait until the existing jobs complete or the number of jobs is less than numberOfInstances
while (($AutomationJobID -ne $oldestJob.JobId) -and ($allActiveJobs.Count -ge $numberOfInstances)) {
    Write-Warning "Waiting as there are currently running $($allActiveJobs.Count) active jobs for this runbook already. Sleeping 30 seconds..."
    Write-Warning "Oldest Job is $($oldestJob.JobId)"

    Start-Sleep -Seconds 30

    $allActiveJobs = Get-AzAutomationJob -AutomationAccountName $automationAccountName `
                                -ResourceGroupName $resourceGroupName `
                                -RunbookName $runbookName | 
                Where-Object { ($_.Status -eq "Running") -or ($_.Status -eq "Starting") -or ($_.Status -eq "Queued")}  

    $oldestJob = $allActiveJobs | Sort-Object -Property CreationTime | Select-Object -First 1
} 

Write-Output "Job can continue..."